Introduction: The treatment option for poorly differentiated stage I anal cancer includes photon radiation therapy with volumetric-modulated arc therapy (VMAT). Intensity-modulated proton therapy (IMPT) has a clinical benefit of reduced normal tissue toxicity over traditional photon therapy. The aim of this study was to evaluate on potential benefits and drawbacks of robust optimized IMPT compared to VMAT plans for anal cancer treatment.
Methods: Data from 20 patients treated with VMAT for anal cancer was used for retrospective IMPT treatment planning. One IMPT plan was created with 2 beam directions, while another was created with 4 beam directions. The composite minimax robust optimization (CMRO) was used for IMPT plan optimization. The quality of the treatment plans was evaluated in terms of target dose coverage and organ at risk (OAR) dose constraints.
Results: Robust target dose coverage was achieved in both IMPT plans in all patients. In general, the dose to OARs was significantly lower in the IMPT plans compared to the VMAT plans: the dose constraint ”Femoral Heads D50 < 30 Gy” was 0.3 Gy in the IMPT plans, while this was 24.1 Gy in the VMAT plans, and the dose constraint ”Bone marrow D30 < 40 Gy” was 8.1 - 9.0 Gy in the IMPT plans, while this was 31.2 Gy in the VMAT plans.
Conclusions: IMPT achieves robust target dose coverage and reduces dose to OARs compared to VMAT in anal cancer treatment, even with only two beams. Because of large dosimetric benefits, IMPT might be considered for anal cancer treatment in the clinic without full model-based decision-making.