2 Dosimetry comparison for brain metastases treatment by stereotactic radiotherapy

Abstract

Introduction Technical capabilities for brain metastases treatment in stereotactic radiotherapy are in-house Dynamic Conformal Arc Therapy (DCA, with non-coplanar arcs DCAr and without DCA0) and non-coplanar Volumetric-Modulated Arc Therapy (VMAT). This study is to decide on the most efficient method use to get the optimum PTV coverage and dose escalation, OAR sparing, especially healthy brain tissue, and minimum monitor units. Methods On Elekta’s Monaco treatment planning system, eight patients with brain metastases and an average target volume of 9.13 cc [3.58–26.5] were selected. Six of these patients were treated with DCAR / DCA0 and two with VMAT on VERSA HD radiation therapy system. For all these patients, the two other planning techniques were carried out retrospectively. Dosimetric analysis tools HI, CI, CO, OCO [1] as well as PTV coverage, PTV overdose, the 50% isodose line and healthy brain constraints ( V 12 Gy 5.9 cc, V 18 Gy 1 cc) enabled this comparison. The selected indicators were compared using non-parametric Wilcoxon’s rank tests. Results Homogeneity index is significantly higher for DCA0, as well as for DCA R compared to VMAT. The overdose for DCA0 is significantly lower than for the other two techniques. For the eight patients, the 50% isodose line is significantly smaller with DCA R . The healthy brain tissue gain (OCO) is significant using VMAT dosimetric planning rather than DCA0 (p 0.05). VMAT significantly saves healthy brain tissue for 12 Gy isodose line compared to DCA0 (p = 0.031). The same applies for DCA R compared to DCA0 (p = 0.047). Concerning average monitor units, there is 1738 UM for VMAT treatment planning against 1449 (−16%) for DCA R and 1359 (−21.8%) for DCA0. Conclusions Comparison between DCA with coplanar ( DCA r ) and non-coplanar arcs (DCA0) shows up that coplanar arcs were of significant interest for overdose at the center of PTV, for a 50% isodose line narrower and help sparing healthy brain tissue. DCA R and VMAT techniques are comparable on this last point. However, using non-coplanar arcs involves considerable additional processing time and controls. The study will be followed up with other patient data to confirm the use of VMAT treatment planning technique on target volumes from 3.58 to 26.5 cc.