318 - Volumetric Changes and Acute Toxicity with 68Ga Prostate-Specific Membrane Antigen vs 18F-Fluciclovine PET/CT Guided Post-Prostatectomy Radiation: Final Analysis of a Randomized Trial

V. R. Dhere¹, D. M. Schuster², S. Goyal³, E. Schreibmann¹, N. Sebastian¹, S. A. Patel⁴, S. Hanasoge¹, J. W. Shelton¹, P. R. Patel¹, B. Hershatter¹, and A. Jani¹; ¹Department of Radiation Oncology, Winship Cancer Institute of Emory University, Atlanta, GA, ²Department of Radiology and Imaging Sciences, Emory University, Atlanta, GA, ³Department of Biostatistics and Bioinformatics Shared Resource, Winship Cancer Institute, Atlanta, GA, ⁴Department of Radiation Oncology, Emory University, Atlanta, GA

Purpose/Objective(s): 68Ga-prostate specific membrane antigen (PSMA) and 18F-Fluciclovine (fluciclovine) PET/CT are approved in post-prostatectomy patients with biochemical recurrence. We evaluated changes in XRT target volume or acute toxicity using PSMA vs fluciclovine in a randomized fashion. We hypothesized that both fluciclovine and PSMA guided XRT would a) significantly change pre-PET XRT volumes and b) show similar toxicity. Materials/

Methods: We performed an IRB-approved, prospective, randomized trial comparing fluciclovine (Arm 1) and PSMA (Arm 2)-guided post-prostatectomy XRT in patients with detectable PSA after prostatectomy. We assessed XRT volumes before (pre-) and after (post-) PET incorporation using the Wilcoxon signed-rank tests. Treatment volumes were rigidly defined based on PET uptake and simultaneous integrated boost to areas of uptake in the prostate bed (70.2-76.0Gy) or pelvis (54.0-56.0Gy) was allowed. Volumes assessed included: prostate bed (CTVpb); pelvic lymph nodes (CTVpln); and volume at 65Gy (V65Gy) for bladder-CTV & rectum. Acute CTCAE v5.0 toxicity was assessed <30 days from treatment completion by Fisher’s Exact tests.
Results: 140 patients were enrolled with 70 randomized to each Arm; 10 Arm 1 and 9 Arm 2 patients did not receive radiation on study (due to extrapelvic uptake or consent withdrawal) and were excluded. There was no significant difference in initial target volumes between Arms (p>0.05 for all CTV measures). Fluciclovine significantly increased mean CTVpb (115.12cc vs 115.74cc, pre- vs post-; p<0.01) and CTVpln (469.73cc vs 472.73cc, pre- vs post-; p<0.01) volumes. PSMA also significantly increased mean CTVpb (121.20 cc vs 121.35cc, pre- vs post-; p<0.01) and CTVpln (465.73cc vs 466.71cc, pre- vs post-; p<0.01) volumes. All patients with PET uptake in the prostate bed and/or pelvis received simultaneous integrated boosts. More fluciclovine pts received prostate bed boosts (45/60 pts vs 26/61 pts, p<0.01) but there was no significant difference in proportion receiving pelvic nodal boosts (10/16 pts vs 5/14 pts, fluciclovine vs PSMA; p=0.14). All patients met rectal V65Gy<35%; 5 patients in each arm exceeded bladder-CTV V65Gy<40%. Rates of G2 GU (17.0% vs 6.7%, fluciclovine vs PSMA; p=0.15) and GI (5.1% vs 1.7%, fluciclovine vs PSMA; p=0.47) toxicity were not significantly different between Arms with no G3+ events.
Conclusion: Though both PSMA and fluciclovine integration 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 pending additional follow-up.