Pathology of Atypical Fibroxanthoma
The term 'Atypical Fibroxanthoma, was introduced by Helwig EB (Texas J Med 1963;59: 664-667 ). Similar cases were reported as Paradoxical fibrosarcoma of the skin or pseudosarcoma by Bourne RG (Med J Aus 1963;1:5 04-510).
Microscopic images of Atypical Fibroxanthoma
- The tumour usually presents as solitary, rapidly growing dome shaped ulcerated and crusted nodule, usually less than 2 cm in diameter, on the head or neck region of the elderly.The lesion usually occurs in sun-damaged or radiation damaged skin.
- A rare clinical variant occurs in younger patients. The tumour is
larger and slow growing and are usually located on the trunk and
- Located in the dermis.
- Expansile growth. The tumour is usually bordered by mononuclear inflammatory cells in the deeper aspect.
- The adjacent adnexal structures are usually compressed. Adnexal structures within the tumour are always intact.
- The overlying epidermis is thin and usually ulcerated with crust formation , but there may be peripheral epidermal acanthosis with formation of 'epithelial collarette'.
- A thin grenz zone may be present separating the tumour from the epidermis or the tumour may stream out from the basal layer.
In the latter case the dermo-epidermal junction is not always clear.
- The tumour is characterized by marked pleomorphism and polymorphism .
- There is an admixture of three main cell types:
Note: Oil red O stain on frozen sections show variable amount of lipid in the polyhedral and giant cells.
- Typical and atypical mitotic figures are noted (one per high power field).
- The epidermis overlying the tumour usually do not show evidence of atypia, junctional activity or atypical melanocytes.
However, epidermis adjacent to the tumour may show evidence of actinic keratosis.
- Dermis adjacent to the main lesion show features of solar elastosis. Telangiectatic blood vessels may be present near the surface.
- In rare cases foci of osteoid and chondroid differentiation and osteoclast like giant cells may be present.
- Spindle cell variant is entirely composed of monomorphic palely eosinophilic spindle shaped cells arranged in fascicles.
There is only mild pleomorphism. (Differential diagnosis: leiomyosarcoma, spindle cell squamous carcinoma and spindle cell melanoma).
Following features help to exclude Atypical Fibroxanthoma from other lesions:
1. Presence of vascular and / or perineural invasion.
2. Extensive necrosis away from the ulcerated surface.
3. Extensively infiltrative growth pattern.
4. Deep extension into the subcutaneous fat.
Vimentin shows diffuse strong cytoplasmic positivity.
Smooth muscle actin shows focal but strong cytoplasmic staining in many cases.
Alpha1- antitrypsin , alpha1- antichymotrypsin , HAM56 (non-specific) are immunopositive.
CD68 (monocyte-macrophage marker) shows some positivity.
CD74 is weakly positive in some cases. (In MFH CD74 is strongly positive).
In a few cases scattered cells stain positively for factor XIIIa.
Focal positivity for CD99 has been reported.
Differential diagnosis of Atypical Fibroxanthoma:
1. Spindle Cell Squamous Cell Carcinoma: Infiltrative tumour and overlying epidermal dysplasia is present. Cytokeratin is positive.
2. Malignant melanoma: S100 protein and HMB45 positive. Spindle cell melanomas are usually amelanotic,infiltrative tumour with uniform cytology and are often associated with desmoplasia. Neural invasion may be present.
3. Leiomyosarcoma: Infiltrative, poorly circumscribed tumour composed of bundles of spindle cells. Primary cutaneous leiomyosarcoma shows minimal cytological atypia.
Metastatic tumours are well circumscribed and demonstrates prominent cellular atypia. Desmin is positive.
Note: Immunohistochemical analysis is absolutely essential to confirm the diagnosis.
4. Malignant fibrous histiocytoma (Pleomorphic sarcoma): Deep soft tissue tumour. CD74 is strongly positive.
6. Reticulohistiocytoma: Composed of epithelioid cells with 'ground glass' cytoplasm.
7. Post irradiation: Features similar to atypical fibroxanthoma may be present in post irradiated skin.
Atypical fibrous histiocytoma of
the skin: clinicopathologic analysis of 59 cases with evidence of
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