3321 - The Guiding Value of PSMA PET/CT Combined with Contrast-Enhanced Prostate MRI for Salvage Radiotherapy in Prostate Cancer Patients with Low PSA Levels: A Single-Center Exploratory Study

Purpose/Objective(s): This study aimed to investigate the diagnostic value of PSMA PET/CT combined with pelvic contrast-enhanced MRI (CE-MRI) in prostate cancer patients prior to salvage radiotherapy, and to analyze the predictive significance of prostate-specific antigen levels at the time of imaging for PSMA-positive findings, with the goal of providing guidance on the selection and timing of imaging examinations.

Materials/Methods: This retrospective study included 58 prostate cancer patients with biochemical recurrence, all of whom underwent PSMA PET/CT and pelvic contrast-enhanced MRI prior to salvage radiotherapy. Prostate-specific antigen at the time of imaging (iPSA) was measured in all patients. The results of both imaging modalities were reviewed in a multidisciplinary team (MDT) consisting of radiation oncology, radiology, and nuclear medicine specialists to determine the presence of positive lesions and to guide treatment planning.

Results: Among the 58 patients, the median iPSA was 0.23 ng/mL (IQR 0.13–0.45; range 0.032–1.36). PSMA PET/CT combined with CE-MRI detected positive lesions in 25 patients (43%), which led to modifications in the extent of salvage radiotherapy and systemic therapy strategies. A total of 29 lesions were identified, including 5 local recurrence sites, 13 pelvic lymph node metastases, 4 rectal mesentery metastases, and 6 pelvic bone metastases.

Compared to PSMA PET/CT alone, the combined use of CE-MRI changed therapeutic decisions in 21% of patients. PSMA PET/CT demonstrated high sensitivity (96.0%) and negative predictive value (95.0%), but a relatively low positive predictive value (31.4%). Eleven patients (19%) were confirmed as PSMA false-positive with CE-MRI by MDT, of whom 10 (91%) had iPSA <0.3 ng/mL. The false-positive rate was significantly higher when iPSA <0.3 ng/mL (27.8% vs. 4.5%, P=0.0387). Stratification by iPSA (Table 1) showed that the positive detection rate reached 50% when iPSA =0.3 ng/mL and exceeded 80% when iPSA =0.5 ng/mL. PSMA positivity increased significantly with higher iPSA levels (p<0.001). Multivariate logistic regression analysis indicated that iPSA remained an independent predictor of PSMA positivity after adjusting for T stage (p=0.003). ROC analysis showed that iPSA predicted PSMA positivity with an AUC of 0.78, with an optimal cutoff value of 0.3185 ng/mL (specificity 85.3%).

Conclusion: In patients with biochemical recurrence, our results suggest that patients with iPSA above 0.3 ng/mL are more likely to have PSMA-positive lesions, and iPSA could help guide the selection for PSMA-PET imaging. When iPSA <0.3 ng/mL, the PSMA false-positive rate is significantly increased; thus, combining PSMA PET/CT with pelvic CE-MRI is recommended to avoid overtreatment.

Table 1 PSMA positivity rates at different stratified iPSA levels

iPSA (ng/ml)	Total	PSMA-positive	PSMA-positive rate (%)
<0.2	23	4	17.4
0.2-0.3	13	5	38.5
0.3-0.5	10	5	50.0
>0.5	12	10	83.3