Dosimetric effect of interfractional needle displacement in prostate high-dose-rate brachytherapy

PurposeTo quantify the dosimetric deviations that would arise from delivering subsequent prostate high-dose-rate fractions with only needle readjustment and no replanning after the first fraction.Methods and MaterialsPatients were treated with either two implant sessions (two 9.5-Gy fractions per session) separated by 2–4 weeks or with one implant session and external beam radiotherapy. After needle placement, needle positions were adjusted under CT guidance, after which dosimetric planning was performed before each fraction. To evaluate the consequence of not replanning before the second fraction, we analyzed the dosimetric parameters of 45 consecutive implants (26 patients). Needles with optimized dwell positions from the first fraction were transferred to the needle positions in the second fraction. Needle displacement between fractions was assessed as well as changes in plan metrics.ResultsAfter adjustment, the mean interfractional needle displacement was 3.5 mm. If replanned, the probability of planning target volume D_90% ≥95% is 100%, prostate V_100% ≥95% is 87%, and urethra V₁₁₅% ≤10% is 78%. If treated without replanning, the probability of planning target volume D_90% ≥95% is 82%, prostate V_100% ≥95% is 53%, and urethra V_115% ≤10% is 69%. Even for implants with minimal needle displacement (<3 mm) and minimal prostate volume change (<3 cc), the dosimetric consequence of not replanning the second fraction would result in 46% of cases with a prostate V_100% <95%.ConclusionThe dosimetric consequences of not replanning the second fraction for prostate high-dose-rate implants results in significantly inferior plan metrics.