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• Single red to pink nodule or polypoid tumor, usually less than 2 cm in diameter and may be ulcerated and covered with serum crust

Contributed by Pavandeep Gill, M.D. and Phyu Aung, M.D., Ph.D.

• Vertex scalp, skin shave:
  • Malignant spindle cell neoplasm predominantly in dermis, consistent with atypical fibroxanthoma, present at tissue edges (see comment)
  • Comment: Sections show a dermal, poorly differentiated pleomorphic neoplasm, characterized by large and bizarre spindled to epithelioid cells with abundant eosinophilic cytoplasm, occasional multinucleation and numerous mitotic figures. Adjacent dermis contains solar elastosis. The overlying epidermis does not have a connection to the tumor. Immunohistochemical studies show that lesional cells are positive for CD68 and negative for CAM5.2, AE1 / AE3, CD31 and S100. Overall, these features support the above interpretation.

• Pleomorphic dermal sarcoma:
  • Histologically similar to atypical fibroxanthoma but larger and shows substantial involvement of the subcutis or beyond, perineural invasion, lymphovascular invasion or necrosis
  • Behaves in a more aggressive manner than atypical fibroxanthoma
• Melanoma:
  • Usually associated with an in situ component in the overlying epidermis
  • Melanin pigment, lymphovascular invasion, perineural invasion, involvement of the subcutis and microsatellites may also be present
  • Lesional cells are positive for melanocytic immunohistochemical markers (e.g. SOX10, S100, MelanA, HMB45, tyrosinase and MITF), although reactivity to melanocytic markers other than S100 and SOX10 may be lost in spindle cell / desmoplastic variants
• Squamous cell carcinoma:
  • Usually associated with a connection to the overlying epidermis and an in situ component
  • Keratin, intercellular bridging, invasion of the subcutis, perineural invasion and lymphovascular invasion may be present
  • Lesional cells are positive for p40 (Am J Surg Pathol 2014;38:1102)
• Angiosarcoma:
  • Usually positive for vascular immunohistochemical markers, such as CD31, CD34 and ERG
• Atypical fibrous histiocytoma:
  • Usually presents in younger patients, lacks prominent actinic changes and may have areas of classic dermatofibroma in the periphery of the tumor
• Leiomyosarcoma:
  • Usually positive for smooth muscle markers, such as smooth muscle myosin and desmin
• Metastatic carcinoma:
  • Should be considered in the differential diagnosis for patients with a known history of carcinoma
  • Lesional cells are usually positive for pancytokeratin and other immunohistochemical markers similar to the primary tumor
• Superficial CD34+ fibroblastic tumor:
  • May overlap with histologic variants of atypical fibroxanthoma
  • Bizarre atypical nuclei but typically only rare mitoses
  • Diffuse CD34 and frequent keratin positivity

The finding of which of the following additional histologic features in the lesion shown above would most likely favor a diagnosis of melanoma over atypical fibroxanthoma?

1. Presence of an in situ component
2. Presence of numerous mitotic figures, including atypical forms
3. Presence of pigment
4. Reactivity for MITF

A. Presence of an in situ component. Choice B is incorrect because numerous mitotic figures, including atypical forms, are seen in both melanoma and AFX. Choice C is incorrect because pigment can be seen in both AFX (hemosiderin) and melanoma (melanin). Choice D is incorrect because reactivity for anti-MiTF may be seen in both melanoma and AFX.

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Reference: Atypical fibroxanthoma

Assuming all other listed markers are negative, a positive immunohistochemical study with which marker is most consistent with a diagnosis of atypical fibroxanthoma?

1. CD10
2. p40
3. S100
4. Smooth muscle myosin

A. CD10. Choice B is incorrect because p40 positivity suggests a diagnosis of squamous cell carcinoma. Choice C is incorrect because S100 positivity suggests a diagnosis of melanoma or neural tumor. Choice D is incorrect because smooth muscle myosin positivity suggests a diagnosis of smooth muscle tumor.

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Reference: Atypical fibroxanthoma