Safe space.

Top Line: What is the optimal dose for prostate SBRT?

The Study: Let’s rephrase that. How can we safely deliver the optimal prostate SBRT dose? Between 32.5 Gy and 40 Gy in 5 fractions, the rate of positive post-treatment biopsy drops from nearly 50% to < 10% while the rate of PSA failure at 5 years drops from 15% to <5%. The problem is that rectal toxicity also increases over this dose range, and it can become considerable at doses over 40 Gy. In a study of dose escalation from 45→ 50 Gy, 100% of patients developed rectal ulceration on surveillance anoscopy and 6.6% developed late grade 3+ rectal toxicity. So, this multi-center phase 2 trial sought to determine if placement of a hydrogel rectal spacer could reduce the rate of rectal ulceration on anoscopy from > 90% to < 70%. Patients (n=44) with low or intermediate risk prostate cancer underwent hydrogel spacer and fiducial marker placement followed by MRI for treatment planning. The goal for rectal spacing was ≥7.5 mm, and this was achieved in 95% of patients. ADT was only allowed for cytoreduction. The CTV included the prostate (no SV) and the PTV was a 3mm isotropic margin. The PTV was prescribed 45 Gy in 5 fractions covering 95% (constraints here). Rectal ulceration was evaluated by direct anoscopy at 1.5, 3, 6 and 9 months after radiation. The overall rate of rectal ulcer development was 14.3%. These were observed at a median of 2.9 months after treatment and typically resolved by 6 months. Symptoms were mild with most ulcers associated with grade 1 toxicity, and there were no grade 3 toxicities. Poor apex coverage of the hydrogel spacer was noted in half of patients with ulcers.

TBL: Using a hydrogel spacer, prostate SBRT dose escalation to 45 Gy in 5 fractions was associated with a low rate of transient rectal ulceration on anoscopy and low acute and late GI toxicity. | Folkert, Int J Radiat Oncol Biol Phys 2021