Response to CyberKnife stereotactic radiosurgery as detected with 18F FDG-PET/CT: Initial experience

582 Objectives: CyberKnife (CK) is a new and promising stereotactic radiosurgery (SRS) technique. Before and after CK therapy, different imaging modalities are used in patient selection and response evaluation. The aim of the current study was to evaluate the utilization of FDG PET/CT in assessing response to CK SRS, in patients with extracranial malignant lesions. Methods: A total of 105 cancer patients were treated with CK SRS for a total of 123 extracranial lesions at our institution between Aug 04 and Sep 07. In 46 of the 105 patients, FDG PET/CT was used for base line evaluation prior to CK SRS therapy. These 46 patients (a total of 57 lesions) treated with CK were followed clinically and by serial FDG-PET/CT to assess response to CK therapy. Serial true whole-body FDG PET/CT scans for the 46 patients were retrospectively selected and reviewed. FDG PET/CT response to CK was categorized into Complete (CR), Partial (PR), Stable (SD) or Progressive Disease (PD). This response was verified by correlation with surgical pathology or clinical follow-up. Results: Of the 46 studied patients, no follow up data was available for three patients. Of the remaining 43 patients with a total of 54 extracranial treated lesions, PET/CT scans revealed CR in 25 (46.3%), PR in 12 (22.2%) and PD in 15 (27.8%); however, were false positive in two (3.7%) lesions. No lesion was considered to be SD by PET/CT. Conclusions: PET/CT provides useful information in assessing response to CK SRS therapy. Our findings need to be validated in a larger prospective trials.