Abstract
Intensity-modulated radiation therapy (IMRT) has been used to spare organs at risk (OARs) in the definitive treatment of anal cancer. However, treatment continues to result in significant hematologic toxicity. In a cooperative trial assessing IMRT (RTOG 0529), the rate of grade 2+ and grade 3+ hematologic toxicity was 73% and 58%, respectively. Intensity-modulated proton therapy (IMPT) has the potential to decrease the integral bone marrow dose and dose to other OARs compared with photon therapy. Computed tomography datasets of 9 patients with anal cancer previously treated with IMRT, volumetric arc therapy (VMAT), or tomotherapy at our institution were used for comparison. Both VMAT and IMPT plans were created for each patient. The IMPT plans were created using a multi-field optimized, split-target technique. The dose to OARs, including bone marrow, bladder, small bowel, large bowel, femoral heads, and genitalia, were compared using a paired t test. The mean bone marrow dose was 17.42 Gy with IMPT plans and 30.76 Gy with VMAT plans (P < .0001). The absolute volume of bone marrow spared 10 and 20 Gy was significantly less with the proton plans. IMPT also showed significant sparing of other OARs, including the small and large bowel, femoral heads, and genitalia. The mean planning target volume receiving at least 95% of the prescribed dose (V95) was similar with IMPT and VMAT plans, 99% and 98%, respectively. IMPT can decrease the mean bone marrow dose compared with VMAT plans by minimizing the low dose spill associated with standard photon treatment. Prospective studies assessing proton therapy for anal cancer are ongoing to evaluate the potential for improvement in hematologic toxicity and the acute tolerance of therapy.
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