Dosimetric Evaluation of Automated Single-Isocenteric vs. Conventional Multi-Isocenteric SRS Technique Using Dynamic Conformal Arcs for the Treatment of Multiple Brain Metastasis

Abstract

Multi-isocenter dynamic conformal arc (MIDCA) SRS technique is commonly utilized to treat brain metastasis. Conventionally, individual isocenters are used to treat each lesion separately, which is time consuming and uncomfortable for the patients. An automated single isocenter dynamic conformal arc (SIDCA) technique to treat all lesions simultaneously could save machine time and help patient to tolerate the shorter treatment time. Our objective of this study is to analyze and compare the quality of treatment plans generated using single-isocenter vs multi-isocenter techniques employing non-coplanar dynamic conformal arcs . Five multiple brain metastasis patients were selected for this study, who were treated with MIDCA on a treatment planning system. Number of brain lesions in each patient ranged from 3 to 10 and there were total 27 targets in all 5 patients. Prescribed doses ranged from 15 Gy to 20Gy depending on tumor volume and location. All patient’s contours were transferred to single isocenter automated multimets element software. Three to four non-coplanar arcs were used for each lesion in MIDCA plans whereas 5 non-coplanar arcs were used for each patient to treat all lesions in SIDCA plans. For dosimetric plan quality evaluation, conformity index (CI), volume of 12 Gy (V12) and volume of 5Gy (V5) were collected for all the plans and compared. Average CI for SIDCA was 1.45 (SD 0.8) versus 1.63 (SD 0.2) for MIDCA. Average V12 for SIDCA 24.3cc (SD 23.3cc) versus 29.8 cc (SD 29.1cc) for MIDCA technique. As number of targets increases V5 and V12 dramatically increased for SIDCA technique, for 10 lesions plans V12 values were 65.7cc and 50.5cc for SIDCA and MIDCA respectively. Similarly, V5 values were 373cc and 272cc for SIDCA and MIDCA respectively. As expected Monitor units were significantly lower for SIDCA as compared to MIDCA hence shorter treatment delivery time. Single isocenter plans done using automated multiments element software were better in conformity index however they were inferior in lower dose volumes such as 12 and 5 Gy. This difference was observed larger when number of brain lesions increased. Single isocenter technique is very convenient and time saving for treating multiple brain mets SRS, however, it could compromise lower dose conformity and potentially could deliver higher volumes of lower doses to normal brain tissue.