Comparison of treatment planning approaches for spatially fractionated irradiation of deep tumors

摘要

Purpose The purpose of this work was to compare the dosimetry and delivery times of 3D‐conformal (3DCRT)‐, volumetric modulated arc therapy (VMAT)‐, and tomotherapy‐based approaches for spatially fractionated radiation therapy for deep tumor targets. Methods Two virtual GRID phantoms were created consisting of 7 “target” cylinders (1‐cm diameter) aligned longitudinally along the tumor in a honey‐comb pattern, mimicking a conventional GRID block, with 2‐cm center‐to‐center spacing (GRIDsub2?cm/sub) and 3‐cm center‐to‐center spacing (GRIDsub3?cm/sub), all contained within a larger cylinder (8 and 10?cm in diameter for the GRIDsub2?cm/sub and GRIDsub3?cm/sub, respectively). In a single patient, a GRIDsub3?cm/sub structure was created within the gross tumor volume (GTV). Tomotherapy, VMAT (6?MV?+?6?MV‐flattening‐filter‐free) and multi‐leaf collimator segment 3DCRT (6?MV) plans were created using commercially available software. Two tomotherapy plans were created with field widths (TOMOsub2.5?cm/sub) 2.5?cm and (TOMOsub5?cm/sub) 5?cm. Prescriptions for all plans were set to deliver a mean dose of 15?Gy to the GRID targets in one fraction. The mean dose to the GRID target and the heterogeneity of the dose distribution (peak‐to‐valley and peak‐to‐edge dose ratios) inside the GRID target were obtained. The volume of normal tissue receiving 7.5?Gy was determined. Results The peak‐to‐valley ratios for GRIDsub2?cm/sub/GRIDsub3?cm/sub/Patient were 2.1/2.3/2.8, 1.7/1.5/2.8, 1.7/1.9/2.4, and 1.8/2.0/2.8 for the 3DCRT, VMAT, TOMOsub5?cm/sub, and TOMOsub2.5?cm/sub plans, respectively. The peak‐to‐edge ratios for GRIDsub2?cm/sub/GRIDsub3?cm/sub/Patient were 2.8/3.2/5.4, 2.1/1.8/5.4, 2.0/2.2/3.9, 2.1/2.7/5.2 and for the 3DCRT, VMAT, TOMOsub5?cm/sub, and TOMOsub2.5?cm/sub plans, respectively. The volume of normal tissue receiving 7.5?Gy was lowest in the TOMOsub2.5?cm/sub plan (GRIDsub2?cm/sub/GRIDsub3?cm/sub/Patient?=?54?cmsup3/sup/19?cmsup3/sup/10?cmsup3/sup). The VMAT plans had the lowest delivery times (GRIDsub2?cm/sub/GRIDsub3?cm/sub/Patient?=?17?min/8?min/9?min). Conclusion Our results present, for the first time, preliminary evidence comparing IMRT‐GRID approaches which result in high‐dose “islands” within a target, mimicking what is achieved with a conventional GRID block but without high‐dose “tail” regions outside of the target. These approaches differ modestly in their ability to achieve high peak‐to‐edge ratios and also differ in delivery times.