Positron emission tomography for response assessment in desmoplastic small round cell tumor

Desmoplastic small round cell tumors (DSRCT) typically have a large stromal component and often are extensively disseminated in the peritoneal cavity at diagnosis. These factors contribute to difficulty in quantifying response to chemotherapy using RECIST or WHO criteria. This study compares the overall disease response to chemotherapy by fluorodeoxyglucose-positron emission tomography (FDG-PET) and computed tomography (CT) in patients with DSRCT.

We conducted a retrospective chart review of 7 patients with DSRCT who were imaged by FDG-PET and CT at diagnosis and after 3 cycles of chemotherapy. Response to chemotherapy was graded according to EORTC metabolic response guidelines and RECIST.

All tumors demonstrated some decrease in SUVmax (51%±21%) and longest diameter (23%±8%) with chemotherapy. The best response achieved by FDG-PET was a partial response in 6 patients and by CT was a partial response in 1 patient. Measured response was concordant between the 2 modalities in 2 patients.

In this small series response measurement by FDG-PET did not always correlate with response measurement by CT. A greater decrease in metabolic activity as compared with size was seen in all patients. Further studies are needed to define the role of FDG-PET in assessing early response of DSRCT to chemotherapy.