ESTRO meets Asia 2024 - Abstract Book

S247

Interdisciplinary – Particle Therapy

ESTRO meets Asia 2024

Anal cancer radiotherapy treatment has a complex target volume surrounded by several critical organs at risk (OARs). Despite intensity-modulated radiotherapy (IMRT), 30-60% of patients would suffer acute grade 3 toxicity. In this study, we review the dosimetric advantage that can be achieved with Intensity Modulated Proton Therapy (IMPT) compared to IMRT.

Material/Methods:

The contours and clinical plans of 13 consecutive patients of anal canal squamous cancer treated using rotational IMRT (Rapid Arc; Varian Medical Systems, Palo Alto, USA) in 2023 were reviewed. Grade 3 acute toxicity was noted in 31% (dermatitis), 15% (Gastrointestinal) and 30% hematological. After Ethics approval, the contours of the target volumes and OARs (bowel, bladder, marrow, bilateral femoral heads, and external genitalia) were reviewed on Eclipse (v16.1) Treatment Planning System (TPS) for suitability for proton therapy planning in concurrence with UK guidelines. The photon clinical plans were re-evaluated for OAR doses and target volume coverage as per the institutional constraints (based on RTOG 0529), and all met the recommended criteria. The scans and contours were exported to the RayStation TPS (v12A) for proton therapy planning. The dose for IMPT planning was prescribed to the CTV with 5mm robustness (planning PTV – p-PTV), keeping the dose prescription similar to the clinical plan (45Gy to the pelvis and 55-60Gy simultaneous boost to any gross nodes and gross disease). Plans were made using four beams (2 anterior obliques to cover the inguinal regions and the anterior portions of the pelvic CTV and two posterior obliques to cover the gross disease and the posterior portion of the pelvic CTV). The proton plan objectives were based on RTOG 0529 and principles of ALARA for OARs without compromising target coverage. The dose-volume parameters, conformity index (CI =V RI /TV where RI or reference isodose volume is defined by ICRU as V95% and TV is the target volume) and homogeneity index (HI = D2%/D98%) were compared for IMRT versus IMPT using student’s t-test. There was a significant decrease in the volume of OARs receiving doses <40Gy (Bowel - V30Gy reduced by 37.37% and V35Gy by 28.4%, p = 0.03 and 0.05 respectively; Bladder V35Gy decreased by 16.06%, p=0.01; Marrow - V10Gy reduced by 38.15%, V20Gy by 26.72%, V30Gy by 13.04%, D mean 24.3:13.6Gy - 44.1% reduction, p<0.01 for all; Femoral heads - V30Gy by 7%, p=0.01; External genitalia - V10Gy by 52.12%, V20Gy by 32.11%, V30Gy by 15.77%, p<0.01 for all). The volumes receiving higher doses (40-50Gy) were not significantly different for all OARs as the volumes receiving these doses by either modality, were very small (Bowel - V45Gy =26.23:25.64cc - IMRT:IMPT, V50Gy = 0.36:0.33cc; Bladder V50Gy = 2.43:2.32%; Marrow - V40Gy = 10.48:7.15%, V45Gy = 2.13:0.66%; Femoral heads - V40Gy = 1.15:0.11%, V44Gy = 0.6:0.04%; External genitalia - V40Gy = 5.79:2.21%). Results:

The target volume coverage was achieved with both modalities while proton also achieved significantly better HI and CI than the corresponding photon plan (Figures 1 & 2).