Atypical fibroxanthoma, AFX

Definition

A superficial pleomorphic tumour usually occurring on sun-damaged skin, indistinguishable histologically from a pleomorphic malignant fibrous histiocytoma. Atypical fibroxanthoma is no longer considered a fibrohistiocytic lesion, but is probably a UV-induced proliferation of dermal fibroblasts. Alternatively, it has been suggested that most cases are variants of squamous cell carcinoma.

Synonyms

Pseudosarcoma of the skin, paradoxical fibrosarcoma, pseudosarcomatous dermatofibroma, pseudosarcomatous reticulohistiocytoma

Epidemiology

Most cases occur in sun-damaged skin on the head or neck of the elderly, less often on the hands. About 25% of cases occur on the limbs of younger adults2. Prior irradiation may be causative.

Clinical features

There is a single nodule. The lesion is often rapidly growing, ulcerated or bleeding. Some lesions are plaque-like1.

Histopathology

There is an expansile dermal nodule which may extend into the subcutis. The epidermis may show atrophy or ulceration. The tumour consists of bizarre cells which may be spindled or round, rarely forming a storiform pattern. There are numerous mitotic figures, some atypical. Necrosis is rare. Intravascular and perineural invasion are not features1. There may be haemosiderin deposition. Rare clear cell and granular variants have been reported. The adjacent skin shows solar elastosis. Significant subcutaneous invasion should exclude a diagnosis of atypical fibroxanthoma.

Variant:

spindle cell non-pleomorphic atypical fibroxanthoma

Immunohistochemistry

Immunohistochemistry is mainly of value in excluding other entities which form the differential diagnosis.

Vimentin	Atypical fibroxanthoma, 26/263	Squamous cell carcinoma	Malignant melanoma
SMA	focally positive
S-100	0/891, 0/162, 0/26(dendritic cells may be positive)3	0/102	9/92
CD10	20/201, 15/162	5/102	3/9(weak immunoreactivity in 3 cases)2
CD99	positive1, 19/263	0/103	1/103
AE1/AE3	0/162, 0/263	10/102	0/92
MNF116	1/891
Desmin	0/263
PGP 9.5	9/26(weak positivity)3	0/103	1/103
CD68	16/263	1/103	5/103
NKIC3	20/263	0/103	10/103
HMB45	0/263

Positivity for CD10 has also been reported in 3/3 malignant fibrous histiocytomas4. Two tumours have been reported positive for both CD10 and MNF116: the histogenesis of these tumours is uncertain4.

Ultrastructure

The features are those of a mesenchymal tumour.

Differential diagnosis

Spindle cell (sarcomatoid) carcinoma: positive for cytokeratins; may be weakly positive for CD102.
Malignant Melanoma: positive for S-100, may be weakly positive for CD102.
Malignant fibrous histiocytoma1
Leiomyosarcoma, primary or metastatic
Atypical benign fibrous histiocytoma: a more likely diagnosis on skin that is not sun-damaged or in young adults.

Prognosis

Behaviour is almost always benign. Recurrences occur in about 5% of cases1 but, if strict diagnostic criteria are applied, metastases almost never occur.

References

1 Mirza B,Weedon D. Atypical fibroxanthoma: a clinicopathological study of 89 cases. Australas J Dermatol 2005; 46:235-8

2 Hultgren Immunohistochemsical staining of CD10 in atypical fibroxanthoma. J Cutan Pathol

3 Monteagudo C, Calduch L, Navarro S, et al. CD99 immunoreactivity in atypical fibroxanthoma: a common feature of diagnostic value. Am J Clin Pathol 2002; 117:126-31

4 Weedon D, Williamson R,Mirza B CD10, a useful marker for atypical fibroxanthomas. Am J Dermatopathol 2005; 27:181

This page last revised 11.1.2007.