Orbital Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in the pediatric population and infrequently occurs in adults. It is a malignant neoplasm that is composed of cells with histologic features of striated muscle in various stages of embryogenesis [1–4]. RMS can arise primarily in the orbit or as secondary extent from the sinuses and nasal cavity. Among the histological subtypes, embryonal is most common in the head and neck region including the orbit while alveolar is more aggressive [1, 2]. Most alveolar rhabdomyosarcomas have translocations involving the PAX and the FOX01 genes, giving rise to fusion genes that contribute to lack of differentiation and proliferation of the tumor cells [3, 5]. May be both intraconal and extraconal. CT shows homogenous soft tissue mass isodense to the extraocular muscles, no calcifications; invasive lesion commonly causing osseous erosion. MRI – T1 signal isointense to normal muscle, hypointense to orbital fat; T2 signal hyperintense relative to the extraocular muscle. Highly vascularized lesion with moderate to marked contrast enhancement. Often low diffusivity on ADC maps indicative of a hypercellular mass [6].