3333 - Comparing PSMA PET/CT and MRI in Prostate Radiotherapy Planning: Which Modality Better Predicts Seminal Vesicle Involvement?

Purpose/Objective(s): In prostate radiotherapy, accurately identifying seminal vesicle (SV) involvement or T3b disease is critical for effective treatment planning. SV invasion indicates more advanced prostate cancer and may necessitate a change in radiotherapy target volume. Missing SV invasion can lead to under-treatment of the cancer and an increased likelihood of future cancer recurrence. Multiparametric magnetic resonance imaging (mpMRI) is recommended as the standard imaging modality pre-biopsy in patients with a high PSA suspicious of prostate cancer and is regarded as the most accurate for T-staging. Recently, positron emission tomography-computed tomography with prostate-specific membrane antigen (PSMA PET/CT) has shown promising results as a tool for staging high-risk prostate cancer. The question is whether one modality is better than the other at predicting SV involvement. The aim of this study, was to compare the accuracy and reliability of PSMA PET/CT and mpMRI in detecting seminal vesicle involvement to optimize prostate radiotherapy planning.

Materials/Methods: A multicenter, international, retrospective study was conducted at Guy’s and St Thomas’ Hospital, London, and San Raffaele institute, Milan. Men who underwent radical prostatectomy for biopsy proven prostate cancer between 2018 and 2023 were identified. Collected data included histopathological findings such as T3b disease, Gleason score, prostate specific antigen (PSA) levels, margin positivity, and pre-operative mpMRI and ⁶⁸Ga-PSMA-11 PET/CT imaging. Whole-mount pathology served as the reference standard. Data analysis was performed in London using statistical software. Statistical analysis involved the Chi-square test and logistic regression, with a p-value of <0.05 considered statistically significant.

Results: Among 268 men (median age 65.36 ±1.427 years), 110 underwent both PSMA PET/CT and mpMRI. Histologically verified SV involvement was found equally in 17.3% of patients who underwent PSMA PET/CT or mpMRI. Sensitivity was 16.7% (95% CI: 10.87%–23.95%) for PSMA PET/CT and 13.7% (95% CI: 9.32%–19.22%) for mpMRI. Specificity was 100% for both modalities. Positive predictive value (PPV) was 100% for both modalities and negative predictive value (NPV) was 37.8% (PSMA PET/CT) vs. 28.5% (mpMRI). Area under the ROC curve (AUC) was 0.586 (PSMA PET/CT) vs. 0.606 (mpMRI). No statistically significant difference in diagnostic accuracy was found between the 2 modalities. Multivariate analysis showed no significant correlation between age, PSA, and T3b disease (p=0.420).

Conclusion: No significant difference was observed between mpMRI and PSMA PET/CT in detecting T3b disease. The key limitation of this study was a small sample size. Further studies are required to refine imaging strategies for prostate radiotherapy planning.