Volumetric-modulated arc stereotactic body radiotherapy for prostate cancer: dosimetric impact of an increased near-maximum target dose and of a rectal spacer

Abstract

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-five-fraction plans only assure 33.2 Gy to ≥95% PTV ([Formula: see text] ≥ 95%). Looking for an improved [Formula: see text], increased near-maximum target dose (D2%) and prostate-rectum spacer insertion were tested.

Methods: For 11 patients, two VMAT plans, with D2% ≤ 37.5 Gy (Hom) or D2% ≤ 40.2 Gy (Het), on each of two CT studies, before or after spacer insertion, were computed. All plans assured [Formula: see text] ≥95%, and <1 cm(3) of rectum, bladder and urethra receiving ≥35 Gy. 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 32 Gy ([Formula: see text]) 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 [Formula: see text]. By linear correlation analysis, spacer insertion was related to the reductions in rectal [Formula: see text] for X ≥ 28 Gy.

Conclusion: A slightly increased D2% or the use of spacer insertion was each able to improve [Formula: see text]. Their combined use assured [Formula: see text] ≥ 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.

Figure 1.

Average dose–volume histograms, with error bars equal to ±1 standard deviation, for rectum and prostate planning target volume (PTV) by comparing Het-Spc vs Hom-NoSpc plans: the enlargement of the therapeutic window by the combined use of spacer insertion and increased accepted D_2% is evident. D_2%, minimum dose to 2% of PTV; Het, plans with D_2% ≤ 40.2 Gy; Hom, plans with D_2% ≤ 37.5 Gy; NoSpc, plans computed on CT scanned before spacer insertion; Spc, plans computed on CT scanned after spacer insertion.