What urethral dose constraints would you use off-trial for reirradiating the prostate using SBRT?   

Do you exclude the urethra (or urethra + 3 mm) from the PTV?

With the increasing volume of these cases that we are treating outside of clinical trials, it would be useful to learn what urethral constraints are used. The PTV dose ranges from 30 to 36 Gy, commonly 34 Gy. I use a combination derived from retrospective series: Urethra + 3 mm V31.5 Gy<50%; V24 Gy<30%; V36 Gy<1cc. This is close to the ongoing French trial, GETUG-AFU 31 (NCT03438552) salvage stereotactic radiation in patients with intraprostatic tumor recurrence after external radiation therapy-study protocol: Urethra + 3 mm V24 Gy<30%; Dmax (35 mm3)<39 Gy, V36 Gy <1 cc. But quite different to the NIH Protocol 17-C-0153 NCT03253744, Dose Escalated Prostate SBRT for Locally Recurrent Prostate Cancer after Prior Radiotherapy, to doses 40 Gy, 42.5 Gy, and 45 Gy and urethral constraint of <105% PTV tumor dose, i.e., 42 Gy, 44.6 Gy and 47.25 Gy for the three respective dose levels.