Dosimetric effect of limited aperture multileaf collimator on VMAT plan quality: A study of prostate and head-and-neck cancers

Abstract

Aim

The aim of study was to evaluate the dosimetric effect of collimator-rotation on VMAT plan quality, when using limited aperture multileaf collimator of Elekta Beam Modulator™ providing a maximum aperture of 21[[ce:hsp sp="0.25"/]]cm[[ce:hsp sp="0.25"/]]×[[ce:hsp sp="0.25"/]]16[[ce:hsp sp="0.25"/]]cm.

Background

The increased use of VMAT technique to deliver IMRT from conventional to very specialized treatments present a challenge in plan optimization. In this study VMAT plans were optimized for prostate and head and neck cancers using Elekta Beam-Modulator^TM, whereas previous studies were reported for conventional Linac aperture.

Materials and methods

VMAT plans for nine of each prostate and head-and-neck cancer patients were produced using the 6[[ce:hsp sp="0.25"/]]MV photon beam for Elekta-SynergyS^® Linac using Pinnacle³ treatment planning system. Single arc, dual arc and two combined independent-single arcs were optimized for collimator angles (C) 0°, 90° and 0°–90° (0°–90°; i.e. the first-arc was assigned C0° and second-arc was assigned C90°). A treatment plan comparison was performed among C0°, C90° and C(0°–90°) for single-arc dual-arc and two independent-single-arcs VMAT techniques to evaluate the influence of extreme collimator rotations (C0° and 90°) on VMAT plan quality. Plan evaluation criteria included the target coverage, conformity index, homogeneity index and doses to organs at risk. A ‘two-sided student t-test’ (p[[ce:hsp sp="0.25"/]]≤[[ce:hsp sp="0.25"/]]0.05) was used to determine if there was a significant difference in dose volume indices of plans.

Results

For both prostate and head-and-neck, plan quality at collimator angles C0° and C(0°–90°) was clinically acceptable for all VMAT-techniques, except SA for head-and-neck. Poorer target coverage, higher normal tissue doses and significant p-values were observed for collimator angle 90° when compared with C0° and C(0°–90°).

Conclusions

A collimator rotation of 0° provided significantly better target coverage and sparing of organs-at-risk than a collimator rotation of 90° for all VMAT techniques.