Dosimetric quality and evolution of edema after low-dose-rate brachytherapy for small prostates: implications for the use of newer isotopes.

PURPOSE

To characterize prostate swelling and dosimetry in patients with small prostate volumes (PVs) undergoing brachytherapy.

METHODS AND MATERIALS

We studied 25 patients with PV <25 cc (range, 15.1-24.8) and 65 patients with PV ≥25 cc (range, 25.0-66.2) based on three-dimensional ultrasound contours who underwent brachytherapy monotherapy with intraoperative planning. Postoperative Days 1 and 30 dosimetry was done by CT-MRI fusion.

RESULTS

Small PVs had greater Day 1 swelling than large PVs (32.5% increase in volume vs. 23.7%, p = 0.04), but by Day 30, swelling was minimal and not significantly different (p = 0.44). Small PVs had greater seed and needle densities at implant (p < 0.001). Rectal and urethral doses were nearly identical by Day 30 (small PV rectum receiving 100% of the prescription dose [145 Gy] [V100] = 0.32 cc; large PV rectum V100 = 0.33 cc, p = 0.99; small PV urethra receiving 150% of the prescription dose [145 Gy] [V150] = 0.20, large PV urethra V150 = 0.20, p = 0.91). Swelling at Day 1 created some cool implants (rate dose that covers 90% of the prostate volume [D90 <140 Gy = 12.0% and 9.4% for the small and large PV groups, respectively, p = 0.71), but Day 30 planning target volume coverage was excellent (rate D90 <140 Gy = 0% for both groups).

CONCLUSIONS

Although smaller prostates have greater Day 1 swelling, good Day 30 dosimetry can be achieved, making them excellent candidates for (125)I seeds (half-life [t½] = 60 days). Smaller prostates may be suboptimal for shorter t½ sources such as (131)Cs (t½ = 9.7 days), in which the majority of the dose may be delivered to an edematous gland, unless the planning is adjusted to anticipate the edema.