Pencil Beam Scanning Proton Radiation Therapy Reduces Dose to the Planned Surgical Skin Flap and Uninvolved Bone in Preoperative Radiation Therapy for Soft Tissue Sarcomas of the Lower Extremity

Abstract

Over 40% of patients treated with preop radiotherapy for soft tissue sarcoma (STS) of the lower extremity develop severe wound complications. Reduced dose to the planned surgical skin flap and uninvolved tissues reduces the rate of severe wound complications. We compared photon and proton treatment planning for target coverage and dose avoidance to the planned surgical skin flap and uninvolved bone and soft tissues for STS of the lower extremity. Plans were generated for 5 patients with deep, lower extremity STS treated with preop radiotherapy. Plans consisted of volumetric arc radiation therapy (VMAT), passively scattered proton radiotherapy (PS) and pencil beam scanning proton radiotherapy (PBS). Target volumes were consistent with guidelines for RTOG 0630. Target volumes and OARs were contoured by a radiation oncologist specializing in sarcoma. Planned surgical flaps were contoured by an orthopedic oncologist. Planning goals were as follows: 1) ensure adequate target volume coverage (CTV D99=100%), 2) achieve dose constraints to the planned surgical skin flap, and 3) avoid uninvolved bone. The prescription dose was 50 Gy in 25 fractions. The average mean dose to the planned surgical flaps was 38.3 Gy RBE (range, 28.1-45.6), 46.1 Gy RBE (range, 38.1-53.1), and 43.4 Gy RBE (range, 33.4-49.1) for the PBS, PS, and VMAT plans, respectively. The mean V30 of the surgical flap with PBS was 69.4% (range, 48.0-87.1%) compared to 86.8% (range, 72.7-98.9%) and 82.8% (range, 55.9-98.4%) with PS and VMAT, respectively. All patients had improved mean flap dose and flap V30 using PBS.Mean uninvolved bone V40 was 6.5% (range, 2.1-10.7%), 22.3% (range, 5.8-43.0%), and 14.6% (range, 2.1-37.8%) using PBS, PS, and VMAT, respectively. Mean bone dose was 17.2 Gy RBE (range 11.8-20.8), 14.4 Gy RBE (range, 4.4-26.1), and 21.7 Gy RBE (range, 13.7-33.3) with PBS, PS, and VMAT plans, respectively. The mean conformality index was 0.91, 0.67, and 0.87 for PBS, PS, and VMAT, respectively. PBS achieved the lowest mean dose and V30 to the planned surgical flap, and the lowest bone V40 compared to PS and VMAT. Further work is necessary to determine if these benefits reduce treatment related side effects.Abstract 3798; Table 1Individual patient dosimetry for each treatment modalityMean dose to flap (Gy RBE)Flap V30 (%)Mean dose to bone (Gy RBE)Bone V40 (%)Pt No.PBSPSVMATPBSPSVMATPBSPSVMATPBSPSVMAT128.138.133.448.072.755.920.89.117.85.214.27.7245.653.149.187.198.992.611.84.414.12.15.82.1342.148.347.174.190.390.519.926.133.310.743.037.8438.346.147.769.784.198.414.520.429.75.429.917.9537.144.839.668.387.976.618.812.113.79.318.77.4 Open table in a new tab