Skin nonmelanocytic tumor

• ICD-O: 8897/1 - smooth muscle tumor of uncertain malignant potential
• ICD-11: 2F9C & XH1EN1 - neoplasms of unknown behavior of connective tissue or other soft tissue & smooth muscle tumor of uncertain malignant potential

• Smooth muscle origin, arising from the erector pili muscles or dermal vasculature (Appl Immunohistochem Mol Morphol 2013;21:132)
• Less aggressive biology compared to subcutaneous and deep leiomyosarcomas (Am J Surg Pathol 2011;35:599)

• Low risk of aggressive behavior
• Local recurrences have been reported
  • Surgical margin is an important predictor of recurrence
  • Subcutaneous extension (between 1 - 4 mm) does not correlate with recurrence (Appl Immunohistochem Mol Morphol 2013;21:132)
• Metastasis is extremely unlikely; other diagnoses should be pursued if a metastatic lesion is detected (Appl Immunohistochem Mol Morphol 2013;21:132)

• Fascicles of relatively uniform, eosinophilic spindled cells with cigar shaped nuclei
• Low to moderate pleomorphism, increased mitoses (ranging from 1 - 28 and 13 - 20 per 10 high power fields)
• Necrosis and perineural invasion are uncommon (Appl Immunohistochem Mol Morphol 2013;21:132)
• At most focal involvement of the subcutaneous tissue

Contributed by David M. Gustafson, M.D. and Catherine G. Chung, M.D.

The image above is seen on a shave biopsy of a 1 cm nodule on the arm of a 50 year old man. A residual histologically identical lesion was seen on the excision specimen with focal involvement of the subcutis. The H&E findings are seen throughout the entirety of the lesion. Immunohistochemical stains for SMA and desmin are strongly positive, while all other markers are negative, including CD10. What is the best diagnosis?

1. Atypical fibroxanthoma
2. Atypical intradermal smooth muscle neoplasm
3. Deep leiomyosarcoma
4. Leiomyoma

B. Atypical intradermal smooth muscle neoplasm. The biopsy shows a spindle cell proliferation with atypia. The subsequent excision had the same morphology with evidence of smooth muscle differentiation by IHC stains. Focal involvement of the subcutis does not rule out atypical intradermal smooth muscle neoplasm. Answer A is incorrect because atypical fibroxanthoma typically has a greater degree of pleomorphism with abundant mitoses. There is little solar elastosis in the background and CD10 (nonspecific) was negative. Answer D is incorrect because leiomyoma should not have this degree of cytologic atypia throughout the entirety of the lesion. Answer C is incorrect because the lesion is nearly entirely confined to the dermis.

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Reference: Atypical intradermal smooth muscle neoplasm

A 65 year old man presents with shortness of breath and multiple lung nodules on imaging. Core needle biopsies consist of pleomorphic spindle cell neoplasm composed of intersecting fascicles of eosinophilic cells with abundant mitoses and tumoral necrosis. Immunohistochemical stains are only positive for SMA and desmin. He has a history of excision of a reported atypical intradermal smooth muscle neoplasm with positive deep margins involving the subcutis. What is the most likely diagnosis?

1. Atypical intradermal smooth muscle neoplasm
2. Benign metastasizing leiomyoma
3. Metastatic leiomyosarcoma
4. Metastatic pleomorphic dermal sarcoma

C. Metastatic leiomyosarcoma. The key point is recognizing that atypical smooth muscle neoplasms are extremely unlikely to metastasize. A revision of the pre-existing diagnosis should be considered. Answer A is incorrect because atypical intradermal smooth muscle neoplasms have an extremely low (essentially nonexistent) risk of metastasis. Answer B is incorrect because leiomyoma should not have significant pleomorphism with associated mitoses or necrosis. Answer D is incorrect because although this is an important differential to rule out given the history of the previous excision, pleomorphic dermal sarcoma usually has nonspecific staining with CD10 or CD68.

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Reference: Atypical intradermal smooth muscle neoplasm