fetal rhabdomyoma

Definition: Rare benign mesenchymal tumor that show immature skeletal muscle differentiation and predilection for the head and neck in children.

Localization

More than 90% of fetal rhabdomyomas occurs in the soft tissue or mucosal sites of the head and neck (most common postauricular site).

 head and neck (8319954)

• parapharyngeal rhabdomyoma (1530111)
• multifocal rhabdomyoma of the neck (2764225)
   cutaneous fetal rhabdomyoma (18300799)

Clinical features

Most patients @<@ 1 year old (medain age 4 y/o). Male to female ratio M:F 2.4:1. The median size is 3 cm.

Mostly present as well defined solitary superficial mass involving soft tissue or mucosa of head and neck.

Macroscopy

Solitary, circumscribed, soft, grey-white to tan-pink mass with glistening cut surface.

Microscopy

Two closely related types can be distinguished by microscopy.

The myxoid classic type is composed of primitive oval or spindle-shaped cells with indistinct cytoplasm, interspersed immature skeletal muscle fibers reminiscent to fetal myotubes seen during the seventh to tenth weeks of intrauterine life, in rich myxoid stroma.

The immature skeletal muscle cells have small uniform nuclei with delicate chromatin and inconspicuous nucleoli with bipolar or sometimes unipolar, eosinophilic cytoplasm.

Cross striations are rare and difficult to identified. The intermediate cellular type is characterized by the presence of numerous differentiated muscle fibers, less conspicuous or absent spindle-shaped mesenchymal cells, and little or no myxoid stroma.

The predominant cells are strap-shaped muscle cells with abundand eosinophilic cytoplasm, centrally located vesicular nuclei, and frequent cross-striations reminiscent of the cells seen in adult rhabdomyomas; many of the cells contain glycogen and are often vacuolated.

Others have prominent ganglion-like rhabdomyoblasts with large vesicular nucleai and prominent nucleoli. In some cases there is mild cellular pleomorphism, but marked cellular atypia is not seen. Transitional forms of myxoid and intermediate types are not rare and in fact age and duration may play a role.

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.

 skeletal muscle cells positive for myoglobin, desmin and smooth muscle antigen.
Genetics

Multiple cases of fetal rhabdomyoma have been reported in patients with nevoid basal cell carcinoma syndrome.

Prognosis

 complete excision is curative.

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