Volumetric Changes and Acute Toxicity With ⁶⁸Ga Prostate-Specific Membrane Antigen Versus ¹⁸F-Fluciclovine Positron...

We evaluated changes in radiation therapy target volume and acute toxicity using ⁶⁸Ga-prostate specific membrane antigen (PSMA) versus ¹⁸F-fluciclovine positron emission tomography (PET)/computed tomography in postprostatectomy patients with biochemical recurrence. We hypothesized that both fluciclovine and PSMA-guided radiation therapy would (1) significantly change pre-PET radiation therapy volumes and (2) show similar toxicity.

We performed an institutional review board-approved, randomized trial comparing fluciclovine (Arm 1) and PSMA (Arm 2)-guided postprostatectomy radiation therapy in patients with detectable prostate-specific antigen after prostatectomy. Treatment volumes were rigidly defined based on PET, and simultaneous integrated boosts to PET uptake in the prostate bed (70.2-76.0 Gy) or pelvis (54.0-56.0 Gy) were allowed. Clinical target volumes (CTVs) included: prostate bed (CTV_PB); pelvic lymph nodes (CTV_PLV); and volumetric constraints for bladder(-CTV) and rectum. Acute genitourinary and gastrointestinal (GI) toxicity (per Common Terminology Criteria for Adverse Events v5.0) was assessed <90 days from treatment.

In total, 140 patients were enrolled with 70 randomized to each arm; 11 Arm 1 and 10 Arm 2 patients did not receive radiation on study and were excluded. Fluciclovine and PSMA incorporation increased both CTV_PB and CTV_PLV (P < .01). More fluciclovine patients received prostate bed boosts (45 of 59 patients vs 26 of 60 patients; P < .01), but there was no difference in proportion receiving pelvic nodal boosts (10 of 15 patients vs 9of 16 patients, fluciclovine vs PSMA; P = .97). Dose constraints were met for most patients. Rates of grade 2 genitourinary (17.0% vs 6.7%, fluciclovine vs PSMA; P = .15) and GI (5.1% vs 1.7%, fluciclovine vs PSMA; P = .47) toxicity were low, with no grade 3+ events. Higher rectal and bladder dose metrics correlated with GI toxicity (P < .05), but use of simultaneous integrated boosts was not associated with acute toxicity.

Although both PSMA and fluciclovine use modestly increased target volumes, significantly more fluciclovine patients received prostate bed boosts. Planning directives were met for most patients, and acute toxicity was mild in both Arms. Analysis of biochemical control, late toxicity, and patient-reported outcomes are forthcoming.