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

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 (GRID(2 cm)) and 3‐cm center‐to‐center spacing (GRID(3 cm)), all contained within a larger cylinder (8 and 10 cm in diameter for the GRID(2 cm) and GRID(3 cm), respectively). In a single patient, a GRID(3 cm) 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 (TOMO(2.5 cm)) 2.5 cm and (TOMO(5 cm)) 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 GRID(2 cm)/GRID(3 cm)/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, TOMO(5 cm), and TOMO(2.5 cm) plans, respectively. The peak‐to‐edge ratios for GRID(2 cm)/GRID(3 cm)/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, TOMO(5 cm), and TOMO(2.5 cm) plans, respectively. The volume of normal tissue receiving 7.5 Gy was lowest in the TOMO(2.5 cm) plan (GRID(2 cm)/GRID(3 cm)/Patient = 54 cm(3)/19 cm(3)/10 cm(3)). The VMAT plans had the lowest delivery times (GRID(2 cm)/GRID(3 cm)/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.