Feasibility and Clinical Implementation of Electron FLASH Radiation Therapy in the Yorkshire Swine Model

PURPOSE/OBJECTIVE(S)

Preclinical studies have shown FLASH radiation therapy (RT) increases the therapeutic index through reduction in normal tissue toxicity but with retained tumor control compared to conventional dose rate (CONV) RT. Dosimetry in FLASH beams is challenging and complex as beam monitoring and proper dosimetry analysis remain uncertain and under investigation. Despite these limitations, clinical translation of FLASH RT has already begun. For translation of FLASH RT from the preclinical stage, it is critical that robust clinical workflows and dosimetry methods be confidently established to ensure patient safety. Here, we present the clinical workflow for the Yorkshire pig, an animal that resembles the body dimension, weight, and biology of a human patient, with the goal to establish standard operating procedures to ensure a safe and robust clinical translation in our upcoming phase I study in cutaneous tumors. The study determines feasibility and safety while finding incidence of dose-limiting toxicities and maximum tolerable dose for future Phase II trials.

MATERIALS/METHODS

All procedures were approved by the institutional animal care and use committee. 6 pigs (40-50 kg) were placed under general anesthesia and underwent CT imaging for radiation therapy simulation purposes. The skin was first shaven, and targets on the dorsolateral flanks were marked with tattoos and BBs for CT visualization. Vacloc immobilization was used to allow for reproducible setup on the treatment couch. A treatment planning model was established for treatment planning and dose evaluation purposes. CONV and FLASH single and fractionated dose regimens were prescribed to the 90% isodose line in a 9 MeV beam. Skin collimation and bolus minimized beam penumbra and increased skin dose. Treatment time and pulse repetition frequency were constant between all FLASH fields. Prescription levels were varied via dose per pulse. Calibration and verification of these settings were performed utilizing a multi-dosimeter method for verification in solid water. Output of the beam was verified on the day of the treatment using beam current transformers. This same multi-dosimeter method was used as in-vivo dosimetry on treatment day and compared to the dose verification ensure full dose was received.

RESULTS

Variation between the three dosimeter methods was found to be within 5% among all pigs within the study. The maximum percent difference between dose verification and dose delivery was 6%. Consideration must be taken in dosimeter readout error due to the surface of the pig skin. FLASH and CONV toxicity results are currently under evaluation and will be published upon completion of the study.

CONCLUSION

Establishing guidelines and protocols for electron FLASH clinical translation is important to instill confidence in patient safety with this new technique. This study has further optimized and developed dosimetry tools and setup to be used in future clinical trials.

Subacute Cutaneous Toxicity with Single-Fraction Electron FLASH RT in Yorkshire Swine

PURPOSE/OBJECTIVE(S)

Information regarding acute/subacute skin toxicity of electron FLASH radiation therapy (RT) is limited. We evaluated short-term safety of electron FLASH for human trials by investigating subacute toxicity compared to conventional dose-rate RT (CONV) in the Yorkshire pig, an animal model known to closely approximate human skin and routinely used for toxicity studies.

MATERIALS/METHODS

Two healthy 50 kg pigs underwent CT imaging for RT treatment planning with field visualization via BBs and tattoos on each dorsolateral flank. Each target received a single fraction of 20, 25 or 30 Gy with FLASH and CONV on opposing sides delivered using a dedicated mobile linear accelerator. FLASH dose rates ranged from 164-245 Gy/sec (12 pulses delivered over 0.122 sec) while the CONV dose rate was set at 0.18 Gy/sec. Doses were verified using thermo- and optically stimulated luminescent dosimeters, and Gafchromic films. We obtained baseline and weekly images up to 98 days post-RT (D98) for blinded toxicity grading by 3 expert radiation oncologists using the modified RTOG radiation dermatitis (RD) scale. We measured erythema and pigmentation indices on those timepoints using a handheld spectrophotometer. We also obtained punch biopsies of targets and non-irradiated controls on D10 and D30 for RNA sequencing and two 6-marker multiplex immunofluorescence analyses of inflammation, immune response, and fibrosis. FLASH and CONV data were compared using repeated measures ANOVA and transcriptomic analyses using DESeq2.

RESULTS

All RT targets developed peak median grade 4 (ulceration, hemorrhage, or necrosis) RD by D84 regardless of FLASH or CONV delivery. However, FLASH targets developed peak RD later than CONV targets after 20 Gy (D84 vs D63), 25 Gy (D84 vs D49) and 30 Gy (D63 vs D42). FLASH induced qualitatively lower mean pigmentation and erythema indices than CONV for all 3 doses. Similarly, peak mean pigmentation indices occurred later with FLASH vs CONV for 20 Gy (D84 vs D63), 25 Gy (D84 vs D49) and 30 Gy (D77 vs D63). However, peak mean erythema indices occurred on the same day for FLASH and CONV (D63 for 20 Gy and D42 for 25 and 30 Gy). Transcriptomic analyses revealed significantly upregulated signals for wound healing (including TGF-beta, cell adhesion and extracellular matrix receptor interaction) and leukocyte infiltration with 20 Gy CONV mostly by D10, while FLASH upregulated those pathways only after 25 or 30 Gy, or by D30, or never at all. Preliminary immunofluorescence data showed FLASH may induce less T cell infiltrate and TGF-beta-expressing macrophages than CONV.

CONCLUSION

Single-fraction electron FLASH resulted in delayed onsets of both subacute cutaneous toxicity and wound healing with leukocytic infiltration signaling than dose-matched CONV based on both subjective and objective metrics of skin injury. Our findings suggest further investigations of optimal dose of electron FLASH for safe clinical translation is warranted, and we have a dose-finding study currently underway.