Ultra-high dose rate (FLASH) radiotherapy: Revolutionizing tumor targeting and microenvironmental dynamics in cancer treatment

To the Editor: Radiotherapy is an essential component of modern cancer treatment modalities, used by nearly 70% of all individuals diagnosed with cancer. However, conventional radiotherapy (CONV-RT) can damage surrounding healthy tissues, limiting the maximum safe dose and potentially reducing treatment efficacy. Recent studies indicate that ultra-high dose rate radiotherapy (FLASH-RT) spares healthy tissues while maintaining the same tumor control.^[1]

The tumor microenvironment (TME), composed of a dynamic network of immune cells, fibroblasts and endothelial cells, plays a crucial role in tumor development, metastasis and therapy resistance. CONV-RT can induce chronic inflammation, fibrosis, hypoxia, vascular damage and immunosuppression in the TME, potentially promoting tumor metastasis and recurrence. In contrast, FLASH-RT has been shown to result in less inflammation and reduced vascular damage in irradiated animal models. This suggests that FLASH-RT may operate through a different mechanism. This article explores the interaction of FLASH-RT with various aspects of the TME, aiming to expand the therapeutic window of existing radiotherapy strategies [Supplementary Figure 1, http://links.lww.com/CM9/C213].

Effects of FLASH-RT on the vasculature: The normalization of the tumor vasculature can restore proper blood vessel functions and may help to prevent cancer cells from acquiring an aggressive phenotype associated with a hypoxic microenvironment. CONV-RT induces dose-dependent damage to the vasculature, particularly the microvascular system. Studies comparing FLASH-RT and CONV-RT at similar doses have found that FLASH-RT prevents the activation of the transforming growth factor-β (TGF-β)/SMAD pathway and protects against acute apoptosis in blood vessels and bronchi. Research on juvenile mouse brain irradiation has shown that CONV-RT compromises the blood-brain barrier by damaging supporting cells and tight junction proteins. However, FLASH-RT preserves the integrity of the blood-brain barrier by maintaining aquaporin 4 expression and astrocytic coverage of the microvasculature. Allen et al^[2] discovered that FLASH-RT, unlike CONV-RT, did not impact crucial vascular characteristics, such as blood vessel volume, endothelial nitric oxide synthase expression, or tight junction proteins like claudin-5 and occludin, which are markers for the efficacy of epithelial cell adhesion. Further studies have shown that FLASH-RT does not cause morphological changes in the blood vessels of lung carcinoma or lead to the collapse of tumor blood vessels. The preservation of vasculature under FLASH irradiation is associated with reduced phosphorylation of myosin light chain (MLC), which affects endothelial cell contraction and limits immune cell infiltration. Notably, using an MLC kinase inhibitor with CONV-RT produced similar protective effects as FLASH-RT, highlighting the MLC pathway as a potential molecular target.^[3]

Oxygen dynamics and FLASH: Hypoxia is a prevalent condition within almost all solid tumors, closely linked to poor prognoses in cancer patients. The widespread hypoxia within the TME leads to instability of hydroxyl (OH·) and hydrogen (H·) radicals, thereby reducing the DNA damage caused by CONV-RT and contributing to radiation resistance. A popular current theory suggests that FLASH-RT’s transient consumption of oxygen leads to temporary hypoxia, which paradoxically protects normal tissues. This mechanism, however, raises concerns about potentially exacerbating hypoxia in tumor tissues, thereby fostering resistance to radiation therapy. Recent studies suggest that FLASH-RT may not exacerbate hypoxia within the tumor microenvironment due to preservation of vasculature. Paul et al^[4] have shown that irrespective of oxygen tension, FLASH-RT does not significantly alter the degree of DNA damage. Kim et al^[5] have revealed that while CONV-RT performs poorly under acute hypoxic conditions induced by clamping tumors implanted in the flanks of a mouse model, FLASH-RT remained effective in killing tumor cells and enhancing tumor control. Based on transcriptomic analysis, FLASH-RT was more efficient than CONV-RT in inhibiting the cell cycle and promoting a switch from oxidative phosphorylation to glycolysis. The p53 effector growth arrest and DNA damage-inducible protein 45 (GADD45) was upregulated after FLASH-RT, which inhibited cyclin B/cyclin dependent kinase 1 (CDK1) complex formation. Additionally, glycolysis inhibition by trametinib enhanced FLASH-RT efficacy regardless of hypoxic or normoxic conditions.

Immune response modulation by FLASH-RT: Tumors adeptly evade immune surveillance by engineering an immunosuppressive microenvironment through the secretion of cytokines such as TGF-β, interleukin (IL)-6 and IL-10. These cytokines recruit immunosuppressive cells like tumor-associated macrophages, myeloid-derived suppressor cells and regulatory T cells (Tregs), which collectively dampen the host’s anti-tumor immune responses.

Within this intricate scenario, FLASH-RT emerges as a novel therapeutic approach with the capacity to counteract the immune evasion mechanisms of tumors. Studies have demonstrated that FLASH-RT reduces inflammation and enhances immune cell infiltration within the TME [Supplementary Table 1, http://links.lww.com/CM9/C213], and in particular boosts the infiltration of cytotoxic CD8⁺ T cells.^[6] This treatment reconfigures the immune landscape of the TME, characterized by a reduction in Tregs and an increase in CD8⁺ T cell proliferation.^[7] Normally, Tregs maintain immune homeostasis and are scant in healthy tissues, but their abundance in many cancers facilitates immune evasion.

Furthermore, FLASH-RT influences the macrophage population within the TME,^[8] significantly reducing CD163⁺ macrophages indicative of a pro-tumor M2 phenotype, while enhancing anti-tumor M1-like macrophages. Advanced studies, including those by Oscar et al^[9] using single-cell RNA sequencing, have illustrated how FLASH-RT distinctively affects the recruitment and behavior of non-resident myeloid clusters differently from CONV-RT, particularly in diffuse midline glioma.

Combining FLASH-RT with monoclonal antibody therapy has proven to be even more beneficial for patient treatment. A notable reduction in programmed death ligand 1 expression on tumor cells was observed in the FLASH-RT treated groups. Integrating FLASH-RT with immunotherapeutic agents, such as anti-programmed cell death-1 antibodies, further enhanced these immunomodulatory effects.^[10] This combination therapy has demonstrated promising results in improving tumor control and reducing metastasis in preclinical models, highlighting the potential of FLASH-RT, both as a monotherapy and in combination with immunotherapy, to effectively modulate the immune environment within the TME. These findings suggest that FLASH irradiation could play a pivotal role in the development of more effective cancer treatment strategies, not only by reversing the immunosuppressive phenotype of tumors but also by enhancing the anti-tumor efficacy of the immune system.

Reduced TGF-βproduction: Radiation is a strong inducer of TGF-β, which plays a dual role in cancer. Specifically, it acts as a tumor suppressor in the early stages but promotes metastasis in the later stages. Research has substantiated the significant correlation between TGF-β expression and metastasis following CONV-RT, emphasizing its role in modulating the relationship among cytotoxic T lymphocytes and Tregs in the TME. In studies on proton FLASH irradiation, a low dose rate (0.2 Gy/s) resulted in a 6.5-fold increase in TGF-β after 24 h, whereas a high dose rate (1000 Gy/s) led to only a 1.8-fold increase. After one month, TGF-β levels in the high-dose-rate group increased by just 0.4-fold, significantly lower than the 2.7-fold increase observed with low-dose-rate irradiation.^[11] Velalopoulou et al^[12] found that while both CONV-RT and pencil beam-scanning FLASH-RT suppress tumors, FLASH-RT was associated with lower TGF-β levels, suggesting its potential to reduce TGF-β-driven metastasis. These findings suggest that FLASH-RT might potentially inhibit tumor metastasis and infiltration caused by TGF-β by suppressing its expression [Supplementary Table 2, http://links.lww.com/CM9/C213].

Clinical trials: FLASH-RT was shown to be safe and feasible in its first application to a patient with multidrug-resistant CD30⁺ T cell cutaneous lymphoma. At 3 weeks, which marked the peak of the skin reactions (grade 1), there was no observed decrease in epidermal thickness, nor was there any disruption of the dermis/epidermis junction. Additionally, there was a loss of hair follicles and a slight increase in vascularization. The FAST-01 trial (NCT04592887) is a prospective, single-center study aimed at evaluating the effectiveness and safety of proton FLASH-RT for palliative treatment of bone metastases. Notably, the trial found that eight out of twelve treatment sites reported either complete or partial pain relief following FLASH therapy. The IMPULSE trial (NCT04986696) is a phase I dose-escalation trial of doses from 22 Gy to 34 Gy in patients with skin metastases from melanoma. The trial is currently in progress. No dose-limiting toxicities have been reported at the first two dosage levels (22 Gy and 24 Gy), which permits the continuation of dose escalation.

Challenges: FLASH-RT is a promising and innovative treatment in oncology, with the potential to benefit many cancer patients in the future, especially those with radioresistant tumors requiring dose escalation or tumors located near organs at risk. However, the full implementation of FLASH-RT in clinical practice will take several years due to the need to overcome various challenges. One of the primary challenges is the extremely high dose rates required by FLASH-RT, which conventional linear accelerators cannot deliver. Achieving these dose rates would necessitate increasing the power of existing clinical accelerators by a thousandfold or more.^[13] This significant obstacle is driving extensive scientific and technological efforts to develop new accelerators, including upgrading existing devices and creating entirely new systems. Additionally, other challenges include determining the optimal parameters for irradiating large volumes, targeting deep-seated tumors, and applying standard fractionation. Clinical trials will be essential to monitor both acute and late toxicity effects in various organs and to evaluate the quality and safety of this new treatment approach.

In conclusion, extensive studies have demonstrated that FLASH-RT effectively kills tumor cells equivalent to CONV-RT [Supplementary Table 3, http://links.lww.com/CM9/C213]. FLASH-RT also shows potential in reducing the remodeling of the TME. There is a growing interest in combining FLASH-RT with other therapies, such as antiangiogenic drugs, immunomodulators and monoclonal antibodies, to potentially enhance anti-tumor efficacy. FLASH-RT offers significant promise in oncology, merging effective tumor control with favorable TME modulation.

Funding

This work was supported by grants from the Nuclear Technology Application Excellent Experts Program of the Second Affiliated Hospital of Soochow University (No. XKTJ-HRC2021002), National Natural Science Foundation of China (No. 12275192), Key Support Discipline Program of Suzhou City (No. SZFCXK202137), Discipline Construction Support Project (No. XKTJ-XK202410), State Key Laboratory of Radiation Medicine and Protection (Nos. GZK1202308, GZK12024046).

Conflicts of interest

None.