Changes in prostate volume during prostate SBRT delivered on an MR-Linac and correlation with acute toxicity.

Purpose: We evaluated prostate volume changes during stereotactic body radiation therapy (SBRT) using serial MRI, identifying variables associated with prostatic swelling and their correlation with acute toxicity. Methods: Fifty-two patients with localized prostate cancer, androgen deprivation therapy naive, underwent SBRT to 40 Gy in five fractions on an MRI-Linear Accelerator with dominant intraprostatic lesion boosts to 45 Gy when present. Whole prostate (WP) and transition zone (TZ) measurements were assessed on the pre-treatment T2 MRI obtained for daily adaptation. Volumes were calculated using the ellipsoid formula. Non-transition zone (nonTZ) measures = WP values - TZ values. Transition zone index (TZI) = TZ volume/WP volume. Acute toxicity and International Prostate Symptom Scores (IPSS) were recorded. Results: Prostate volume increased significantly over the first four fractions, peaking at fraction 4 with mean percent and absolute changes of 21 % and 7.8 cc, respectively. Standardized TZ measures were strongly associated with WP volume (β per SD 10.60-12.78; all p < 0.001), whereas the only nonTZ dimension weakly associated was anteroposterior (β per SD 1.78). Each standard deviation increase in baseline TZ parameters doubled to tripled the odds of significant swelling (≥10 cc) (all p ≤ 0.011). The interaction of baseline TZI with later fractions was significantly associated with swelling (fraction 4: β = 12.06, p = 0.020; fraction 5: β = 10.96, p = 0.036), but not baseline TZI alone. Neither Grade 2+ genitourinary toxicity nor IPSS changes were associated with prostate measures or TZI. Conclusions: Prostate volume significantly increases during SBRT, primarily corresponding with TZ volumetric changes. Baseline TZ measurements most strongly predict high-volume swelling. Acute toxicity was not associated with volumetric change.