Absolute volume of active bone marrow and total bone marrow spared in anal cancer patients using intensity modulated proton versus volumetric arc therapy

Objective Patients undergoing chemoradiation for anal cancer are at risk of hematologic toxicity (HT) with rates of grade 3–4 HT as high as 60%. Studies have demonstrated a correlation between HT and the dose to pelvic bone marrow (PBM) as well as the positive emission topography (PET)-defined active pelvic bone marrow (APBM) spared. We hypothesize that the use of intensity modulated proton therapy (IMPT) can reduce the dose to the PBM and APBM compared to volumetric-modulated arc therapy (VMAT). Methods Comparative VMAT and IMPT plans were generated from eight patients originally treated with chemoradiation for anal cancer. The PBM volume was contoured as per Mell et al (IJROBP 2008). The APBM volume was defined as all regions within the PBM with a standardized uptake value (SUV) > total body mean SUV. IMPT plans used multi-field optimization and split-target technique. Data was analyzed using paired sample t tests. Results The volume of PBM and APBM spared was significantly greater with IMPT plans compared to VMAT plans. IMPT plans increased the mean volume of APBM spared 10 Gy (485 vs 91 cc, p = 0.0005), 20 Gy (569 vs 117 cc, p = 0.0005), and 40 Gy (815 vs 759 cc, p = 0.04). Conclusion The volume of PBM and APBM spared has been shown to be strong predictors of acute hematologic toxicity. IMPT planning results in significantly greater PBM and APBM sparing compared to VMAT. The use of IMPT for anal cancer may result in lower HT rates in patients undergoing definitive chemoradiation through improved bone marrow sparing.