1Department of Radiation Oncology, Soonchunhyang University Seoul Hospital, Soonchunhyang University College of Medicine, Seoul, Korea
2Department of Radiation Oncology, Seoul National University Hospital, Seoul National University College of Medicine, Seoul, Korea
3Cancer Research Institute, Seoul National University College of Medicine, Seoul, Korea
*Corresponding author: Hak Jae Kim,
Department of Radiation Oncology, Seoul National University Hospital, Seoul
National University College of Medicine, 101 Daehak-ro, Jongno-gu, Seoul 03080,
Korea, E-mail: khjae@snu.ac.kr
• Received: August 21, 2024 • Accepted: September 23, 2024
This is an Open-Access article distributed under the terms of the
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FLASH radiotherapy (FLASH-RT) is an innovative approach that delivers ultra-high
dose rates exceeding 40 Gy in less than a second, aiming to widen the
therapeutic window by minimizing damage to normal tissue while maintaining tumor
control. This review explores the advancements, mechanisms, and clinical
applications of FLASH-RT across various radiation sources. Electrons have been
predominantly used due to technical feasibility, but their limited penetration
depth restricts clinical application. Protons, offering deeper tissue
penetration, are considered promising for treating deep-seated tumors despite
challenges in beam delivery. Preclinical studies demonstrate that FLASH-RT
reduces normal tissue toxicity in the lung, brain, skin, intestine, and heart
without compromising antitumor efficacy. The mechanisms underlying the FLASH
effect may involve oxygen depletion leading to transient hypoxia, reduced DNA
damage in normal tissues, and modulation of immune and inflammatory responses.
However, these mechanisms are incompletely understood, and inconsistent results
across studies highlight the need for further research. Initial clinical
studies, including treatment of cutaneous lymphoma and bone metastases, indicate
the feasibility and potential benefits of FLASH-RT in patients. Challenges for
clinical implementation include technical issues in dosimetry accuracy at
ultra-high dose rates, adaptations in treatment planning systems, beam delivery
methods, and economic considerations due to specialized equipment requirements.
Future directions will involve comprehensive preclinical studies to optimize
irradiation parameters, large-scale clinical trials to establish standardized
protocols, and technological advancements to overcome limitations. FLASH-RT
holds the potential to revolutionize radiotherapy by reducing normal tissue
toxicity and improving therapeutic outcomes, but significant research is
required for real-world clinical applications.
Radiotherapy (RT) is a crucial component of antitumor therapies, and
approximately 30% of cancer patients in Korea undergo RT [1]. RT must strike a balance between efficiently killing
tumor cells and minimizing damage to normal tissue [2]. This constraint complicates the administration of an
adequate tumoricidal dose, presenting a significant challenge, especially in the
context of repeated RT [3,4]. Although numerous biological studies
have been conducted to prevent or mitigate RT-induced acute and late toxicities,
there have also been significant advancements in radiation technology over the
past few decades, such as intensity-modulated RT, stereotactic body RT, and
adaptive RT [5–10]. Moreover, technologies aided by
artificial intelligence have been integrated into these advancements [11]. Despite these improvements, the
radiation oncology community continues to explore new methods to expand the
therapeutic window, as there remain unmet medical needs.
One of these innovative approaches is FLASH radiotherapy (FLASH-RT), which
delivers irradiation at an ultra-high dose rate (UHDR) exceeding 40 Gy in less
than a second [12]. This method can
significantly shorten treatment times compared to conventional RT. In 1959,
Dewey and Boag first observed that the radiosensitivity of Serratia
marcescens decreased when exposed to 1.5 MV X-rays at a UHDR of
10–20 kilorads/2 µs, thereby protecting the bacteria compared to
exposure at conventional dose rates—a phenomenon now known as the FLASH
effect [13]. The term FLASH-RT was first
introduced in a 2014 study by Favaudon et al. [14]. Subsequent preclinical experiments with mammalian cells and
animal models have shown that UHDR irradiation, compared to conventional RT,
provides a similar antitumor effect while also protecting normal tissue [12]. In 2019, a case report detailed the
treatment of a patient with T-cell cutaneous lymphoma using FLASH-RT, noting
that the approach was feasible and resulted in favorable outcomes for both the
lymphoma and normal skin [15].
When cells and tissues are irradiated, a series of physical, chemical, and
biological reactions occur. However, after FLASH-RT, these reactions do not
advance to the biochemical phase [16].
Several radiobiological hypotheses have been proposed to explain the
differential effects of FLASH-RT on normal and tumor tissues, including oxygen
depletion, DNA damage, and the immune/inflammatory response [17]. Nevertheless, the exact mechanism of
action of FLASH-RT is still not well understood.
Objectives
The purpose of this review is to explore the revolutionary advancements and
underlying mechanisms of FLASH-RT in cancer treatment. By integrating findings
from both preclinical and clinical studies, this review aims to highlight the
therapeutic potential and challenges of FLASH-RT, ultimately bridging the gap
between cutting-edge radiobiological research and clinical application.
Ethics statement
As this study is a literature review, it did not require institutional review board
approval or individual consent.
FLASH radiotherapy beam delivery devices
FLASH-RT is theoretically feasible with all contemporary RT equipment; however, most
research has primarily utilized pulsed electron beams [18]. In this session, we will introduce the characteristics of
FLASH-RT according to different radiation sources. However, implementing FLASH-RT in
clinical settings using current RT modalities presents technical challenges,
including the need for multiple beam directions to ensure tumor conformity [19].
Electrons
The initial studies on FLASH-RT were performed using low-energy electrons (~25
MeV) from either experimental or medical linear accelerators [20]. Conventional C-arm and intraoperative
RT devices have also been successful in achieving UHDR [21]. These devices are readily available for UHDR, and
several vendors are developing electron beam FLASH-dedicated linear accelerators
[18]. However, the clinical
application of this technology is restricted by the inherent properties of
electrons, which include a low penetration depth of only a few centimeters, a
short source-target distance (~50 cm), and a significant lateral penumbra [18,21]. Consequently, only skin cancers or tumors located within
2–3 cm of the body surface are currently suitable for treatment with
FLASH-RT [22].
The use of very high energy electrons (VHEE, 50–250 MeV) has been
suggested as an effective method for delivering therapeutic doses to tumors deep
within the body using external electron beam FLASH-RT [22]. VHEE beams demonstrate relative insensitivity to body
inhomogeneities compared to protons [23].
Although VHEE beams are associated with high entrance and exit doses, employing
multi-directional beams can offer a benefit by sparing the skin [23,24]. However, due to the technical challenges associated with
electron acceleration, VHEE research is limited to a few facilities, including
the Photo Injector Test facility at Deutsches Elektronen-Synchrotron in Zeuthen
and the European Organization for Nuclear Research Linear Electron Accelerator
for Research facility [21].
Photons
Currently used linear accelerators in RT are unsuitable for photon beam-based
FLASH-RT [18]. The primary reason is the
high inefficiency in converting the electron beam to a photon beam, largely due
to electron heat deposition [21,22]. This inefficiency necessitates the
generation of a factor of 1,000 more electrons than current equipment can
handle, presenting a significant challenge that must be overcome. Furthermore,
technology that can accelerate this vast quantity of electrons and convert them
into photons is also required [22].
Unlike high-energy beams, X-rays with energies <1 MeV can achieve FLASH
conditions through a synchrotron [18].
The European Synchrotron Radiation Facility was the first to demonstrate that a
UHDR synchrotron light source could reduce brain injury in mice following
whole-brain irradiation [25]. Johns
Hopkins University successfully achieved UHDR (40–240 Gy/s) using two 150
kVp fluoroscopy systems [26]. Development
projects for new accelerators specifically designed for conventional,
high-energy photon UHDR beams are currently in progress. Notable examples
include the superconductive linac (6–8 MeV) from the Chengdu THz Free
Electron Laser group and the Pluridirectional High-energy Agile Scanning
Electronic RT platform from the Stanford Linear Accelerator Center [18,21].
Protons
Protons, unlike electrons and photons, possess a unique physical property known
as the Bragg peak, where they deposit most of their energy at a specific depth
just before stopping. This characteristic enables proton therapy to concentrate
the dose on the tumor site while reducing exposure beyond the Bragg peak,
thereby minimizing the risk of side effects [27]. Clinical isochronous cyclotrons utilized in proton therapy can
deliver intensities exceeding 60 Gy/s at a fixed energy [18]. With proton energies surpassing 200 MeV, these
cyclotrons are capable of treating deep-seated tumors [21]. As a result, protons are considered the most advanced
technology for the clinical application of FLASH therapy [18].
Proton therapy employs pencil beam scanning to accurately target tumor volumes.
However, a significant limitation of proton-based FLASH therapy stems from this
method of beam delivery, as existing technology cannot accommodate the rapid
energy modulation needed. In clinical settings, it is difficult to achieve FLASH
conditions across the entire tumor due to the current speed of 3D volumetric
scanning. Each energy change in the scanning process takes about one second,
which is too slow to satisfy FLASH criteria. To achieve FLASH conditions
throughout the entire tumor, the scanning speed must be increased by at least
two orders of magnitude [18].
Researchers are actively working to overcome this challenge and bring this
technology into routine clinical use. Hybrid active-passive systems featuring
patient-specific 3D range modulators are currently being tested in clinical
facilities and offer extremely rapid irradiation times (<1 s).
Additionally, laser-driven accelerators are under development at
Helmholtz-Zentrum Dresden-Rossendorf [18].
Heavy ions
The depth-dose distribution of heavy ions exhibits a Bragg peak similar to that
observed with protons; however, unlike protons, heavy ions also display a tail
region [21]. Heavy ions offer additional
advantages in RT, including a sharper lateral penumbra and higher relative
biological effectiveness [18,21]. Despite these advantages, the global
number of heavy ion centers remains limited. Consequently, research on FLASH-RT
using heavy ions has been relatively sparse, partly due to the technical
challenges associated with the synchrotron accelerators needed to achieve UHDR
[21]. Currently, the Heidelberg Ion
Beam Therapy Center has achieved dose rates exceeding 50 Gy/s using both helium
and carbon ions. Meanwhile, Gunma University Hospital has reached dose rates up
to 195 Gy/s with carbon ions, and the GSI Helmholtz Center for Heavy Ion
Research has exceeded 100 Gy/s [18].
In vivo studies on FLASH radiotherapy
The burgeoning interest in UHDR FLASH-RT is driven by recent preclinical studies that
have shown its potential to protect various normal tissues (Table 1). These promising results have sparked a surge of
in vivo research focused on understanding the underlying
mechanisms and improving clinical applications. RT often results in significant
toxicity, which complicates the administration of high doses to tumors [28–30]. Given FLASH-RT's ability to preserve normal tissues, it
could facilitate dose escalation, thus improving tumor control while maintaining
similar levels of normal tissue toxicity. However, it is crucial to acknowledge that
not all studies have demonstrated the FLASH effect, with some yielding negative
outcomes. This underscores the urgent need for additional research to better
comprehend the factors that affect the efficacy of FLASH-RT.
Table 1.
Summary of in vivo studies demonstrating the effect of
FLASH on normal tissues
K, 103; M, 106; HMGB1, high mobility group box
protein 1; cGAS, cyclic guanosine monophosphate–adenosine
monophosphate synthase; STING, stimulator of interferon genes; ROS,
reactive oxygen species; SOBP, spread-out Bragg peak; TNF, tumor
necrosis factor; SMA, smooth muscle actin.
1)Used to assess the effects on normal tissue.
2)To investigate the differences in FLASH-induced neuropreservation at
various dose rates, intermediate dose rates of 1, 3, 10, 30, 100, and
500 Gy/s were also used.
3)Delivered only to the right hemisphere.
Lung
The first groundbreaking proof of concept study in 2014, using a lung fibrosis
mouse model, demonstrated that 17 Gy FLASH-RT prevented lung fibrosis and
radiation-induced acute apoptosis in blood vessels and bronchi. At 30 Gy,
FLASH-RT was less fibrogenic than conventional RT, although it still caused some
pulmonary fibrosis [14]. The morphology
of lung tissue exposed to 20 Gy of either single or 10-pulse (fractionated)
FLASH resembled that of non-irradiated tissue, with only minimal neutrophil
infiltration [31]. This finding was
consistent with observations from 30 Gy FLASH whole-thorax irradiation [32]. In contrast, the conventional RT group
showed additional changes, including thickening of the alveolar septum and
interstitial hemorrhage [31].
The study conducted by Fouillade et al. provides compelling evidence that FLASH
irradiation offers protective effects on lung tissue compared to conventional RT
[33]. The researchers observed that
FLASH irradiation significantly reduced DNA damage in irradiated lung tissues
and decreased the proliferation of lung progenitor cells following injury.
Through single-cell RNA sequencing and histological analyses, it was revealed
that FLASH reduced the activation of pro-inflammatory genes, diminished
progenitor cell proliferation, and curtailed stem cell senescence, all factors
contributing to lung fibrosis. Notably, lungs treated with FLASH showed a
greater potential for tissue regeneration, exhibiting fewer signs of persistent
DNA damage and senescence compared to those treated with conventional RT.
However, these protective effects were absent in
Terc–/– mice, which have notably shortened
telomeres and deficient telomerase activity, underscoring the critical role of
telomere integrity and progenitor cell populations in harnessing the full
benefits of FLASH.
In a study comparing thorax-irradiated mice, the FLASH group exhibited an 81%
lower risk of mortality compared to the conventional group [32]. By the end of the follow-up period,
survival rates were 100% in the control group, 90% in the FLASH group, and 50%
in the conventional group.
Brain
Montay-Gruel et al. were the first to show that after 10 Gy whole-brain
irradiation in mice, spatial memory was impaired at conventional dose rates, but
remained unchanged at dose rates of ≥100 Gy/s [34]. This preservation of memory was attributed to the
reduced impact of FLASH-RT on neurogenesis in the subventricular zone of the
hippocampus compared to conventional dose rates. Montay-Gruel and colleagues
also demonstrated this neuroprotective effect using synchrotron-generated X-rays
for FLASH-RT [25]. Additionally,
radiation-induced reactive astrogliosis was less severe following FLASH
treatment than with conventional RT [25,35]. In later experiments,
FLASH-RT was associated with long-term neurocognitive benefits over a 6-month
period, reduced neuroinflammation, and preserved neuronal structure [36]. Not only was astrogliosis less
pronounced in the FLASH group compared to conventional RT, but there was also no
significant increase in the number of activated microglia in the hippocampus or
brain cortex [36,37]. The dendritic area, branches, and length, which were
significantly reduced after conventional RT compared to the control, were
maintained in the FLASH-irradiated brain [36]. Similar results were observed in the number, density, and
volume of dendritic spines.
Although the juvenile mouse brain is known to be radiosensitive, the FLASH effect
was still observed [38,39]. Further research has shown that
FLASH-RT not only spares the neurogenic niche but also preserves pituitary
function, as evidenced by stable growth hormone levels [38]. Even with hypofractionated regimens, it prevented
radiation-induced neurocognitive complications in the normal brain [39,40]. Other researchers have also validated the relationship between
FLASH-RT and reduced cognitive deficits in both spatial and non-spatial object
recognition, as well as associated neurodegeneration, at the
electrophysiological, molecular, and structural levels [39,41,42].
In experiments involving proton-based FLASH, no significant differences were
observed in locomotion, exploratory behavior, spontaneous activity, or anxiety
levels among the control, FLASH, and conventional RT groups [43]. However, in the object recognition
task used to assess memory, both the control and FLASH groups demonstrated good
recall of the familiar object, in contrast to the rats that received
conventional RT. Analysis of the immune cell populations in the brain parenchyma
revealed that infiltration of peripheral immune cells (CD45high)
occurred irrespective of the dose rate. Additionally, there was a 4-fold
reduction in microglia compared to non-irradiated tissue, with no variations
noted across different dose rates.
These beneficial effects of FLASH might result from reduced production of
reactive oxygen species or less pronounced increases in pro-inflammatory
cytokines following FLASH-RT [36,41]. They may be associated with the
preservation of cerebrovascular integrity through the protection of tight
junctions or aquaporin-4 levels [37,39,44].
Skin
FLASH-RT has the potential to mitigate radiation-induced skin reactions, which
could significantly reduce both acute and late skin reactions in the treatment
of head and neck cancers and extremity soft tissue sarcoma when integrated into
future clinical practice [45–48]. When murine skin was subjected to UHDR
proton therapy, the skin reaction score was lower compared to that observed with
conventional proton therapy [45–47]. There were
fewer instances of epidermal necrosis, skin stem cell depletion, hair follicle
atrophy, inflammation, epidermal hyperplasia, myofiber atrophy, and bone
remodeling [46,47,49,50]. The potential benefits of FLASH-RT
have also been demonstrated in mini-pigs; however, severe late skin necrosis,
which was volume-dependent, developed but eventually resolved [51,52]. Regarding lymphedema, no differences were noted in the
incidence or progression between the dose rates; however, the severity was
greater in mice that received conventional RT [46]. This reduction in side effects translated into a survival
benefit for mice that received ≤40 Gy, although no survival difference
was observed between the two dose rates at the 45 Gy dose [45,46].
Zhang et al. reported the oxygen dependence of the FLASH effect [48]. FLASH irradiation improved skin
contraction (25–30 Gy), epidermal thickness (25 Gy), and collagen
deposition (25 Gy). However, when irradiation occurred in a 100% oxygen
environment or under hypoxic conditions induced by restricting blood flow
through leg constriction, the tissue-sparing effect of FLASH was lost.
In transcriptome analysis, pathways such as apoptotic signaling, keratinocyte
differentiation, and cornification were upregulated in the group that received
conventional proton therapy. Conversely, these changes were not observed in the
UHDR group [46]. TGF-β1 expression
was also observed at lower levels following FLASH-RT [46,47]. The levels
of chemokine ligand-1 and granulocyte-colony stimulating factor increased, while
those of granulocyte-macrophage colony-stimulating factor decreased following
conventional RT [47]. The
granulocyte-macrophage colony-stimulating factor/granulocyte-colony stimulating
factor ratio, which inversely correlates with tissue toxicity, was reduced in
the conventional group, suggesting increased tissue toxicity [53]. In the FLASH group, IL-6 levels rose,
although no significant differences in IL-6 levels were noted between the 57
Gy/s and 115 Gy/s dose rates [47].
However, when proton irradiation was delivered at 930 Gy/s, no differences were
observed in the levels of TGF-β1, IL-1α, IL-1β, and tumor
necrosis factor-α in the blood [50].
Intestines
FLASH experiments using electrons demonstrated more favorable crypt survival at
doses between 7.5–12.5 Gy after whole-abdominal irradiation than after
conventional dose rate irradiation [54].
However, this sparing effect decreased as the number of FLASH pulses increased
or as the interval between pulses extended, leading to a longer delivery time.
When 15 Gy of proton irradiation was administered to the whole abdomen, there
was a smaller reduction in proliferating cells within the jejunum crypts in the
FLASH group than in the conventional RT group [55,56]. After intestinal
irradiation at 15 or 18 Gy, the extent of intestinal fibrosis was similar to
that observed in non-irradiated tissue [32,55].
Some studies have shown that abdominal FLASH irradiation reduces mortality in
mice suffering from radiation-induced gastrointestinal syndrome compared to
conventional irradiation [32,57]. This protective effect is believed to
stem from FLASH irradiation's ability to decrease chromosomal damage and
apoptosis in the crypt base columnar cells of the jejunum, thereby helping to
preserve intestinal function and epithelial integrity [57]. Additionally, the beneficial impact of FLASH X-rays
may be associated with differing inflammatory responses, including reduced
activation of the cyclic guanosine monophosphate–adenosine monophosphate
synthase-stimulator of interferon genes (cGAS-STING) pathway and changes in the
redox status within the intestinal crypts [58,59].
Fecal samples were utilized for gut microbiome analysis, revealing that overall
α-diversity and evenness declined across all irradiated groups, although
richness decreased solely in the conventional group [54]. In the β-diversity analysis, the cluster of the
FLASH-treated group was closer to that of the control group, suggesting fewer
alterations in the microbiome.
Heart
The impact of FLASH-RT on the heart remains largely unexplored. Until recently,
research in this area continued to be scarce, with the heart being an uncharted
area in FLASH-RT studies. It was not until 2024 that the first study addressing
the effects of FLASH-RT on cardiac tissue was published, marking a significant
advancement in our understanding of how this innovative RT might influence
cardiac function.
A recent study investigated the impact of proton FLASH-RT aimed specifically at
the cardiac apex, delivering a precise 40 Gy dose [60]. The research utilized γH2AX staining to
evaluate DNA damage, which was found to be limited to the lower third of the
heart, with no impact on adjacent tissues. Bulk RNA sequencing of cardiac tissue
revealed distinct pathway regulations based on the treatment approach. In the
FLASH-RT group, pathways related to cytoplasmic translation, mitochondrion
organization, and adenosine triphosphate synthesis were upregulated. In
contrast, pathways involved in tissue morphogenesis and the regulation of
developmental growth were downregulated. A key finding was that FLASH-RT reduced
cardiac inflammation and profibrotic responses, leading to decreased myocardial
fibrosis. Unlike conventional RT, FLASH-RT maintained heart functionality at
levels similar to those of non-irradiated controls.
Tumors
Tumor cell killing was not altered, as reported by Favaudon et al., who found
that both xenograft human tumor and syngeneic orthotopic lung tumor models
exhibited equivalent tumor growth inhibition when comparing FLASH-RT (4.5 MeV
electrons, 60 Gy/s) with conventional RT [14]. Similarly, other studies employing various tumor models and
FLASH sources have demonstrated comparable levels of histological tumor cell
damage, regardless of the dose rate and fractionation [31,40,43,46,47,49,55–57,61–67]. In some
instances, tumor growth was even more delayed with FLASH-RT than with
conventional RT [32]. In a separate study
using 250 MeV proton beams, no difference in lung tumor diameter was observed
between 18 Gy FLASH (60 Gy/s) and conventional irradiation; however, there was a
significant reduction in proliferating tumor cells following FLASH, indicating a
meaningful decrease in lung tumor burden [68]. The survival of tumor-bearing mice was found to be equivalent
to or better with FLASH-RT compared to conventional RT [14,32,40,56,66].
Although numerous studies have explored the interaction between the tumor immune
microenvironment (TIME) and FLASH-RT, the results have so far been varied and
inconsistent [43, 61, 62, 65, 68]. Some researchers have observed that FLASH retains its antitumor
efficacy even in severely immunodeficient mice, suggesting the existence of an
antitumor mechanism that may function independently of the immune response
[65]. FLASH-RT has been shown to
enhance cytotoxic T-cell infiltration into tumors and reverse the
immunosuppressive phenotype [68]. There
was an increase in CD8+ T-cell recruitment to the tumor, accompanied
by a decrease in the infiltration of immunosuppressive regulatory T-cells
(Treg). Additionally, macrophage polarization shifted towards an M1-like
phenotype, which facilitated increased lymphocyte infiltration in lung tumors.
Furthermore, FLASH-RT suppressed the expression of programmed death-1 (PD-1) and
its ligand (PD-L1).
In an orthotopic glioma rat model, tumor-infiltrating lymphocytes, including
CD4+ and CD8+ T-cells, increased at both conventional
dose rates and UHDR (226 MeV proton, 257 Gy/s) [43]. Interestingly, Treg levels also increased in both groups.
Additionally, there were observed increases in natural killer cells and B cells,
suggesting that cranial irradiation activates adaptive immunity. However, in the
FLASH group, no increase in tumor myeloid cells was noted.
A very recent study using an orthotopic syngeneic mouse model of brainstem
diffuse midline glioma explored high-resolution profiling of the TIME following
FLASH (9 MeV electron, 90 Gy/s) and conventional dose-rate RT [61]. The methods employed included
single-cell RNA sequencing and flow cytometry. Analysis of CD45+
cells revealed that both the FLASH and conventional groups displayed similar
proportions of immune subsets, with microglia as the predominant population. As
an acute effect of RT, both FLASH and conventional irradiation triggered a type
1 interferon (IFN1) response in microglia. However, by day 10 post-RT, the FLASH
group exhibited a dose-rate-dependent reduction in the IFN1 response in
microglia, indicating a distinct temporal pattern and suggesting that microglial
activation by FLASH was transient during the early stages. Regarding
non-resident myeloid cells, such as macrophages and dendritic cells, which
represented a minor fraction of the TIME, an early IFN1 response was observed in
the conventional group, but in the FLASH group, this response was not clearly
defined until day 10 post-RT. Despite these temporal immune changes, no
significant differences in tumor control were noted between the dose rates,
highlighting an area for future research.
Several trials have explored FLASH-RT in animals with cancer, using electrons in
the 4.5-12 MeV range [51,52,69–71]. In one study,
seven cats with T1/2N0M0 squamous cell carcinoma of the nasal planum received 30
Gy of radiation [52]. All remained
tumor-free for one year, with only one case showing progression thereafter.
Another trial involved six cats with locally advanced T2/T3N0M0 tumors treated
with 25–41 Gy, achieving an 84% progression-free survival rate at 16
months [51]. Additionally, a
collaborative effort between researchers from Denmark and Sweden applied
FLASH-RT to canine cancer patients with superficial malignant tumors [69–71]. The treatment was effective, although it was associated with a
potential risk of osteoradionecrosis.
Clinical studies with FLASH radiotherapy
The clinical application of FLASH-RT was first demonstrated in 2018 when a
75-year-old patient with multi-resistant CD30+ T-cell cutaneous lymphoma
received treatment at Lausanne University Hospital in Switzerland (Table 2) [15]. A skin tumor measuring 3.5 cm was exposed to 15 Gy of radiation in
just 90 milliseconds using a 5.6-MeV linac. The tumor began to shrink 10 days after
treatment with FLASH-RT, achieving a complete response by day 36, which was
sustained for five months. Regarding adverse effects, the patient experienced
asymptomatic grade 1 epithelitis and grade 1 edema in the surrounding skin, which
had previously undergone extensive RT. Optical coherence tomography showed no
reduction in epidermal thickness or disruption of the basal membrane, except for a
slight increase in vascularization. Subsequently, the patient underwent two
additional treatments of 15 Gy each at different sites on the same day (dose rates,
166 Gy/s and 0.08 Gy/s, respectively) [72].
Over the next 2 years, both treatment sites exhibited similar levels of acute and
late skin toxicity, with no differences in tumor response noted (Table 2).
15 Gy in a single fraction - Right
elbow: 0.08 Gy/s - Left distal arm: 166 Gy/s
- Follow-up of 2 years - Rapid,
complete, and durable tumor response - Grade 1 acute
epithelitis at both treated sites - Mild late radiodermatitis
at both treated sites
1–3 symptomatic bone metastases in
the extremities (except for the feet, hands, or wrists)
10 (12 sites)
Protons (250 MeV, 51–61 Gy/s)
8 Gy in a single fraction
- Median follow-up of 4.8 months (range,
2.3–13.0) - Average patient time on the treatment
couch 18.9 minutes (range, 11–33) - No device-related
treatment delays - Transient pain flares (2–9 days
post-FLASH) in 4 of the 12 sites (33%) - Pain relief in 8 of
the 12 sites (67%) - No pain in 6 of the 12 sites
(50%) - No grade ≥3 FLASH-related toxicity
The FAST-01 trial, a pioneering first-in-human study of FLASH-RT, involved 10
patients with symptomatic bone metastasis [73]. This trial, presented at the 2022 Annual Meeting of the American
Society for Radiation Oncology, suggested that FLASH-RT could be a promising
treatment for particularly resistant tumors. It targeted one to three painful bone
metastases in the extremities, administering an 8 Gy single fraction to 12
metastatic sites using a FLASH-enabled proton therapy system at a dose rate of
≥40 Gy/s. The primary outcomes, which included workflow feasibility and
radiation-related toxicities, demonstrated favorable results comparable to those of
conventional RT (as detailed in Table 2).
Among the 12 treated metastatic lesions, pain was completely alleviated at six
sites, and symptoms partially improved at two sites.
Several clinical trials involving FLASH-RT have recently been initiated and are
currently recruiting patients: NCT04986696 (phase I, metastatic melanoma),
NCT05524064 (phase I, bone metastases, FAST-02), and NCT05724875 (phase II, skin
cancers; Table 3). These trials mark a
significant step forward in investigating the safety and efficacy of FLASH-RT,
potentially providing cancer patients with faster and less toxic treatment
options.
Table 3.
Overview of ongoing clinical trials involving FLASH radiotherapy
The biological mechanisms by which FLASH irradiation reduces damage to non-malignant
tissues while maintaining effective tumor control, as compared to conventional
irradiation, remain under active investigation and are not yet fully understood.
Several hypotheses have been proposed to explain these differential effects, each
with its own limitations. In this section, we introduce three key biological
mechanisms: oxygen depletion, DNA damage, and immune/inflammatory response.
Additionally, other emerging hypotheses, such as minimal mitochondrial damage or the
preservation of normal flora induced by FLASH, are also under consideration [74]. A deeper understanding of these mechanisms
is essential for optimizing FLASH-RT and successfully translating its benefits into
clinical practice.
Oxygen depletion hypothesis
The oxygen depletion hypothesis is currently the most widely accepted theory. It
is based on the principle that oxygen acts as a critical radiosensitizer in RT;
thus, tissues with a high oxygen supply are more radiosensitive [75]. FLASH irradiation rapidly depletes
oxygen, leaving insufficient time for oxygen to be replenished from the
surrounding circulating blood [74]. This
results in acute hypoxic conditions that lead to transient radioresistance,
thereby sparing normal tissue [76].
Conversely, tumors, with their inherently abnormal blood vessels, are already
adapted to hypoxic conditions. This adaptation explains why the dose rate does
not significantly impact the tumor cells' susceptibility to radiation
[17].
A limitation of this hypothesis is that while FLASH-RT resulted in greater oxygen
consumption compared to conventional RT, it did not completely deplete all the
oxygen [77]. Furthermore, the oxygen
levels associated with higher cell survival rates following FLASH-RT vary
significantly across experiments, ranging from severely hypoxic conditions
(<0.5%) to oxygen-rich environments like those found in the lungs [74]. This variability suggests that the
oxygen depletion hypothesis may not fully explain the FLASH effect.
An alternative explanation has been proposed, suggesting that reactive oxygen
species, which serve dual roles as signaling and damaging agents within cells,
may interact with molecules involved in redox metabolism. This interaction could
potentially play a pivotal role in the FLASH effect [18].
DNA damage hypothesis
Cell fate after irradiation is primarily determined by DNA damage, specifically
unrepaired DNA double-strand breaks [78].
Several studies have shown that DNA damage is less severe after FLASH
irradiation [33,37,57]. This
reduction in DNA damage helps preserve stem and progenitor cells across various
tissues, consequently decreasing toxicity [33,34,38,46,55,56]. However, while this effect accounts for the sparing of normal
tissue, it does not completely explain the sustained antitumor activity.
Although the precise mechanisms are still not fully understood, it is possible
that differences in the activation of downstream pathways after DNA
damage—such as DNA repair pathways, the cGAS-STING pathway, or the immune
system—between normal and tumor cells could contribute [17].
Immune and inflammatory hypothesis
FLASH significantly reduces the duration of radiation exposure, which is
anticipated to decrease the volume of irradiated blood and aid in preserving
circulating immune cells from depletion [50,79]. However, several
in vivo studies have yielded negative results, showing no
significant difference in the circulating immune cell populations between FLASH
and conventional dose rates [43,59,80]. Instead, while further detailed research is necessary, it is
generally observed that there is an increase in cytotoxic T-cell infiltration
into tumors [43,68]. Conversely, the persistence of the antitumor effect in
immunocompromised animals suggests that this effect cannot be solely attributed
to the immune response [65].
FLASH also reduces TGF-β and pro-inflammatory gene expression, as well as
the release of pro-inflammatory cytokines, thereby mitigating stress response
and inflammation [33,41,46,47,58–60,81]. This contributes to the preservation
of normal tissue, exemplified by the reduction of neuroinflammation in the
normal brain, which in turn supports the maintenance of neurological function
[25,35–37]. Given that the
TGF-β pathway is a pharmacological target in cancer therapy, the
reduction of TGF-β expression induced by FLASH could enhance antitumor
activity, similar to the effects of TGF-β antagonists [82].
Current cancer treatment is witnessing a revival of interest in immunotherapy,
particularly in its integration with RT [83]. In this context, the immune response elicited by FLASH provides
compelling insights that may herald a new phase in radioimmunotherapy. First,
FLASH reduces the expression of PD-1 and PD-L1, thereby inhibiting the
tumor's ability to evade the immune system [68,84]. In a study
using an ovarian cancer mouse model, abdominopelvic irradiation followed by PD-1
therapy led to enhanced tumor control in both conventional and UHDR settings,
without an increase in toxicity compared to using FLASH alone [85]. This treatment also resulted in a
lower Treg-to-T-effector ratio and a higher level of CD8+ T-cell
infiltration within the tumor. While immunotherapy alone often yields only
modest response rates, these findings are noteworthy as they indicate that
FLASH-RT can significantly improve the effectiveness of PD-1/PD-L1
inhibitors.
Challenges in the clinical application of FLASH radiotherapy: current issues and
future directions
Recent preclinical studies and ongoing clinical trials have advanced the clinical
application of FLASH-RT significantly. However, numerous challenges must be overcome
before it can be routinely implemented in clinical settings. Key considerations for
preclinical studies include: (i) research has been limited to a small number of
normal tissues, which may lead to unexpected side effects when FLASH-RT is used
clinically; (ii) the extent of the protective effect varies based on tissue type and
physical parameters; (iii) there are inconsistent results among different studies;
and (iv) most studies have utilized high single doses, necessitating further
research to determine if the FLASH effect is achievable with lower doses and
fractionated regimens [17,81]. Addressing these issues is essential for
the successful integration of FLASH-RT into standard clinical protocols.
In addition to these challenges, it is important to note that some studies have not
observed the beneficial effects of FLASH [80,
86-88]. One study compared high dose-rate synchrotron broad-beam RT
(37–41 Gy/s) with a mean photon energy of 124 keV to conventional RT
(0.05–0.06 Gy/s) with 93 keV [86]. It
found that synchrotron broad-beam RT did not demonstrate the FLASH effect of sparing
normal tissue compared to conventional RT [86]. The irradiated mice exhibited weights below normal compared to control
mice and experienced disruption of normal crypt-villus units following abdominal
irradiation. Additionally, cranial irradiation led to neurological deficits, while
thoracic partial irradiation caused inflammatory responses and long-term lung
damage.
In a mouse model investigating radiation-induced lymphopenia, both cardiac and
splenic irradiation were administered using 20 MeV electron FLASH-RT (35 Gy/s) and
conventional RT (0.1 Gy/s) [80]. For both
cardiac and splenic irradiation, researchers employed a multi-fraction regimen of 2
Gy (or 1 Gy) per day over 5 days, as well as a single fraction of 10 Gy (or 5 Gy).
The findings indicated a decrease in CD3+, CD4+,
CD8+, and CD19+ lymphocytes, regardless of the dose rate
or fractionation regimen used. Notably, the FLASH-RT group showed a more significant
reduction in lymphocyte counts following splenic irradiation compared to the
conventional RT group. In a model of gastrointestinal mucosal injury following
whole-abdominal irradiation, acute gastrointestinal toxicity was more severe in the
FLASH-RT group after a 16 Gy single fraction. All mice in the FLASH-RT group died
within 7 days, whereas those in the conventional RT group survived until day 15.
A recent study demonstrated the absence of tissue-protective effects with FLASH-RT
[87]. After partial abdominal FLASH
proton irradiation at a rate of 120 Gy/s, survival rates were notably lower in the
FLASH group compared to those in the conventional RT group at doses ranging from
15.1 to 18 Gy. Additionally, measurements of proliferating crypt cells and the
thickness of the muscularis externa revealed no significant differences. Similarly,
there were no variations in circulating lymphocyte counts. These findings indicate
that the effectiveness of FLASH irradiation may be subject to multiple influencing
factors and that FLASH irradiation could potentially result in adverse outcomes if
not properly managed.
When using zebrafish embryos and proton-based FLASH irradiation, no significant
protective effect was observed [88]. A
comparison of FLASH irradiation (100 Gy/s) using 224 MeV protons with conventional
RT (5 Gy/min) revealed no differences in embryonic survival attributable to the
varying proton dose rates. Apart from a decreased incidence of pericardial edema
following FLASH irradiation, there were no differences in the rate of embryo
malformations, specifically spinal curvature, between the two irradiation
methods.
The negative results observed in these studies underscore the importance of
thoroughly examining the underlying factors. It is possible that the low dose rates
and the specific experimental setup played a role in these outcomes [89]. Determining the optimal dose rate to
preserve the integrity of normal tissue remains an unresolved issue. Future research
should focus on identifying the most effective dose, dose rate, pulse, and fraction
size to reduce complications in normal tissues for specific organs [80]. These experiments should be carefully
designed to mirror clinical treatment scenarios, ensuring that the results are
relevant to real-world applications. Given these challenges, ongoing research and
sustained attention are crucial to effectively address these issues and advance the
field.
One technical issue pertains to dosimetry. Current dosimetry protocols and equipment,
designed for much lower dose rates than those used in FLASH, struggle with accurate
measurements at UHDR [90]. The ion chambers
typically employed in clinical settings are significantly affected by ion
recombination at UHDR, resulting in substantial uncertainties [21,90]. Another
challenge involves the development of treatment plans that can accurately deliver
the desired dose at UHDR to the specific target location. To address this,
modifications are necessary in the treatment planning system to not only calculate
and display the dose distribution in patients but also evaluate the 3D dose rate
distribution [21]. Additionally, the beam
delivery system needs further development. For optimal conformity to the RT target,
beams are usually delivered from multiple angles, which requires the use of rotating
gantry systems instead of fixed gantry setups in FLASH-RT [19].
From an economic perspective, FLASH-RT is currently available at only a few
institutions, and the equipment required for proton therapy, suitable for treating
deep tumors, is extremely expensive. Electron therapy, on the other hand, is only
effective for superficial tumors. Photon equipment, which is more widely used
globally and less costly than proton therapy, can treat deep-seated tumors.
Therefore, it is essential to develop photon-based FLASH-RT equipment to make this
treatment more accessible and economically viable [12].
Conclusions
FLASH-RT represents an exciting avenue for improving therapeutic outcomes in
oncology, characterized by its ability to deliver UHDR radiation while minimizing
damage to normal tissues. This approach has shown promise in both preclinical and
initial clinical studies, offering efficacy in tumor control and reduced toxicity.
Despite these positive findings, numerous biological and technical challenges
remain. The precise mechanisms underlying the FLASH effect are complex and not yet
fully understood, necessitating further investigation into the oxygen depletion
hypothesis and other potential explanations. Additionally, implementing FLASH-RT in
clinical settings requires improvements in dosimetry, treatment planning, and beam
delivery systems to meet the specific requirements of UHDR. Future research and
clinical trials are essential to address these challenges and validate the long-term
safety and effectiveness of FLASH-RT across a broader range of cancers. As this
technology evolves, it holds the potential to revolutionize RT, offering more
effective and less toxic treatment options for patients.
Authors' contributions
Project administration: Kim HJ
Conceptualization: Kim HJ
Methodology & data curation: not applicable
Funding acquisition: Kim HJ
Writing - original draft: Kim JS
Writing - review & editing: Kim JS, Kim HJ
Conflict of interest
No potential conflict of interest relevant to this article was reported.
Funding
This work was supported by the National Research Foundation of Korea (NRF) grant
funded by the Korea government (MSIT) (2023R1A2C1002539) to Hak Jae Kim.
Data availability
Not applicable.
Acknowledgments
Not applicable.
Supplementary materials
Not applicable.
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K, 103; M, 106; HMGB1, high mobility group box
protein 1; cGAS, cyclic guanosine monophosphate–adenosine
monophosphate synthase; STING, stimulator of interferon genes; ROS,
reactive oxygen species; SOBP, spread-out Bragg peak; TNF, tumor
necrosis factor; SMA, smooth muscle actin.
1)Used to assess the effects on normal tissue.
2)To investigate the differences in FLASH-induced neuropreservation at
various dose rates, intermediate dose rates of 1, 3, 10, 30, 100, and
500 Gy/s were also used.
15 Gy in a single fraction - Right
elbow: 0.08 Gy/s - Left distal arm: 166 Gy/s
- Follow-up of 2 years - Rapid,
complete, and durable tumor response - Grade 1 acute
epithelitis at both treated sites - Mild late radiodermatitis
at both treated sites
1–3 symptomatic bone metastases in
the extremities (except for the feet, hands, or wrists)
10 (12 sites)
Protons (250 MeV, 51–61 Gy/s)
8 Gy in a single fraction
- Median follow-up of 4.8 months (range,
2.3–13.0) - Average patient time on the treatment
couch 18.9 minutes (range, 11–33) - No device-related
treatment delays - Transient pain flares (2–9 days
post-FLASH) in 4 of the 12 sites (33%) - Pain relief in 8 of
the 12 sites (67%) - No pain in 6 of the 12 sites
(50%) - No grade ≥3 FLASH-related toxicity
Overview of ongoing clinical trials involving FLASH radiotherapy
NCT identifier
Cancer
Design
Population
Estimated enrollment
Radiation source
Treatment
Primary endpoint
Status1) (study start date)
NCT04986696
Malignant melanoma
Phase I Non-randomized Dose
escalation
Multiple skin metastases PD after
systemic treatment
46
Electrons
7 dose levels (22, 24, 26, 28, 30, 32, and
34 Gy in a single fraction)
MTD or RP2D
Recruiting (July 1, 2021)
NCT05524064
Bone metastasis
Phase I Single arm
1–3 symptomatic bone metastasis in
the thorax
10
Protons
8 Gy in a single fraction
Toxicity Patient-reported pain
relief Pain medication use
Recruiting (March 8, 2023)
NCT05724875
Skin cancer
Phase II Randomized
T1-2N0M0 cutaneous squamous cell carcinoma
or basal cell carcinoma
60
Electrons
FLASH-RT vs. Conventional RT
(T1, 22 Gy in a single fraction; T2, 30 Gy in 5 fractions)
Skin toxicity (≥grade
3) Local control rate
Recruiting (June 22, 2023)
NCT, National Clinical Trial; PD, progressive disease; MTD, maximum
tolerated dose; RP2D, recommended phase II dose; RT, radiotherapy.
1)From https://clinicaltrials.gov/ (accessed on July 26,
2024).
Table 1.
Summary of in vivo studies demonstrating the effect of
FLASH on normal tissues
K, 103; M, 106; HMGB1, high mobility group box
protein 1; cGAS, cyclic guanosine monophosphate–adenosine
monophosphate synthase; STING, stimulator of interferon genes; ROS,
reactive oxygen species; SOBP, spread-out Bragg peak; TNF, tumor
necrosis factor; SMA, smooth muscle actin.
Used to assess the effects on normal tissue.
To investigate the differences in FLASH-induced neuropreservation at
various dose rates, intermediate dose rates of 1, 3, 10, 30, 100, and
500 Gy/s were also used.
Delivered only to the right hemisphere.
Table 2.
Clinical experiences with FLASH radiotherapy
Table 3.
Overview of ongoing clinical trials involving FLASH radiotherapy
NCT, National Clinical Trial; PD, progressive disease; MTD, maximum
tolerated dose; RP2D, recommended phase II dose; RT, radiotherapy.
From https://clinicaltrials.gov/ (accessed on July 26,
2024).
