Dosimetric advantages of non-coplanar volumetric modulated arc therapy (VMAT) using biaxially rotational dynamic radiation therapy compared with coplanar VMAT in prostate stereotactic body radiation therapy with focal boost

PURPOSE: To identify dosimetric advantages of biaxially rotational dynamic radiation therapy (BROAD-RT) for stereotactic body radiation therapy (SBRT) with focal boost for non-metastatic prostate cancer (PCa), compared with coplanar volumetric-modulated arc therapy (co-VMAT). MATERIALS AND METHODS: BROAD-RT is a unique beam delivery technique, which facilitates sequential non-coplanar beam delivery without the need to rotate the couch or reposition the patient. For 15 patients with non-metastatic PCa, two different plans (BROAD-RT and co-VMAT) were created, and these plans and dosimetric indices were compared. The prescribed dose was 35 Gy in 5 fractions to the whole prostate gland, and that to the intra-prostatic dominant lesions (IPDLs) was increased up to 50 Gy. Quality assurance (QA) was performed for BROAD-RT and co-VMAT plans from the 15 patients, and the calculated and measured dose distributions were evaluated according to global gamma analysis using ArcCHECK. RESULTS: The dose coverage of the target volumes and the high-dose exposure to organs at risk (rectum, bladder, and urethra) were not significantly different between BROAD-RT and co-VMAT plans. The normal tissue dose outside of the planning target volume (PTV) (maximum dose 2 cm away from PTV [D2 cm], and 30%, 50%, or 70% isodose volume divided by the volume of PTV [R30, R50, and R70]) were significantly lower in the BROAD-RT plan (p < 0.001, p < 0.001, p < 0.001, and p < 0.001, respectively). In QA, average (± standard deviation) passing rates were 98.2 ± 0.7% for BROAD-RT and 99.0 ± 1.0% for co-VMAT. CONCLUSION: Non-coplanar VMAT via BROAD-RT improved the dose distribution, mainly outside of the PTV and for some of the organs at risks, in prostate SBRT with focal boost compared with coplanar VMAT. Therefore, as BROAD-RT enables practical implementation of non-coplanar VMAT, it is considered a promising radiotherapy method of SBRT with focal boost for non-metastatic PCa.