Localization
head and neck (8319954)
cutaneous fetal rhabdomyoma (18300799)
Synopsis
sex ratio: 1/1
Age: 3 days to 58 years of age (median, 4.5 years)
median tumor size : 3.0 cm (range, 1.0 to 12.5 cm)
well-defined, solitary masses arising within soft tissue
"classic" FRM
- bland, primitive spindled cells associated with delicate, elongated skeletal muscle cells reminiscent of fetal myotubules that were haphazardly arranged in an abundant fibromyxoid stroma.
"intermediate" FRM
- greater degree and greater number of cells with skeletal muscle differentiation
- large, ganglion cell-like rhabdomyoblasts with vesicular nuclei and prominent nucleoli, interlacing ribbon or strap-like rhabdomyoblasts with deeply acidophilic cytoplasm
- broad bundles of more delicate spindled rhabdomyoblasts arranged in fascicles simulating smooth muscle
- occasional plexiform pattern with infiltration of adipose tissue and skeletal muscle
- focal intimate association with peripheral nerves
- rare areas of fibroblastic proliferation
Mitoses were not found or 1 to 14 mitoses/50 high-power fields.
Marked nuclear atypia, anaplasia, and a "cambium layer" uniformly absent.
Immunochemistry
The FRMs typically stained for myoglobin, desmin, and muscle-specific actin with focal or rare staining for vimentin, smooth muscle actin, S-100 protein, glial fibrillary acidic protein, and Leu-7. Cytokeratin, epithelial membrane antigen, and CD68 antigen (with KP1) were not detected.
Differential diagnosis
rhabdomyosarcomas
- The mitotic rates of FRM as well as certain histologic features overlap with rhabdomyosarcoma; the lack of marked nuclear atypia is an important distinguishing feature.
References
Kapadia SB, Meis JM, Frisman DM, Ellis GL, Heffner DK. Fetal rhabdomyoma of the head and neck: a clinicopathologic and immunophenotypic study of 24 cases. Hum Pathol. 1993 Jul;24(7):754-65. PMID: 8319954