# Differences in residual lesion detection after treatment of metastatic prostate cancer based on dual-tracer PET/CT

## TL;DR

This study compares two PET/CT imaging methods for detecting residual prostate cancer after treatment, finding that one method is more effective in identifying remaining tumors.

## Contribution

The study provides new comparative evidence on the effectiveness of 68Ga-PSMA-11 versus 18F-FDG PET/CT for residual lesion detection in treated metastatic prostate cancer.

## Key findings

- 68Ga-PSMA-11 PET/CT showed higher detection rates (100%) for post-treatment primary tumors compared to 18F-FDG (92%).
- 68Ga-PSMA-11 had significantly higher SUVmax and TBR values for primary tumors, bone metastases, and lymph node metastases.
- Combined use of both tracers could enhance detection of residual disease in treated metastatic prostate cancer patients.

## Abstract

18F-fluorodeoxyglucose (18F-FDG) positron-emission tomography/computed tomography (PET/CT) and 68Ga-prostate-specific membrane antigen (68Ga-PSMA) PET/CT are widely used imaging modalities for the diagnosis and management of prostate cancer. However, comparative data on their performance in evaluating residual disease in treated metastatic prostate cancer remain limited. This study aimed to compare the efficacy of 68Ga-PSMA-11 and 18F-FDG PET/CT in detecting residual lesions after therapy in patients with metastatic prostate cancer.

We retrospectively analyzed 26 metastatic prostate cancer patients who underwent both 68Ga-PSMA-11 PET/CT and 18F-FDG PET/CT at Nanyang Central Hospital between January 2023 and June 2025. A composite reference standard incorporating histopathology (when available) and follow-up contrast-enhanced CT or MRI was used to confirm metastatic lesions. Lesion-based detection rates and tumor-to-background ratios (TBR) were compared between the two tracers.

In post-treatment primary tumors, detection rates were 92% for 18F-FDG and 100% for 68Ga-PSMA-11. 68Ga-PSMA-11 demonstrated significantly higher SUVmax (median 17.08 vs 5.05, IQR 10.76-24.67 vs 3.90-6.38; P < 0.001, r = 0.81) and TBR (median 26.98 vs 4.81, IQR 17.00-38.97 vs 3.72-6.08; P < 0.001, r = 0.85) than 18F-FDG. For bone metastases, detection rates were 95% and 100%, respectively, with 68Ga-PSMA-11 again exhibiting superior SUVmax and TBR (8.29 ± 5.02 vs 33.77 ± 23.51, P < 0.001, d = 0.95; 6.56 ± 3.97 vs 33.08 ± 23.03, P < 0.001, d = 1.03). The per-lesion detection rate for lymph node metastases was 91% for 18F-FDG PET/CT and 96% for 68Ga-PSMA-11 PET/CT. SUVmax and TBR were significantly higher with 68Ga-PSMA-11 (17.02 (9.40-27.07) vs 4.17 (3.55-5.17), P = 0.005, r = 0.80; 26.88 (14.85-42.77) vs 3.96 (3.38-4.93), P = 0.002, r = 0.85).

68Ga-PSMA-11 PET/CT shows higher tracer avidity and tumor-to-background contrast than 18F-FDG PET/CT for detecting residual primary tumors, lymph node, and bone metastases after therapy in metastatic prostate cancer. 18F-FDG PET/CT may serve as a complementary modality, and combined use of both tracers could enhance the detection of residual disease in treated patients.