(UroToday.com) The 2023 Society of Nuclear Medicine and Molecular Imaging (SNMMI) Annual Meeting held in Chicago, IL between June 24th and 27th, 2023 was host to a session on urologic malignancies. Dr. Saurav Jha presented the results of an analysis comparing 68Ga-PSMA-PET/CT and 18F-FDG-PET/CT in the metastatic work up of renal cell carcinoma (RCC).
18F-FDG-PET/CT has demonstrated improved sensitivity, compared to conventional imaging, for the detection of distant metastases among patients with high-risk RCC, providing both anatomic and metabolic information.1 Prostate-specific membrane antigen (PSMA) is expressed on the endothelial cell surface of neo-vasculature of various solid malignant tumors, including clear cell RCC. There has been emerging evidence that PSMA-PET/CT may provide more accurate staging for patients with metastatic RCC.2 As such, the objective of this study was to compare 18F-FDG-PET/CT and 68Ga-PSMA-PET/CT with respect to semi-quantitative parameters and the ability to detect primary site lesions, as well as metastatic deposits in soft tissue sites and bones.
This study included patients with histopathologically and/or radiologically-confirmed RCC, who subsequently underwent both 18F-FDG and 68Ga-PSMA-PET/CTs. Patients received intravenous injections of 10 mCi of 18F-FDG and 3-5 mCi of 68Ga-PSMA and subsequently underwent the corresponding PET/CT scans at 60- and 45-minutes post-injection, respectively. The acquired whole body PET/CTs were analysed both visually and quantitatively by two experienced nuclear medicine physicians.
This analysis included 27 patients, of whom 22 were male. The mean age was 55 years. All 27 patients underwent both 18F-FDG and 68Ga-PSMA-PET/CTs within at least one week of each other.
Primary site lesions were detected in only 12/27 patients, with all lesions demonstrating concordance on both PET/CT modalities. The mean SUVmax of the primary site lesion was higher with 18F-FDG (5.2 versus 3.4) and also demonstrated a higher mean tumor-to-background ratio (5.4 versus 3.7).
A total of 510 metastatic soft tissue lesions were determined to be positive in 25/27 patients, per conventional imaging with CT or MRI. Of these 510 metastatic lesions, 93% were detected on 18F-FDG PET/CT and only 68% were detected on 68Ga-PSMA PET/CT. Among these 25 patients with detectable metastatic soft tissue lesions, there was evidence of imaging findings concordance (FDG and PSMA PET/CT) in 7/25 patients only, with imaging findings discordance noted in the remaining 18. A greater number of lesions was detected by FDG PET/CT in 15/18 discordant cases, whereas PSMA-PET/CT detected a large number of metastatic deposits in the remaining three discordant cases.
In one patient with noted brain metastases, the lesions were noted to be more easily appreciated on PSMA-PET/CT, owing to the physiologic uptake of FDG in the brain, with resultant lower tumor-to-background ratios (TBRs). PSMA may also assist with detecting mediastinal lymph node metastases, whereby PSMA avidity was useful in differentiating infective from metastatic lesions (histologically confirmed). The SUVmax and TBRs for metastatic soft tissue lesions, by imaging modality, were as follows:
- FDG PET/CT: 5.7 and 3.1, respectively
- PSMA PET/CT: 3.7 and 2.3, respectively
With regards to bone metastases, 53 lesions were detected in nine patients on conventional imaging. FDG PET/CT detected 98% of the lesions, whereas PSMA PET/CT detected 96%. Among these nine patients with identifiable bone metastases, concordance was noted in 7/9 patients. The mean SUVmax and TBRs for metastatic bone lesions, by imaging modality, were as follows:
- FDG PET/CT: 8.1 and 3.7, respectively
- PSMA PET/CT: 6.1 and 3.9, respectively
The authors concluded that moderate to intense uptake of 68Ga-PSMA was observed in primary as well as metastatic RCC lesions. 68Ga-PSMA-PET/CT may have value as an adjunctive imaging modality to 18F-FDG PET/CT for lesion characterization and staging. Furthermore, PSMA-PET/CT offers the potential advantage of being a future theranostic tool for patients with limited therapeutic options and with favorable PSMA parameters (e.g., high SUV levels).
Presented by: Saurav Jha, MD, Co-Program Director, Cardiothoracic Imaging Fellowship · Associate Professor of Radiology at the Hospital of the University of Pennsylvania, Philadelphia, PA
Written by: Rashid K. Sayyid, MD, MSc – Society of Urologic Oncology (SUO) Clinical Fellow at The University of Toronto, @rksayyid on Twitter during the 2023 Society of Nuclear Medicine and Molecular Imaging (SNMMI) Annual Meeting, Chicago, IL, Sat, June 24 – Tues, June 27, 2023.
References:- Wang H, et al. Meta-analysis of the diagnostic performance of [18F]FDG-PET and PET/CT in renal cell carcinoma. Cancer Imaging, 2012;12(3):464-74.
- Udovicich C, et al. Impact of Prostate-specific Membrane Antigen Positron Emission Tomography/Computed Tomography in the Management of Oligometastatic Renal Cell Carcinoma. Eur Urol Open Science, 2022;44:60-8.