Utility of FDG-PET for Residual Lesions After Chemotherapy for Metastatic Seminoma
In an analysis of International Global Germ Cell Cancer Group Registry data reported in the Journal of Clinical Oncology by Cathomas et al, fluorodeoxyglucose positron-emission tomography (FDG-PET) was found to have a low positive predictive value for vital tumor in residual lesions after chemotherapy in patients with metastatic seminoma.
The study involved data from 90 retrospectively identified patients with metastatic seminoma with FDG-PET–positive residual lesions after chemotherapy. Patients with elevated alpha-fetoprotein or nonseminomatous histology were not included in the analysis.
FDG-PET–Positive Findings
Median follow-up was 29 months (interquartile range [IQR] = 10–62 months). Residual masses were located in the retroperitoneum (77%), pelvis (16%), mediastinum (17%), or lung (3%), with median diameter of the largest residual mass being 4.9 cm (range = 1.1–14 cm). Median time from the last day of chemotherapy to FDG-PET was 6.9 weeks (IQR = 4.4–9.9 weeks).
Management after positive FDG-PET included repeated imaging in 51 patients (57%), resection in 26 (29%), biopsy in 9 (10%), and radiotherapy in 4 (4%). In the 26 resected specimens, histology was necrosis in 21 (81%) and vital seminoma in 5 (19%). No biopsy showed vital seminoma.
Thus, among 41 patients with further evaluation of positive scans by either resection or biopsy, vital seminoma was found in 7 (17%). In addition to necrosis, as the most common finding in false-positive FDG-PET, other histologies associated with false-positive findings included sarcoidosis, fibrosis, inflammation, and benign tumors.
Relapse or progression occurred in 15 patients (17%) after a median of 3.7 months (IQR = 2.5–4.9 months) and was identified in 11 (22%) of the 51 patients undergoing repeated imaging, 2 (8%) of the 26 undergoing resection, and 2 (22%) of the 9 undergoing biopsy. All patients but one with relapse were successfully treated with salvage therapy.
FDG-PET Positive Predictive Value
The positive predictive value for FDG-PET for vital tumor was 23% in the entire population. Positive predictive values were 29% for patients with definite FDG-PET positivity, 19% when FDG-PET was performed > 6 weeks after the end of chemotherapy, 32% when standardized uptake value was ≥ 4, and 22% when residual lesions were ≥ 3 cm.
The investigators concluded, “FDG-PET has a low [positive predictive value] for vital tumor in residual lesions after chemotherapy in patients with metastatic seminoma. This cautions against clinical decisions based on PET positivity alone.”
The study was supported by Krebsliga Graubünden.
Richard Cathomas, MD, of the Division of Oncology/Hematology, Kantonsspital Graubünden, Chur, Switzerland, is the corresponding author for the Journal of Clinical Oncology article.
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