In volumetric-modulated arc (VMAT) prostate stereotactic body radiotherapy (SBRT), dose coverage of the planning target volume (PTV) becomes challenging when the sparing of rectum, bladder, and urethra are strictly pursued. Our current 35Gy-in-five-fractions plans only assure 33.2Gy to ≥95%PTV (V(PTV)33.2≥95%). Looking for an improved V(PTV)33.2, increased near-maximum target dose (D2%) and prostate-rectum spacer insertion were tested.
For eleven 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 V(PTV)33.2 ≥95%, and less than one cubic centimetre 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 18Gy, 28Gy, and 32Gy (V(r)X), was further tested by linear correlation analysis.
By hypothesis testing, the increased D2% was associated with improvements in target coverage, while spacer insertion was associated with improvements in both target coverage and rectal V(r)X. By linear correlation analysis, spacer insertion was related to the reductions in rectal V(r)X for X≥28Gy.
A slightly increased D2%, or the use of spacer insertion, were each able to improve V(PTV)33.2. Their combined use assured V(PTV)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.
Br J Radiol. 2015 Aug 3:20140736. [Epub ahead of print]
Ruggieri R1, Naccarato S1, Stavrev P1, Stavreva N1, Fersino S1, Giaj Levra N1, Mazzola R1, Mancosu P2, Scorsetti M2, Alongi F1.
11 Radiation Oncology, Ospedale 'Sacro Cuore - don Calabria', Negrar (VR), Italy.
22 Radiotherapy and Radiosurgery, IRCCS Istituto Clinico Humanitas, Rozzano (MI), Italy.