A Feasibility Study: Can the Dose to Recurrent Sacral Chordoma be Safely Escalated to 84.6Gy (RBE) Using Pencil-Beam-Scanning Proton Therapy and Volumetric-Modulated Arc Therapy?

Abstract

Chordomas locally recurrent after surgery are difficult to control. Increasing the total radiation dose has the potential of increasing local control. We investigated the feasibility of dose escalation (84.6Gy RBE, 1.8Gy RBE/fraction) to the gross-tumor-volume (GTV) for recurrent chordomas while respecting dose constraints to organs-at-risk (OARs) using different pencil beam scanning (PBS) beam spot sizes and beam angles as well as VMAT planning. Several patients with recurrent sacral chordomas were selected retrospectively. These patients were originally treated at our institution with pre-operative RT with 50.4Gy RBE, followed by surgery and post-operative boost of 21.6Gy RBE. In this study, GTV is contoured on the recurrent CT image. Treatment plans were generated to provide dose escalation (84.6Gy RBE) to the recurrent GTV with a 3 mm margin. PBS plans were generated using intensity-modulated proton therapy (IMPT) with two sets of beams: one with posterior-anterior (PA) only, and the other with a left- and a right-posterior-oblique beam (LP and RP). Two different spot sizes were evaluated: 8 mm to 15 mm sigma versus 2.3 mm to 4.4 mm sigma in air at isocenter with energies from 220 MeV to 90 MeV. VMAT plans used four partial arcs. OAR dose constraints are listed in the Table and used as optimization objectives and constraints for IMPT and VMAT. The use of posterior-oblique beams reduced the maximum and mean dose to small bowel and bladder compared to a single PA beam. PBS (spot size of 2.3 mm to 4.4 mm) provided the best mean and maximum dose to OARs and improved GTV coverage. Neither PBS with PA beam using spot size of 8 mm to 15 mm, nor the VMAT, plans met the maximum dose constraints for bladder, small bowel and skin. Intergluteal-fold dose was lower with two posterior-oblique beams. VMAT plans had similar maximum and mean dose to some OARs compared to spot size of 8 mm to 15 mm. High dose radiation (84.6Gy RBE) can be delivered to GTV while respecting dose constraints to OARs with PBS using spot size of 2.3 mm to 4.4 mm. PBS generally provides lower mean OARs dose compared to VMAT. Institutions with larger spot size should compare PBS and VMAT plans with different PBS beam angles for certain OARs. Institutions with smaller spot size, two posterior-oblique beams may be favorable compared to a single PA beam.Abstract 3762Maximum dose (Gy RBE)OAR dose constraintsPBS-PA 8 mm-15 mmPBS-PA 2.3 mm-4.4 mmPBS-LP-RP 8 mm-15 mmPBS-LP-RP 2.3 mm-4.4 mmVMATGTV84.987.687.687.587.589.0Sigmoid77.467.066.866.866.465.7Bladder60.066.752.928.635.665.3Small Bowel57.66457.157.955.265.8Rectum77.476.170.977.768.772.7Cauda77.424.10.529.31.23.9Skin rim6674.174.065.262.389.1Mean dose (Gy RBE)PBS-PA 8 mm-15 mmPBS-PA 2.3 mm-4.4 mmPBS-LP-RP 8 mm-15 mmPBS-LP-RP 2.3 mm-4.4 mmVMATGTV85.78685.786.285.7Sigmoid21.015.619.29.821.6Bladder5.04.60.30.319.8Small Bowel2.53.61.20.89.5Rectum29.112.331.214.430.4Cauda2.80.04.70.11.9Skin rim4.57.55.83.44.4 Open table in a new tab