Background and Purpose: The shrinking effect of androgen deprivation therapy (ADT) and radiotherapy (RT) on prostate gland volume is a known clinical finding. Until now, it is not known which part of the prostate shrinks more. We examined patients with and without ADT undergoing intensity-modulated RT (IMRT) and performed 3-dimensional measurements of the peripheral zone (PZ) and central gland (CG) with magnetic resonance imaging (MRI).
Methods and Materials: Prostate gland volumes of PZ and CG between planning MRI and first available follow-up MRI were retrospectively determined in 44 patients with localized prostate carcinoma. A total of 24 patients had ADT with a median time interval of 5 months (range, 1.5-24 months). Median time interval between both MRI time points was 132 days (range, 104-224 days). Two observers performed PZ and CG delineation in consensus using planimetry. Volume changes over time were determined and compared.
Results: Patients who had ADT showed smaller prostate volume in the first MRI (mean [SD], 32 [16.7] mL), which was still present after IMRT (28.1 [16.7] mL). Patients who had no ADT started with 44.6 (16.9) mL and showed 37.5 (13.9) mL after IMRT. Shrinking effect in PZ was significantly larger than in CG for all patients (−18.3% vs −6.3%, P < 0.05).
Conclusions: Because, typically, most tumors are located in PZ and this area also shows the largest shrinkage effect after IMRT, this should be taken into account for planning purposes. Notably, there are only minor differences in the relative shrinking effects between patients with and without ADT, although they start with different volumes.