• Highly differentiated form of embryonal rhabdomyosarcoma composed of 50%+ elongated spindle cells, and associated with a good prognosis

• First described in 1992; usually boys in paratesticular, head or neck regions (Am J Surg Pathol 1992;16:229) or adults in non-paratesticular regions (Am J Surg Pathol 2005:29:1106, Virchows Arch 2006;449:554)
• 6% of embryonal rhabdomyosarcoma

• 15 year old boy with paratesticular mass (Case of the Week #145)
• Cardiac tumor (Pathol Res Pract 2010;206:325)

• Favorable prognosis in children after excision, more aggressive in adults (Amer J Surg Pathol 2005;29:1106)

• Firm, fibrous tumor with tan-yellow, whorled cut surface resembling leiomyoma

• 50% of more of tumor cells should be spindled for this diagnosis
• Low grade, fascicular or storiform pattern of uniform, relatively differentiated elongated spindle cells
• Scattered spindled or polygonal rhabdomyoblasts with brightly eosinophilic cytoplasm and pleomorphic nuclei are also present.
• Cytologic features similar to smooth muscle tumors with blunted central nuclei and tapered ends, but with pale indistinct cytoplasm
• Cross-striations are rare
• Low mitotic activity

Relatively differentiated spindle cells

Spindle cells resemble leiomyosarcoma

Spindle cells resemble MFH

Figure 1

Figure 2

Figure 3

Bladder tumor

Fig 1: somewhat bland spindle cells are characteristic
Fig 2: may be paucicellular with fibrotic stroma containing undifferentiated round and spindle cells mixed with differentiating rhabdomyoblasts with abundant eosinophilic cytoplasm
Fig 3: desmin+ rhabdomyoblasts are usually present, but often sparse

15 year old boy with paratesticular mass (Case of Week #145):

Myogenin

Desmin

Smooth muscle actin

AE1/AE3

• Numerous spindle cells and large fragments of cytoplasmic processes with cross-striations (Acta Cytol 2005;49:331)

• Desmin, myogenin, muscle specific actin, smooth muscle actin, vimentin (Arch Pathol Lab Med 2006;130:1454)
• Also titin (marker of terminal differentiation)

• Caldesmon, S100, PLAP, AE1/AE3, CD117 (Zhonghua Zhong Liu Za Zhi 2008;30:141)

• Fibrosarcoma: herringbone pattern, may have similar morphology but no rhabdomyoblasts, negative for skeletal muscle markers
• Infantile fibromatosis: deep location, fascicles of spindle cells, no cross striations, no undifferentiated cells
• Leiomyosarcoma: usually high grade, cigar shaped nuclei, mitotic figures and necrosis, no rhabdomyoblasts, often positive for caldesmon, negative for myoglobin
• Neuromuscular hamartoma of soft tissue: usually age < 2 years, affects brachial plexus or sciatic nerve, multinodular growth with connective tissue separating nodules, no rhabdomyoblasts, muscular component is positive for desmin and muscle specific actin, neural component is positive for S100
• Rhabdomyoma: benign tumor of skeletal muscle differentiation, no rhabdomyoblasts, no pleomorphism, no necrosis

This information is intended for physicians and related personnel, who understand that medical information is often imperfect, and must be interpreted in the context of a patient's clinical data using reasonable medical judgment. This website should not be used as a substitute for the advice of a licensed physician.

All information on this website is protected by copyright of PathologyOutlines.com, Inc. Information from third parties may also be protected by copyright. Please contact us at copyrightPathOut@gmail.com with any questions (click here for other contact information).