Effect of protons on bone marrow and small-bowel exposure for patients receiving neoadjuvant radiotherapy for resectable rectal cancer.

Abstract

669 Background: Determine the potential role for protons in the preoperative treatment of resectable rectal cancer. Methods: Eight consecutive patients with resectable (T2-T3) rectal cancers underwent 3D conformal photon (3DCRT); intensity-modulated photon (IMRT); and 3D conformal proton (PT) treatment planning. Initial target volumes (PTV1) were contoured using the RTOG anorectal atlas guidelines. Boost target volumes (PTV2) consisted of the gross rectal tumor plus a uniform 2cm expansion. Plans delivered 45Gray (Gy) or Cobalt Gray Equivalent (CGE) to the PTV1 and a 5.4 Gy (CGE) boost to the PTV2. Ninety-five percent of PTVs received 100% of the target dose and 100% of the PTVs received 95% of the target dose. Standard normal tissue constraints were utilized. Wilcoxon paired t-tests were performed to compare various dosimetric points between the 3 plans for each patient. Results: All plans met all normal-tissue constraints and were isoeffective in terms of PTV coverage. Proton plans offered significantly reduced median normal-tissue exposure over the 3DCRT and IMRT plans with respect to: pelvic bone marrow (PBM) at the V5Gy, V10Gy, V15Gy and V20Gy levels and small bowel (SB) at the V10Gy and V20Gy levels. Proton plans also offered significantly reduced median normal-tissue exposure over the 3DCRT plans with respect to the SB at the V30Gy and V40Gy levels. Conclusions: 1.) By reducing bone marrow exposure, protons may reduce the acute hematologic toxicity of neoadjuvant chemoradiation and increase the likelihood of uninterrupted chemotherapy delivery. Bone marrow sparing may also facilitate the delivery of salvage chemotherapy for patients who go on to develop hematogenous metastasis. 2.) By reducing small bowel exposure, protons may reduce toxicity and possibly facilitate the use of more aggressive concomitant chemotherapy with radiotherapy. [Table: see text]