ASCO 2017: FDG PET scan (PET) positive residual lesions after chemotherapy (chemo) for metastatic seminoma: Results of an International Global Germ Cell Cancer Group (G3) registry

Chicago, IL ( Residual disease is a frequent finding after chemotherapy for metastatic testicular seminoma, seen in 55-80% of patients. Watchful waiting is recommended for residual lesions < 3 cm or lesions ≥3 cm with negative FDG-PET scan. In seminoma postchemotherapy residual masses of up to 3 cm represent necrosis in 97-100% and FDG-PET is not recommended. FDG-PET has a high negative predictive value to discriminate viable tumor from necrosis. Positive predictive value for FDG-PET has been shown to be low: 25% if performed before 6 weeks after the end of chemotherapy and 69% after this cut off. There is lack of data regarding the optimal management strategy for FDG-PET positive residual lesions.

Dr. Cathomas presented a retrospective study within the G3 Group identifying 90 patients from 18 centers in 9 countries with metastatic seminoma and residual PET positive lesions after chemotherapy. Patients with non-seminomatous components at diagnosis were excluded. The authors analyzed the post PET management chosen and its impact on relapse and survival.

Overall the median follow-up was 28.1 (IQR 9.6-62) months. Median age at diagnosis was 41 (range 19 – 69) years. The primary tumor was gonadal in 67 pts (75%), retroperitoneal in 10 pts (11%) and mediastinal in 12 pts (13%). 64 patients (71%) had elevated LDH and 54 patients (60%) had elevated HCG. The median diameter of the largest residual mass was 4.9 (range 1.1 – 14) cm mainly located in the retroperitoneum (77%), pelvis (16%), mediastinum (17%) or lung (3%). Median time from last day of chemotherapy to PET scan was 6.9 (IQR 4.4 – 9.9) weeks.

Post PET management was repeated imaging in 51 pts, resection in 26 pts, biopsy in 9 pts, and radiotherapy in 4 pts. The histology of the resected specimen was necrosis only in 25 patients (78%) and vital seminoma in 7 cases (22%). No biopsy revealed vital seminoma. Relapses occurred after a median of 3.7 (IQR 1.7 – 9.3) months in 15 pts. Site of relapse was the area of residual disease in 14 pts (93%) and additional distant relapse in 3 patients (20%). All relapsed pts received successful salvage chemotherapy apart from one who died from treatment.

In summary, 17% of seminoma patients with a positive post chemotherapy FDG-PET relapsed and only 7/32 of the resected specimens (22%) seminoma was found. Relapses occur rapidly and can be successfully salvaged. Biopsies were not diagnostic in any of the reported cases. FDG-PET has a very low positive predictive value even if performed 6 weeks after the end of chemotherapy (19%). The authors recommend repeating an FDG-PET scan 8-12 weeks after a positive first FDG-PET scan. Resections of PET positive residuals may be performed after a 2nd positive scan, but represent an overtreatment for 70-80% of patients.

Presented By: Richard Cathomas, SAKK - Swiss Group for Clinical Cancer Research, Coordinating Center, Bern, Switzerland

Written By: Hanan Goldberg, MD, Urologic Oncology Fellow (SUO), University of Toronto, Princess Margaret Cancer Centre
Twitter: @GoldbergHanan

at the 2017 ASCO Annual Meeting - June 2 - 6, 2017 – Chicago, Illinois, USA