3353 - Could PSMA PET/CT post Definitive Radiation Therapy (RT) and Androgen Deprivation Therapy (ADT) Detect Viable Residual Tumor with a Non-Rising PSA?

12:45pm - 02:00pm PT

Hall F

Screen: 25

POSTER

Presenter(s)

Alina Goltser, MD - Sheba Medical Center, Ramat Gan, Dan

I. Tamir, A. Goltser, I. BarOrian, R. Lewin, J. Mattout, M. ben-Ayun, S. Dubinski, J. Kraitman, D. Benjamin, I. Weiss, Y. Lawrence, Z. Symon, and L. Hammer; Radiation Oncology Department, Sheba Medical Center, Ramat Gan, Israel

Purpose/Objective(s):

Follow-up PSMA PET/CT is indicated only for rising PSA or new symptoms after definitive RT. Rates of local failure in prostate cancer following definitive RT are 13% in high-risk patients, and PSA nadir =0.1ng/mL within six months after RT completion was previously shown to be prognostic following RT and ADT. We hypothesized that routine post-treatment PSMA PET/CT, even with a non-rising PSA, could enable early diagnosis of residual viable tumor.

Materials/Methods:

We analyzed a retrospective cohort of patients with localized or node positive PCa treated with definitive RT and ADT between 2014 and 2023 in a single institution. Evaluable patients completed PSMA PET/CT at baseline and during follow up, despite no measurement of rising PSA. Endpoints were disease recurrence on PSMA PET/CT, defined as pathological uptake or the presence of a new site of involvement, as compared to PSMA PET/CT at baseline. The rate of PSA nadir within six months in the study cohort was documented to evaluate the association with disease recurrence.

Results:

The study cohort included 100 patients with localized or node positive PCa, as confirmed by PSMA PET/CT at diagnosis. Patients were treated with definitive RT and ADT, at a median age of 76 years (IQR, 7.75), and a median baseline PSA of 10.89 ng/ml (range, 1.09,333). Divided by risk group, 5% had unfavorable intermediate, 71% had high-risk, and 24% had regional node-positive disease. All patients were treated with ADT for a median duration of 18 months (range, 4-24), with concurrent Abiraterone in 25%. PSMA PET/CT was performed at a median of 10.5 months (range, 6-18) from completion of RT. At the time of PSMA PET/CT, 88% of patients were actively treated with ADT, while PSA remained at nadir. Disease recurrence rate on PSMA PET/CT was 9%: local residual disease in five patients, nodal metastases in two, a bone lesion in one, and combined local and distant recurrence in one patient. When evaluating patients with PSA nadir =0.1 ng/mL within six months post-RT, five out of 17 (29%) patients had early disease recurrence on PSMA PET/CT, compared to 4 out of 83 (5%) with a PSA nadir<0.1 ng/mL.

Conclusion:

Our findings support the hypothesis that routine post-treatment PSMA PET/CT can detect residual viable tumor, even in the absence of a rising PSA. Early recurrence was identified in 9% of patients, aligning with the literature on disease recurrence rate. Within the limitation of a small cohort, Recurrence was predominantly in those with a PSA nadir =0.1 ng/mL within six months post-RT. These results support the utilization of PSMA PET/CT in early detection of disease recurrence in locally advanced PCa with non-rising PSA. Prospective studies are warranted to further validate these findings.