3012 - Development of Organs at Risk Constraints for Proton-Based Intensity Modulated Treatment for Prospective Randomized Trial in Anal Cancer

R. Krishnatry^1,2, R. Punde³, A. Jain⁴, S. Gudi⁵, and R. Engineer⁶; ¹Homi Baba National Institute,, Mumbai, India, ²Tata Memorial Centre, Homi Bhabha National Institute, Mumbai, India, ³Tata Memorial Centre, MUMBAI, India, ⁴TATA MEMORIAL CENTRE, MUMBAI, India, ⁵Department of Radiation Oncology, Tata Memorial Centre, Homi Bhabha National Institute, Mumbai, India, ⁶HBNI University, Mumbai, India

Purpose/Objective(s): 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. Multi-institute data on proton therapy could not translate dosimetric advantage into a clinical advantage. We review the dosimetric advantage that can be achieved with Intensity Modulated Proton Therapy (IMPT) compared to previously IMRT-treated patients with known toxicity. Materials/

Methods: After Ethics approval, contours and clinical plans of 13 consecutive patients of anal cancer treated using rotational IMRT in 2023 were reviewed. Grade 3 acute toxicity was noted in 31% (dermatitis), 15% (GI) and 30% haematological. The photon clinical plans were evaluated for OAR doses and target volume coverage as per the institutional constraints (derived from RTOG 0529). The contours (target and OARs: bowel, bladder, marrow, femoral heads, and external genitalia) were reviewed for suitability for IMPT plan and exported to the treatment planning system for IMPT planning. The dose for IMPT planning was prescribed to the CTV with 5mm robustness (planning PTV), keeping the dose prescription similar to the previous clinical plan (45Gy to the pelvis and 55-60 Gy SIB to any gross disease). Plans were made using four beams (2 anterior obliques covering inguinal regions and the anterior part of pelvic CTV and two posterior obliques to cover the GTV and the posterior part of CTV, by split technique). The proton plan objectives were based on RTOG 0529 and the principles of ALARA for OARs without compromising target coverage. The dose-volume parameters, conformity index (CI =VRI/TV) and homogeneity index (HI: D2%/D98%) were compared using student’s t-test.
Results: There was a significant decrease in the volume of OARs receiving doses <40Gy (Bowel -- V30Gy reduced by 37.37%, V35Gy by 28.4%; Bladder V35Gy by 16.06%; Marrow -- V10Gy by 38.15%, V20Gy by 26.72%, V30Gy by 13.04%, Dmean 44.1%; Femoral heads -- V30Gy by 7%; External genitalia V10Gy by 52.12%, V20Gy by 32.11%, V30Gy by 15.77%, p<0.01). The volumes receiving higher doses (40-50Gy) were not significantly different as the volumes receiving these doses by either modality, were very minimal (e.g: Bowel -- V45Gy =26.23:25.64cc -- IMRT: IMPT, V50Gy = 0.36:0.33cc). Although target coverage was acceptable for the photon plan, CI and HI significantly improved with protons. The dosimetric constraints for Proton IMPT were derived for the prospective clinical study as mean + 1SD as acceptable and mean +2SD as the upper limit from the IMPT plans, which were similarly lower compared to IMRT constraints as the differences observed above.
Conclusion: Despite minimal high-dose volumes with photon IMRT, significant GI & haematological toxicity was observed, possibly related to the mid & low-dose spill, which Proton-based IMPT can significantly reduce. This study derived constraints for a prospective proton versus photon 259andomized study, which will be reported in future.