F-18 FDG PET and PET/CT Evaluation of Response to Chemotherapy in Bone and Soft Tissue Sarcomas

Abstract

F-18 FDG PET has been used to grade sarcomas and assess response to therapy in advanced disease. Certain chemotherapy agents are thought to induce an inflammatory response in the tumor bed that can make interpretation of post-therapy FDG PET scans difficult. A review of our experience with PET in assessing therapy response in osseous and soft tissue sarcomas (OSTS) is presented. This is a retrospective study (January 1999 to December 2004) of 14 patients with histologic diagnosis of OSTS, who had 2 consecutive PET examinations for evaluation of chemotherapy response. The group included 8 men and 6 women, with age range of 18 to 56 years (average, 36 +/- 14). Semiquantitative assessment of FDG uptake was performed by calculating maximum standard uptake value (SUVmax) before and after treatment. The response to therapy was assessed independently by tumor necrosis at post-therapy surgery and according to European Organization for Research and Treatment of Cancer (EORTC) criteria for PET. The follow-up PET examinations were performed at an interval of 28 to 166 days (average, 90 +/- 45). All patients ended the ifosfamide regimen at 7 to 36 (average, 16 +/- 9) days before the follow-up PET scans. Five of them received methotrexate, adriamycin, and/or cisplatin as well. Based on the EORTC criteria alone, 3 patients (21.4%) had progression of disease (increase in SUVmax of 29%-69%; mean, 48% +/- 20%), 5 patients (35.7%) had stable disease, and 6 patients (42.8%) had partial response (decrease in SUVmax of 27%-84%; mean, 62% +/- 23%). Across all patients, the tumor necrosis postchemotherapy ranged from 5% to 100% (mean, 64% +/- 34%). In 8 patients (57.1%) the tumor necrosis correlated with the SUVmax changes. However, for 3 patients, the SUVmax changes indicated partial response despite necrosis of fewer than 90% of the surgical specimens, whereas 3 patients with >90% tumor necrosis had SUVmax changes indicative of stable disease. The pathologically determined degree of necrosis postneoadjuvant chemotherapy was concordant with PET-assessed EORTC classification of response in 57.1% of the cases. However, a significant number of patients had discrepancies, which may be in part explained by chemotherapy-induced inflammation. The latter should be considered during post-therapy PET interpretation in OSTS.