Changes in prostate volume during neo-adjuvant hormone therapy and definitive radiotherapy.

Abstract

e628 Background: Neo-adjuvant hormone therapy (NA-HT) produces profound changes in prostate vasculature and volume. There is little data available on how prostate volumes changes during radiotherapy (RT) after NA-HT. This is highly relevant in the context of adaptive RT and focal boosting. Methods: Eleven patients with intermediate- or high-risk prostate cancer, receiving 3 months NA-HT plus 60 Gy RT in 20 fractions, underwent four multiparametric MRI scans. These were performed before and after NA-HT, then during the third week of RT and 8 weeks after RT. The prostate was contoured on each scan by a radiation oncologist with experience in prostate MRI and reviewed by a second radiation oncologist. Statistical analysis was performed using Spearman correlation and the unpaired t-test with Welch’s correction. Results: One patient declined the post-treatment scan. NA-HT induced a dramatic mean volume reduction of 47%, range -27% to -64%. Volume changes during NA-HT inversely correlated with volume change during RT (r -0.755, p=0.01). Patients with ≥50% vs. <50% reduction during NA-HT experienced significantly different responses to RT with +29.4% vs. -9.1% (p=0.006) mean volume changes respectively from pre-RT size by week three. Absolute volume pre-NA-HT (p=0.64) or pre-RT (0.29) was not predictive for subsequent change. There was a small mean volume change post RT of -5%; range -20% to +15%. Conclusions: In this study a reduction in volume during RT is seen in patients with smaller than 50% prostate shrinkage due to NA-HT. Other patients experienced a mean volume increase of nearly one third by week three. This would be equivalent to an increase of around 4mm for an initial diameter of 40mm. In case of small treatment margins, the radiotherapy should be adjusted for these changes. [Table: see text]