Assessing Tumor Response and Detecting Recurrence in Metastatic Renal Cell Carcinoma on Targeted Therapy: Importance of Size and Attenuation on Contrast-Enhanced CT

OBJECTIVE: The aim of this study was to improve response assessment in patients with metastatic renal cell carcinoma (RCC) on antiangiogenic targeted therapy by evaluating changes in both tumor size and attenuation and by detecting unique patterns of contrast enhancement on contrast-enhanced CT (CECT). MATERIALS AND METHODS: Tumor long-axis measurements and volumetric mean tumor attenuation of target lesions on CECT images were correlated with time to progression in 53 patients with metastatic clear cell RCC treated with first-line sorafenib or sunitinib. The frequencies of specific patterns of tumor progression were assessed. The data were used to develop new imaging criteria, the size and attenuation CT (SACT) criteria. CECT findings were evaluated using the SACT criteria, Response Evaluation Criteria in Solid Tumors (RECIST), and modified Choi criteria, and the Kaplan-Meier method was used to estimate survival functions. RESULTS: One or more target metastatic lesions had decreased attenuation of >or=40 HU in 59% of patients with progression-free survival of >250 days (n=44) after initiating targeted therapy; 0% of patients with earlier disease progression (n=9) had this finding. A favorable response based on SACT criteria had a sensitivity of 75% and specificity of 100% for identifying patients with progression-free survival of >250 days, versus 16% and 100%, respectively, for RECIST and 93% and 44% for the modified Choi criteria. CONCLUSION: Objectively measuring changes in both tumor size and attenuation on the first CECT study after initiating targeted therapy for metastatic RCC markedly improves response assessment. Distinct patterns of disease recurrence are seen in patients with metastatic RCC on targeted therapy.