Skeletal Muscle Metastases on FDG PET/CT Imaging

Background: Skeletal muscle metastases (SMM) are a rare occurrence despite the fact that skeletal muscle comprises more than 50% of total body mass. When present, most are asymptomatic and are often found incidentally on imaging. Several studies have reported isolated cases of patients with SMM. The goal of our study was to perform a comprehensive retrospective analysis of the incidence, frequency, and source of SMM in patients with malignancies who underwent whole-body FDG PET/CT imaging for the workup for their malignancy. Methods: The records of 8492 consecutive patients referred for oncologic imaging at our institution during the period from January 2006 to December 2009 were reviewed. Patients who had SMM on FDG PET imaging were further assessed for collaborating evidence on other imaging modalities, pathology from tissue sampling, and follow-up scans. The incidence of SMM in our setting was calculated, and their association with various primary malignancies was also evaluated. Results: A total of 73 patients with SMM were identified on PET/CT imaging (incidence, 0.86%). Lung cancer was the most frequent source (25 patients [34%]), followed by gastrointestinal tract malignancies (13 patients [18%]), breast cancer (10 patients [14%]), genitourinary malignancies and lymphoma (6 patients [8%] each), melanoma (5 patients [7%]), and other miscellaneous malignancies (3 patients [4%]). Primary source remained unknown in only 5 patients (7%). Most patients with SMM generally had markedly widespread involvement which led to an unusual appearance on FDG PET/CT imaging. However, since most of these patients had stage IV malignant disease by virtue of visceral metastases elsewhere as well, they were generally not upstaged. Conclusions: Our study confirmed the rare occurrence of SMM. Furthermore, FDG PET/CT can potentially detect unsuspected SMM. Interestingly, in our patient population, lung cancer was found to be the most frequent malignancy metastasizing to the skeletal muscles. Knowledge about the presence and appearance of widespread SMM may aid in appropriate interpretation of FDG PET/CT scans.