What are the histologic characteristics of alveolar rhabdomyosarcoma?

Updated: Feb 01, 2019
  • Author: Amelia F Drake, MD; Chief Editor: Arlen D Meyers, MD, MBA  more...
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The alveolar subtype makes up about 31% of all cases of rhabdomyosarcoma. It is most frequently observed in adolescents and in patients whose primary sites involve the extremities, the trunk, and the perianal and/or perirectal region. On microscopy, these tumors have the appearance of club-shaped tumor cells arranged in clumps and outlined by fibrous septa. In the center, the clusters are arranged loosely, and therefore, they appear in an alveolar pattern (see the image below). These cells stain intensely with eosinophilic stain. Cross-striated malignant rhabdomyoblasts are observed in 25% of cases, which is less frequent than what is observed with the embryonal form.

Alveolar rhabdomyosarcoma is evidenced by uniform Alveolar rhabdomyosarcoma is evidenced by uniform cell population consisting of cells with a high nuclear-to-cytoplasmic ratio. The cells are arranged in variably sized nests separated by fibrous tissue septa. In places, the cells appear loosely dispersed, mimicking a pulmonary alveolar pattern. Image provided by Scott Kilpatrick, MD, Department of Pathology, University of North Carolina Hospitals.

Like embryonal rhabdomyosarcoma, alveolar rhabdomyosarcoma has distinct molecular characteristics. A unique translocation occurs between the FKHR gene on chromosome 13 and either the PAX3 gene on chromosome 2 (70%) or the PAX7 gene on chromosome 1 (30%). Individuals with the PAX7 translocation are younger and may have longer event-free survival than those with the PAX3 translocation. Unlike embryonal rhabdomyosarcoma, alveolar rhabdomyosarcoma commonly demonstrates gene amplification, and its DNA content is typically tetraploidy. [9, 10, 11]

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