Dosimetic quality and evolution of edema after prostate brachytherapy for small prostates: Implications for the use of newer isotopes.

Abstract

232 Background: Prostate brachytherapy is often avoided in men with small prostate volumes (PV) due to concerns about suboptimal dosimetry. We characterized prostate swelling and dosimetry in patients with small PVs compared with large PVs. Methods: We studied twenty-five patients with PV <25 cc (range 15.1-24.8) and sixty-five patients with PV >=25 cc (range 25.0-66.2) who underwent brachytherapy. Gland size was based on the contoured volume on a 3D transrectal ultrasound before the procedure. Ultrasound-guided brachytherapy was performed under intra-operative planning with loose Iodine-125 seeds to a prescription dose of 145 Gy. Patients underwent CT and MRI scans on post-implant Days 1 and 30 for dosimetric study. MRI was used to contour the prostate and then fused with the CT for dosimetry. Results: Small PVs had greater Day 1 post-implant swelling than patients with large PVs (32.5% increase in volume vs. 23.7%, p=0.04) but Day 30 swelling was minimal and not significantly different (4.4% increase in volume vs. 1.6%, p=0.44) for small and large PVs respectively. Small PVs had greater seed and needle density at implant (p<0.001). Small PV patients had larger Day 1 hot spots in the prostate at (V150 = 59.6% vs. 53.0, p=0 .022) which resolved by Day 30 (V150 = 70.9% vs. 67.1, p=0.20). Rectal and urethral doses were nearly identical by Day 30 (Small PV RV100 = 0.32 cc, Large PV RV100 = 0.33 cc, p=0.99; small PV UV150 = 0.20, large PV UV150 = 0.20, p=0.91). Day 1 swelling created cold areas (rate D90>140 Gy = 88.0% and 90.6% for the small and large PV groups respectively, p=0.71), but day 30 dosimetry was excellent (rate D90>140 Gy = 100% for both groups). Conclusions: While smaller prostates have more immediate post-operative swelling, good Day 30 dosimetry can be achieved in small prostates, which makes them excellent candidates for treatment with I-125 seeds (t½ = 60 days). Due to the increased short-term swelling, small prostates may be suboptimal candidates for implants done with shorter half-life sources such as cesium-131 (t½ = 9.7 days), where the majority of the dose may be delivered to an edematous gland.