Dosimetric characteristics of IMRT and VMAT techniques for spatially fractionated radiation therapy

Abstract

Object

This study aimed to evaluate the differences in plan quality metrics between three main methods of radiotherapy: Volumetric Modulated Arc Therapy (VMAT), Intensity-Modulated Radiation Therapy - Sliding Window (IMRT-SW), and Intensity-Modulated Radiation Therapy - Multiple Static Segments (IMRT-MSS) for spatially fractionated radiation therapy (SFRT) planning in the radiotherapy department of Tianjin Union Medical Center using the Eclipse 15.5 treatment planning system.

Methods

SFRT plans of ten Patients were designed using coplanar IMRT-MSS,IMRT-SW and VMAT techniques, A total of 45 plans were created with collimator angles set to 315°, 0°, 45°, and 90°, utilizing evenly distributed fields within 360°. VMAT plans employed four full arc, while IMRT plan were divided into four groups based on field spacing of 30°, 22.5°, 15°, and 10°.Plan quality was assessed and compared using metrics include Peak-to-valley dose ratio (PVDR), peak dose, lattice spheres coefficient of variation(CV), Conformity Index(CI), Gradient Measure(GM), Mean Dose to Normal Tissue(D_mean,NT), gamma passing rates(GPR), monitor units (MU), and the MU Percentage for each Collimator Angle.

Results

VMAT group exhibited a significantly elevated PVDR and peak dose, with the worst-case CV values. VMAT demonstrates a progressive enhancement in PVDR superiority compared to IMRT as the lattice point number decrease(≤10–15 points). The IMRT-MSS group generally shows higher PVDR than the IMRT-SW group. VMAT group demonstrated significant advantages in CI, GM, MU efficiency, and acceptable GPR, while the IMRT-MSS cohort outperformed IMRT-SW in these parameters. Collimator angles of 45° and 315° were found to be particularly significant in SFRT plan.

Conclusion

VMAT demonstrates overall superior performance compared to IMRT, particularly with reduced lattice point density or in superficial convex-shaped tumors. For SFRT of large-volume tumors (lattice point counts>15), we recommend VMAT implementation with controlled lattice points for improved PVDR.