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[(18)F]fluorodeoxyglucose positron emission tomography (FDG PET) is used to evaluate the metabolic activity of intermediate- to high-grade soft tissue sarcomas prior to the initiation of chemotherapy. FDG PET and MRI are repeated after chemotherapy to assess treatment response and to correlate this with the histologic response. This case report highlights discrepant results between the final histologic diagnosis (tumor response) and the percent change in maximum standard uptake variable (SUV(max)). Further investigation of the resected specimen identified the significant difference in tumor response assessment to be due to the heterogeneity of the tumor and heterogeneity in its response to treatment. FDG PET imaging has an important role in evaluating tumor metabolic activity and has played an increasingly important part in evaluating the response to neoadjuvant chemotherapy in sarcomas. Evaluating the heterogeneity of the tumor at baseline and after response to therapy can provide insight into prognosis and treatment planning.

C]Thymidine (TdR), a PET tracer for cellular proliferation, may be advantageous for monitoring brain tumor progression and response to therapy. Kinetic analysis of dynamic TdR images was performed to estimate the rate of thymidine transport (K1t) and thymidine flux ( KTdR) into brain tumors and normal brain. These estimates were compared to MRI and pathologic results. Methods: Twenty patients underwent sequential [ 11 C]CO2 (major TdR metabolite) and TdR PET studies with arterial blood sampling and metabolite analysis. The data were fitted using the five-compartment model described in the companion article. Results: Comparison of model estimates with clinical and pathologic data shows that K1t is higher for MRI contrast enhancing tumors ( p < .001), and KTdR increases with tumor grade ( p < .02). On average, TdR retention was lower after treatment in high-grade tumors. The model was able to distinguish between increased thymidine transport due to blood‐ brain barrier breakdown and increased tracer retention associated with tumor cell proliferation. Conclusion: Initial analysis of model estimates of thymidine retention and transport show good agreement with the clinical and pathological features of a wide range of brain tumors. Ongoing studies will evaluate its role in measuring response to treatment and predicting outcome. Mol Imaging (2002) 1, 145‐150.

Purpose Response to neoadjuvant chemotherapy is a significant prognostic factor for the Ewing sarcoma family of tumors (ESFTs). [18F]fluorodeoxyglucose (FDG) positron emission tomography (PET) is a noninvasive imaging modality that accurately predicts histopathologic response in several malignancies. To determine the prognostic value of FDG PET response for progression-free survival (PFS) in ESFTs, we reviewed the University of Washington Medical Center experience. Patients and Methods Thirty-six patients with ESFTs were evaluated by FDG PET. All patients received neoadjuvant and adjuvant chemotherapy. FDG PET standard uptake values before (SUV1) and after (SUV2) chemotherapy were analyzed and correlated with chemotherapy response, as assessed by histopathology in surgically excised tumors. Thirty-four patients had both SUV1 and SUV2. Results The mean SUV1, SUV2, and ratio of SUV2 to SUV1 (SUV2:1) were 7.9 (range, 2.3 to 32.8), 2.1 (range, 0 to 4.3), and 0.36 (range, 0.00 to 1.00), respectively. Good FDG PET response was defined as SUV2 less than 2.5 or SUV2:1 ≤ 0.5. FDG PET response by SUV2 or SUV2:1 was concordant with histologic response in 68% and 69% of patients, respectively. SUV2 was associated with outcome (4-year PFS 72% for SUV2 < 2.5 v 27% for SUV2 ≥ 2.5, P = .01 for all patients; 80% for SUV2 < 2.5 v 33% for SUV2 ≥ 2.5, P = .036 for localized at diagnosis patients). SUV2:1 ≤ 0.5 was not predictive of PFS. Conclusion FDG PET imaging of ESFTs correlates with histologic response to neoadjuvant chemotherapy. SUV2 less than 2.5 is predictive of PFS independent of initial disease stage.

2-[11C]Thymidine (TdR), a PET tracer for cellular proliferation, may be advantageous for monitoring brain tumor progression and response to therapy. Kinetic analysis of dynamic TdR images was performed to estimate the rate of thymidine transport ( K1t) and thymidine flux ( KTdR) into brain tumors and normal brain. These estimates were compared to MRI and pathologic results. Methods: Twenty patients underwent sequential [11C]CO2 (major TdR metabolite) and TdR PET studies with arterial blood sampling and metabolite analysis. The data were fitted using the five-compartment model described in the companion article. Results: Comparison of model estimates with clinical and pathologic data shows that K1t is higher for MRI contrast enhancing tumors ( p > .001), and KTdR increases with tumor grade ( p > .02). On average, TdR retention was lower after treatment in high-grade tumors. The model was able to distinguish between increased thymidine transport due to blood–brain barrier breakdown and increased tracer retention associated with tumor cell proliferation. Conclusion: Initial analysis of model estimates of thymidine retention and transport show good agreement with the clinical and pathological features of a wide range of brain tumors. Ongoing studies will evaluate its role in measuring response to treatment and predicting outcome.

9005 Background: Criteria for tumor response that rely on changes in size may underestimate the true response rate in soft tissue sarcomas (STS). We report an interim analysis of a prospective stud...