Immunohistochemical studies performed on the pancreas FNA specimen were negative for LCA expression but were diffusely positive for myogenin and myoD1 (Fig. 6-8). The diagnosis in this case was first made on the forearm biopsy which showed similar morphologic and immunohistochemical features to that seen in the pancreatic FNA, with abundant small round blue cells containing PAS-positive cytoplasmic vacuoles (Fig. 9-10). The patient in this case was diagnosed with stage IV alveolar rhabdomyosarcoma with pancreatic and bone marrow metastasis. Molecular studies performed on paraffin-embedded tissue confirmed the presence of a PAX3-FOXO1A fusion transcript within the tumor cell population. The patient was treated non-surgically with combination chemotherapy. Unfortunately, the patient developed intracranial recurrence in the form of carcinomatous meningitis and was converted to comfort measures only.
Alveolar rhabdomyosarcoma is a rare tumor which typically presents as a rapidly growing extremity or axial mass. Adolescents and young adults are most often affected, though the tumor can occur across all age groups. The most common sites of alveolar rhabdomyosarcoma metastasis includes the lungs, liver and bones.  Pancreatic metastasis of alveolar rhabdomyosarcoma is extremely rare, with only a few cases reported. Alveolar rhabdomyosarcoma is considered to be a high grade tumor that carries a poorer prognosis than most other rhabdomyosarcomas.
Histologically, alveolar rhabdomyosarcoma is composed of a monotonous population of primitive cells. In typical lesions, the neoplastic cells are separated into nests by fibrovascular septations. The periphery of the nests is classically discohesive with a “picket fence” pattern. A solid variant can also occur, which may only exhibit classic “alveolar” pattern focally. Multi-nucleated wreath-like cells may be present. Myogenin and myoD1 are consistently expressed in alveolar rhabdomyosarcoma due to the rhabdomyoblastic origin of the tumor. In contrast to embryonal rhabdomyosarcoma, the nuclear myogenin and myoD1 expression seen in alveolar rhabdomyosarcoma is usually strong and diffuse. Tumors may also express keratins and neuroendocrine markers, which may constitute a diagnostic pitfall if interpreted in isolation.
Alveolar rhabdomyosarcoma is known to harbor recurrent translocations, with the largest subset harboring t(2;13)(q35;q14) and a smaller subset harboring t(1;13)(p36;q14). These translocations result in both PAX3-FOXO1 and PAX7-FOXO1 fusion products, respectively, which act as transcriptional activators. The presence of either translocation is considered specific for alveolar rhabdomyosarcoma, though fusion-negative cases can occur in up to 30% of cases.