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

Abstract

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 cm³ /19 cm³ /10 cm³ ). 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.

Keywords: deep-seated tumors; grid therapy; spatially fractionated radiotherapy.