Microscopic description:

Ill-defined nests of primitive round to oval tumor cells growing in a discohesive fashion

Usually has an alveolar pattern of growth surrounded by dense fibrous septa

Solid variants are composed entirely of sheets of densely packed cells

Gross and microscopic description:

Positive for desmin, vimentin and muscle specific actin

MyoD1 and myogenin are highly specific markers in alveolar rhabdomyosarcoma

May be confused for neuroendocrine tumors as there is variable expression of synaptophysin, chromogranin, and CD56

Discussion:

Primitive sarcoma which exhibits limited skeletal muscle differentiation

2nd most common rhabdomyosarcoma, and accounts of 31% of all rhabdomyosarcomas

Occurs in all ages, with a predilection for adolescents and young adults

Most commonly arise in the extremities, head and neck

60% of cases have a characteristic t(2;13)(q35;q14) translocation, resulting in FOXO1A-PAX3 fusion gene

Resultant chimeric protein produces a novel transcription factor which promotes cellular proliferation, support of cellular survival, and inhibition of differentiation

References:

Bahrami A, et al. Aberrant expression of epithelial and neuroendocrine markers in alveolar rhabdomyosarcoma: a potentially serious diagnostic pitfall. Mod Pathol. 2008;21(7):795-806

Linardic, CM. PAX3-FOXO1 Fusion Gene in Rhabdomyosarcoma. Cancer Lett. 2008;270(1):10-18

Goldblum JR, Folpe AL, Weiss SW. (2014) Enzinger & Weiss’s Soft Tissue Tumors. (6th Ed.) Philadelphia, PA: Elsevier Saunders.

CASE IMAGES