Desmin

• Atrophic skeletal muscle fibers staining with desmin.
• Atrophic skeletal muscle fibers staining with desmin. Note the cross-striations.
• Desmin highlighting atrophic skeletal muscle fibers being infiltrated by a desmin-negative sarcoma.
• Desmin highlighting atrophic skeletal muscle fibers which are infiltrated by desmin-negative sarcoma.
• Desmin expression in angiomyolipoma (PEComa).
• Desmin expression in a rare example of angiomyolipoma with epithelial cyst. Some of the ovarian-like stroma cells are also positive.
• Desmin expression in a rare example of angiomyolipoma with epithelial cyst. Some of the ovarian-like stroma cells are also positive.
• Desmin expression in angiomyolipoma (PEComa).
• Desmin expression in alveolar rhabdomyosarcoma.
• Desmin expression in alveolar rhabdomyosarcoma.
• Desmin expression in alveolar rhabdomyosarcoma.
• Desmin expression in alveolar rhabdomyosarcoma.
• Desmin expression in desmoplastic small round cell tumor(DSRCT).
• Desmin expression in desmoplastic small round cell tumor(DSRCT).
• Desmin expression in desmoplastic small round cell tumor(DSRCT).
• Desmin expression in mammary myofibroblastoma.
• Desmin expression in mammary myofibroblastoma.
• Desmin expression in reactive mesothelium.
• Rare cells stain for desmin in tenosynovial giant cell tumor, diffuse type (a.k.a. - pigmented villonodular synovitis).

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Desmin is an intermediate filament that is expressed in both smooth and skeletal muscle myocytes. It is also expressed in neoplasms with smooth or skeletal muscle differentiation.

Staining Pattern

• Cytoplasmic (because it is an intermediate filament, desmin often has a fibrillary staining quality).

Expression in Normal Tissues

• Smooth Muscle
• Skeletal Muscle
• Myofibroblasts (usually only express actin and vimentin, but may occasionally express desmin)
• Endometrial stroma

Expression in Neoplastic Entities

Benign

Tumor name	% Positive Staining (Sensitivity)	Number of Cases	Specificity	Comments
Tenosynovial giant cell tumor, localized type (aka giant cell tumor of tendon sheath)				scattered cells stain
Tenosynovial giant cell tumor, diffuse type (aka pigmented villonodular synovitis)				scattered cells stain
Leiomyoma
Rhabdomyoma

Intermediate/Uncertain Malignant Potential

Tumor name	% Positive Staining (Sensitivity)	Number of Cases	Specificity	Comments
PEComa

Malignant

Tumor name	% Positive Staining (Sensitivity)	Number of Cases	Specificity	Comments
Rhabdomyosarcoma
Leiomyosarcoma
Angiomatoid Fibrous Histiocytoma

Additional Comments:

• Rhabdomyosarcoma
  • Desmin is positive in rhabdomyosarcoma, but a positive desmin is NOT enough to confirm skeletal muscle differentiation. Myogenin and/or MyoD1 (specific markers of skeletal muscle differentiation) should also be performed to confirm the diagnosis of rhabdomyosarcoma.
  • Desmin is often focal and dot-like in sclerosing rhabdomyosarcoma
• Leiomyosarcoma
  • Some leiomyosarcomas do not express desmin or express it only focally/weakly

Expression in Non-Neoplastic Entities

• Some reactive myofibroblastic proliferations may express focal desmin.
• Reactive mesothelium

Practical Uses / Panels

• Desmin may be used as part of a very useful panel for initial work up of a high grade spindle cell neoplasm: desmin, Smooth Muscle Actin (SMA) , Pancytokeratin (AE1/AE3), and S100 Protein. This panel serves to rule out rhabdomyosarcoma, leiomyosarcoma, sarcomatoid carcinoma (or other cytokeratin positive sarcoma), and spindle cell melanoma respectively. If all stains in the panel are negative (and if the morphology fits), the high grade sarcoma is probably best classified as a pleomorphic undifferentiated sarcoma (previously referred to as malignant fibrous histiocytoma [MFH]). It is important to note that pleomorphic undifferentiated sarcomas may show focal or weak expression of any of these four antigens, and thus care should be taken before classifying a high grade spindle cell tumor as a leiomyosarcoma or a metastatic sarcomatoid carcinoma on the basis of very focal staining.

Common Pitfalls

• As mentioned above, tenosynovial giant cell tumor expresses desmin, and may sometimes be over-called as rhabdomyosarcoma.
• The sclerosing variant of rhabdomyosarcoma only show weak or focal desmin expression, often with a perinuclear "dot" pattern. Myogenin may also show only very focal expression in these tumors, although MyoD1 is usually diffusely positive.
• Only half of epithelioid smooth muscle tumors show desmin expression.