CT-Based Quantification of Prostate Volume Change after LHRH-Agonist Androgen Deprivation: A Prospective, Three-Reader Study for Radiotherapy Planning

Introduction : ADT is routinely combined with radiotherapy (RT) for intermediate- and high-risk prostate cancer. While prostate shrinkage may facilitate planning, prospective CT-based, patient-level estimates over short, workflow-relevant intervals are scarce. Methods: We conducted a prospective study of 47 patients starting luteinizing hormone–releasing hormone agonist (LHRHa) therapy (leuprolide, 6-month depot). Prostate volumes were independently contoured by three blinded radiation oncologists on paired CT scans at baseline and ~8 weeks post-injection. The primary outcomes were the mean relative volume change and the proportion achieving a clinically relevant reduction (≥15%). PSA and testosterone were recorded at both time points; correlations and exploratory univariable logistic regression for ≥15% reduction were performed at the patient level. Results: Mean relative volume reduction ranged from −18.5% to −21.3% across observers; ≥60% of patients met the ≥15% threshold (RT-A 61.7%, RT-B 66.0%, RT-C 74.5%). PSA and testosterone decreased substantially (e.g., median PSA from 9.64 to 1.84 ng/mL) and were moderately correlated (Spearman ρ=0.43, p=0.002; Pearson r=0.51, p< 0.001). No baseline clinical, histologic, or biochemical variable reached statistical significance for predicting ≥15% volume reduction; % PSA change showed a non-significant trend (OR 1.03; 95% CI 1.00–1.07; p=0.076). Conclusions: Short-course LHRHa induced consistent CT-measured cytoreduction, with most patients achieving ≥15% shrinkage within 8 weeks. Prostate downsizing was reproducible across readers and accompanied by marked PSA and testosterone declines, although biochemical responses did not predict volumetric change. These findings support incorporating a short neoadjuvant “window” before RT simulation and highlight the need for larger studies to refine predictors and compare agonist vs antagonist trajectories.