ESTRO 2025 - Abstract Book

S3920

Radiobiology - Normal tissue radiobiology

ESTRO 2025

2126

Mini-Oral Safety and cutaneous toxicity of hypofractionated FLASH and conventional radiotherapy in a clinically relevant swine skin model Elise Konradsson 1 , Kevin Liu 1 , Safee Baig 1 , Alan Lopez Hernandez 1 , Brett Velasquez 1 , Stephanie Mayor 2 , Kayla Samuel 2 , Traci Viscarra 2 , Krystal Garrow 2 , Erica Moore 2 , William Norton 2 , Jody Swain 2 , Albert C Koong 3 , Steven H Lin 3 , Emil Schüler 1 , Devarati Mitra 3 1 Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, USA. 2 Department of Veterinary Medicine and Surgery, The University of Texas MD Anderson Cancer Center, Houston, USA. 3 Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, USA Purpose/Objective: In preparation for our institution’s first clinical trial comparing FLASH radiotherapy (RT) and conventional (CONV) RT for post-operative treatment of primary cutaneous melanoma, this study aims to evaluate the safety and maximum tolerated dose of a 5-fraction hypofractionated treatment regimen in a clinically relevant swine skin model. Material/Methods: Three Yorkshire-Landrace swine were treated with either FLASH-RT (mean dose rate 200 Gy/s) or CONV-RT (mean dose rate 0.1 Gy/s) using the Mobetron electron accelerator (IntraOp, San Jose, CA, US). Six treatment fields on the lateral flanks of each pig were irradiated with 5 fractions over 10 days using 1 cm bolus and skin collimation. Treatment parameters varied across fields, including field sizes (4, 7, or 10 cm), treatment modes (FLASH or CONV), and fraction doses (6, 7, or 8 Gy to the depth of dose maximum). Additionally, two single-fraction reference fields (18 Gy) were irradiated on each hind limb, and two unirradiated control patches on the shoulder were monitored. Pre-treatment dose validation was performed using a multi-dosimeter approach including radiochromic film, TLD, and alanine. FLASH irradiations were delivered in 4 pulses with a dose-per-pulse of 1.5-2 Gy, for a total treatment time of 33 ms. Follow-up for 4 months included weekly photographs for toxicity scoring by a blinded expert radiation oncologist using a RTOG modified radiation dermatitis (RD) scale (score 0-4) and erythema index measurements using a Mexameter MX18 spectrophotometer (Courage+Khazaka electronic GmbH, Köln, Germany). Results: In vivo film dosimetry confirmed that the prescribed doses were delivered accurately for each treatment field (total doses within 3% of the intended dose). At total doses of 30 and 35 Gy, only minor erythema and RD were observed, with a maximal dermatitis score of 2/4 for both FLASH-RT and CONV-RT. At 40 Gy, significantly greater erythema and RD were observed along with moist desquamation (for FLASH-RT) and ulceration (for CONV-RT). No dependence on field size was found at any dose level, and no trend of a FLASH sparing effect was observed. Conclusion: This study indicates no difference in clinical toxicity between hypofractionated FLASH-RT and CONV-RT up to 4 months post-treatment using a 5-fraction regimen and a clinically relevant workflow. A total dose of 40 Gy was poorly tolerated with both FLASH-RT or CONV-RT. Future work will include tissue collection and late time point fibrosis measurements, as well as an expanded cohort to further investigate dose- and volume dependent skin toxicity in this model.

Keywords: FLASH, ultra-high dose rate, skin toxicity