ESTRO 2025 - Abstract Book

S2721

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

Conclusion: Broad UHDR electron beams and CDR photon IMRT/VMAT can be combined to deliver HUC dose distributions for selected patient cohorts with a dosimetric plan quality comparable to the one of standard-of-care treatments while delivering the bulk dose within sub-seconds utilizing UHDR electrons. Devices based on proven and accessible technology, such as UHDR-converted clinical linacs [2], could enable HUC delivery in future. HUC treatments offer therefore potential to reduce technical burdens for achieving dosimetrically conformal UHDR delivery for the initial clinical exploration of FLASH-RT. References: [1] Böhlen TT, Germond J-F, Bourhis J, Vozenin M-C, Ozsahin EM, Bochud F, et al. Normal Tissue Sparing by FLASH as a Function of Single-Fraction Dose: A Quantitative Analysis. International Journal of Radiation Oncology*Biology*Physics 2022;114:1032–44. https://doi.org/10.1016/j.ijrobp.2022.05.038. [2] Dal Bello R, Von Der Grün J, Fabiano S, Rudolf T, Saltybaeva N, Stark LS, et al. Enabling ultra-high dose rate electron beams at a clinical linear accelerator for isocentric treatments. Radiotherapy and Oncology 2023;187:109822. https://doi.org/10.1016/j.radonc.2023.109822. Digital Poster ESTRO-ACROP guidelines in postmastectomy radiation after immediate reconstruction: Dosimetric Comparison of 3D-CRT versus VMAT planning TAHANI H. NAGEETI 1 , UMME SALAMA M 1 , DUAA ALHAWI 2 , OMER KALANTAN 2 , ELHAM RASHAIDI 2 , NESREEN SHORBAJI 2 1 RADIATION ONCOLOGY, KING ABDULLA MEDCAL CITY- HOLLY CAPITAL, MAKKAH, Saudi Arabia. 2 MEDICAL PHYSICS, KING ABDULLA MEDCAL CITY- HOLLY CAPITAL, MAKKAH, Saudi Arabia Purpose/Objective: To evaluate target volume coverage and organs at risk (OARs) sparing comparing 3D-Conformal Radiation (3D-CRT) vs. Volumetric Arc Treatment (VMAT) planning based on contouring guidelines of European Society for Therapeutic Radiology and Oncology Advisory Committee in Radiation Oncology Practice (ESTRO-ACROP) for implant sparing and target volume delineation in post-mastectomy radiation therapy (PMRT) after immediate breast reconstruction (IBR). Material/Methods: Ten eligible patients were identified via a retrospective chart review. The CT-simulation scans were used to contour target volumes applying ESTRO-ACROP guidelines and OARs. For each case, 3D-CRT and VMAT plans were generated to evaluate the best achievement of target volume coverage and minimal dose to OARs. Results: There was a significant difference towards VMAT for PTV coverage by 90% and 95% isodose line. The VMAT, compared to 3D-CRT, showed a lower mean (PTV-105%) (8.1 vs. 17.8), (p-value 0.000034). The OARs sparing showed a significant difference in favor of VMAT for ipsilateral lateral lung V20 (p-value 0.007048), both lungs' mean dose (p value 0.019021), and heart mean dose (p-value 0.000076). The 3D-CRT plan showed lower thyroid mean dose (19.27 vs 23cc), (p-value 0.0235), and contralateral breast Dmax, D5 and D10, p-values (0.04088, < 0.00001 and< 0.00001), respectively. In the implant doses, there was no statistical difference between Dmax (54.59Gy vs. 54.14Gy), while there was a statistically significantly lower mean implant dose for VMAT (43.83Gy) vs. 3D-CRT (50.81Gy), (p-value < 0.00001). Keywords: FLASH, mixed beams, treatment planning 165