3244 - Changes in Prostate Volume during Prostate SBRT Delivered on an MR-Linac and Correlation with Acute Toxicity

Purpose/Objective(s):

Limited data are available among patients undergoing prostate SBRT as to the change in prostate volume from treatment-induced edema assessed on serial MRI measurements after each treatment. In this study we evaluated the extent of the prostate volume change observed after each fraction, variables associated with prostatic swelling and correlation with acute toxicity.

Materials/Methods:

This study included 52 patients with localized prostate cancer who underwent SBRT treated every other day on an MRI-Linear Accelerator to 40 Gy in 5 fractions with dominant intraprostatic lesions (DIL) boosted to 45 Gy (if present). To assess changes in prostate volume related to SBRT, without confounding variables, patients treated with androgen deprivation therapy were excluded. Whole prostate (WP) and transitional zone (TZ) measurements in cranio-caudad (CC), anterior-posterior (AP), and transverse (TR) were measured by a single radiologist on a T2 MRI obtained immediately after each treatment fraction. Volumes were calculated using an ellipsoid formula. Transitional zone index (TZI) was defined as TZ volume/WP volume. Acute CTCAE genitourinary (GU) and CTCAE gastrointestinal (GI) toxicity were recorded during treatment, at 3 and 6 months follow ups. IPSS was recorded at baseline, 3 and 6 months follow ups. Linear mixed-effects models were used to analyze changes in volume with time-variant and -invariant variables.

Results:

Whole prostate volume significantly increased over the first four fractions, peaking at fraction 4 before slightly decreasing at fraction 5. The average percent change in volume from fraction 1 to fraction 4 was 21% (range: -2% – +64%). Initial WP volume exhibited a significant effect on volume changes (ß = 0.08, p < 0.001). Treatment of DIL did not correlate with changes in WP volume (p=0.658). The change in TZ measurements were key determinants of WP volume, with CC_TZ contributing the most to overall WP volume changes (CC_TZ ß=11.14, AP_TZ ß=8.62, TR_TZ ß=8.90, p < 0.001). The change in TZ volume represented the vast majority of increase in total prostate volume (ß = 0.95, p < 0.001). TZI at fraction 1 did not predict change in WP volume, but at fractions 4 and 5 it became significantly associated with WP volume increases (Fraction 4, ß = 12.06, p=0.020 and Fraction 5, ß = 10.96, p=0.036). Neither development of Grade 2+ toxicity (p=0.310) nor IPSS score changes were associated with changes in WP volume or TZI.

Conclusion:

Prostate volume significantly increases during a hypo-fractionated SBRT regimen and corresponds with TZ volumetric changes. Acute toxicity was not associated with change in prostate volume when using real-time adaptive planning. These findings could impact upon treatment planning which may need to account for inter-fractional prostatic swelling, especially when tight planning target volume margins are used.