Objective: In volumetric-modulated arc therapy (VMAT) prostate stereotactic body radiotherapy (SBRT), dose coverage of the planning target volume (PTV) becomes challenging when the sparing of rectum, bladder and urethra is strictly pursued. Our current 35-Gy-in-fivefraction plans only assure 33.2 Gy to ≥95% PTV (VPTV33:2 ≥95%). Looking for an improved VPTV 33:2 , increased near-maximum target dose (D2%) and prostate-rectum spacer insertion were tested. Methods: For 11 patients, two VMAT plans, with D2%≤37.5Gy (Hom) or D2%≤40.2Gy (Het), on each of two CT studies, before or after spacer insertion, were computed. All plans assured VPTV 33:2 ≥95%, and ,1 cm3 of rectum, bladder and urethra receiving ≥35Gy. By hypothesis testing, several dose-volume metrics for target coverage and rectal sparing were compared across the four groups of plans. The impact of spacer insertion on the fractions of rectum receiving more than 18, 28 and 32Gy (VrX) was further tested by linear correlation analysis. Results: By hypothesis testing, the increased D2% was associated with improvements in target coverage, whereas spacer insertion was associated with improvements in both target coverage and rectal VrX. By linear correlation analysis, spacer insertion was related to the reductions in rectal VrX for X≥28Gy. Conclusion: A slightly increased D2% or the use of spacer insertion was each able to improve VPTV 33:2 . Their combined use assured VPTV 33:2 ≥98% to all our patients. Spacer insertion was further causative for improvements in rectal sparing. Advances in knowledge: For VMAT plans in prostate SBRT, the distinct dosimetric usefulness of increased D2% and of the use of spacer insertion were validated in terms of target coverage and rectal sparing.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging