Preliminary Dose Constraints for Testing Intensity Modulated Proton Therapy in Anal Cancer Trials

The target volume for radiotherapy of anal cancer is complex, with the added challenge of it being surrounded by several critical organs at risk (OARs). In this study, we review the dosimetric advantage that can be achieved with intensity modulated proton therapy (IMPT) over intensity modulated radiotherapy (IMRT) and derive the dose constraints for OARs.

Ten consecutive patients of anal cancer treated using rotational IMRT (Rapid Arc; Varian Medical Systems, Palo Alto, USA) in 2023 were reviewed. 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 for adherence to contouring guidelines. The clinical plans were re-evaluated for OAR doses and target volume coverage.The scans with structure sets were exported to RayStation treatment planning system (v12A) for intensity modulated proton therapy (IMPT) planning. The dose for IMPT planning was prescribed to the CTV with 5 mm robustness, keeping dose prescription similar to the clinical IMRT plan (45-50 Gy to pelvis and 55-60 Gy simultaneous boost to any gross disease and gross nodes). The 4 beam class solution was identified as best for IMPT planning. The IMPT plan objective was set 10% to 20% below the lower threshold of RTOG 05-29 and UK guidelines, followed by principles of ALARA for OARs without compromising target coverage. The dose-volume parameters, conformity index (CI) and homogeneity index (HI) were compared for IMRT versus IMPT using student's t-test.

Target volume coverage was well achieved with both IMRT and IMPT. However, V100% of GTV_p and GTV_n and V95%, V98%, V99%, V100% of CTV_Pelvis and CTV_TumourBoost were significantly better with IMPT. HI and CI of target volumes were also significantly improved with IMPT. There was a significant decrease in volume of OARs receiving doses <40 Gy. Volumes receiving higher doses (40-50 Gy) were very small and not significantly affected with use of IMPT.

IMPT offers improved target coverage with reduction in low-intermediate dose spill to OARs. Based on these results, dose constraints for IMPT planning have been generated for future clinical trial validation.