Volumetric Modulated Arc Therapy for Stereotactic Body Radiation Therapy of the Prostate

Abstract

system. Treatment was delivered using 6/10 MV beam with one single 360 o arc containing 20 sectors. The MLC performed a full dynamic sweep in each sector. All the calculations were performed using a Monte Carlo algorithm, including the QA plans. Plans wereverified using the cylindrical ArcCheck QA device. AQA criterion of 3%-3mm was used to determine the plan passing rate. Results: In all cases at least 95% of the PTV received the prescription dose. The resulting coverage yielded a median prostate mean dose of 53.7 Gy [52.2 - 55.2 Gy] and a median PTV mean dose of 53.5 Gy [51.8 - 54.5 Gy]. Due to the proximity of PTV, the most stringent dose constraints were for the bladder wall and anterior rectal wall. The median bladder wall V25Gy was 9.3cc [8.8 - 13.8] and the median anterior rectum wall V50Gy was 1.9cc [0.7 - 3.5]. The urethra maximum dose was limited to less than 52.5 Gy. The median treatment time was 10:06 min [8:24 - 12:35] and the median number of MUs was 3630 [3137 4620] which indicated that most of the MUs were efficiently delivered at a high dose rate during most of the treatment (max dose rate 480 MU/min). The number of MLC segments decreased progressively as experience built up and the median was 185 [166 1108]. QA passing rates were fully satisfactory and above our threshold of 90% with a median of 95.95% [93.4 - 98.6%]. Conclusions: Highly conformal plans were generated and delivered using Monaco and VMAT. Very short treatment times were achieved, thereby improving patient comfort and reducing the risk of intra-fractional motion associated with long treatments times. QA results show excellent agreement between measurements and plans. Patients tolerated treatments well acutely.