Skeletal muscle metastases: Role of FDG PET/CT imaging and CECT.

1900 Learning Objectives Demonstrate the utility of PET/CT and CECT in detecting SMM in different malignancies. INTRODUCTION: Positron Emission Tomography (PET) with 18 F-fluorodeoxyglucose (FDG) and Computed Tomography (CT) imaging has become the principal imaging modality for evaluation of metastatic disease. Skeletal Muscle Metastases (SMM) are uncommon despite the fact that skeletal muscle comprises greater than 50% of total body mass.(1) The apparent rarity of SMM has previously been attributed to the complex resistance of the musculature to metastatic invasion by hematogenous spread.(2) Furthermore, most SMM are asymptomatic and often discovered incidentally on imaging.(1) Physiological skeletal muscle activity can also mask metastatic disease. OBJECTIVE: The goal of our study is to summarize PET/CT findings in multiple cases of SMM in different malignancies and demonstrate the utility of Contrast Enhanced CT (CECT) in evaluation of those lesions. METHODS: PET/CT of 25 patients with 10 different confirmed malignancies and suspected SMM were retrospectively evaluated and examined for enhancement on CECT. RESULTS: 25 cases of either biopsy-confirmed or presumed cases of SMM included lung, breast, lymphoma, melanoma, thyroid, gastrointestinal, multiple myeloma, chordoma, endometrial, and leiomyosarcoma. On CT, 10 cases were enhancing while the remainder were either non-enhancing or without available CT scans for comparison. 11 cases of SMM had corresponding biopsies for pathological examination. 1 case was possibly a peripheral nerve sheath tumor. 14 cases were unconfirmed and presumed to be SMM based on FDG PET/CT and CECT imaging. CONCLUSION: SMM are relatively uncommon and most patients are asymptomatic. Only 11 cases of SMM were biopsied in our study, which is likely because biopsying such lesions would not have changed clinical management. Furthermore, SMM were only incidentally detected in the setting of known metastatic disease and other sites such as lymph nodes were preferentially biopsied. Enhancement of these lesions on CECT should be carefully reviewed, as it could be subtle.