Fossa fuel.

Top Line: We like talking about prostate stereotactic body radiation (SBRT) so much that we’ll talk about it when there is no prostate.

The Study: We’ve previously seen a phase 1 dose escalation trial of prostate fossa SBRT that showed encouraging acute toxicity results. But we’re learning there’s much more to post-prostatectomy SBRT than acute toxicity. Here we have another phase 1 dose escalation study from City of Hope. Treatment planning and delivery was based on CT with gold fiducials, full bladder, and a rectal balloon. The clinical target volume (CTV) was essentially a minimalist version of your standard post-prostatectomy CTV and included the vesico-urethral anastomosis and distal bladder neck. The inferior bladder behind the pubic symphysis was not included, though. The CTV also didn’t dip into the posterior bladder but still extended superiorly up to the seminal vesicle remnants. The 2mm PTV was then prescribed doses escalating from 35 Gy → 45 Gy in 5 fractions. There was no dose-limiting acute CTCAE grade 3+ toxicity (in line with prior studies). Also similar to the prior trial, most acute toxicity was rectal and generally manageable with conservative therapy. While acute GU toxicity seemed favorable, 38% of patients had grade 2+ late GU toxicity and 15% had grade 3+ toxicity. Interestingly, two of these were ureteral (not urethral) stenosis requiring stent placement.

TBL: The growing body of early phase data on prostate fossa SBRT shows favorable acute toxicity, but underlines the importance of long-term follow-up of late GU toxicity and the open question of efficacy. | Sampath, Int J Radiat Oncol Biol Phys 2019