Laboratory Studies
The following laboratory tests may be needed:
- Panels of antibodies
- Electron microscopy
- DNA content quantification
Other Tests
Dermoscopy (see Histologic Findings)
Histologic Findings
The pathologic appearance of atypical fibroxanthoma (AFX) belies the usually excellent prognosis. As shown in the image below, this nonencapsulated dermal tumor is composed of large, fibrocytic, spindle-shaped and anaplastic cells arranged in a haphazard fashion, occasionally in fascicles, and usually with an increased number of mitotic figures. Large histiocytic cells may form bizarre multinucleated giant cells that frequently contain lipid, contributing to the tumor's name. Phagocytosis of erythrocytes has been demonstrated, resulting in hemosiderin pigmentation within a lesion and causing clinical confusion with malignant melanoma. Granular and clear cell variants have recently been reported.
Microscopic view of atypical fibroxanthoma. Note the large abnormal-appearing cells in a field of spindle cells. Courtesy of Capt James Steger, MC, USN, US Naval Hospital, San Diego. Electron microscopy
Electron microscopy suggests a fibrohistiocytic nature in the tumor. A transition from fibroblast to large giant cells can be seen, with intermediate forms that exhibit features of both cell types. Pathologic findings in AFX appear to be more related to MFH than to either dermatofibroma or dermatofibrosarcoma protuberans (DFSP). Delicate cytoplasmic fibrils were seen in one case studied with electron microscopy, but these fibrils were not considered to arise from myofibroblasts and findings did not support a muscle origin for AFX.
Immunohistochemistry
Panels of specific antibodies together with the tumor's histologic pattern help differentiate AFX from other types of spindle cell skin lesions. Specific and critical differences of antibody reactivity (R) and nonreactivity (N) are demonstrated in the Table.
Table. Antibody Panels in Tumors (Open Table in a new window)
| Antibody | AFX | MFH | SCC | DFSP | Spindle MM* | Leiomyosarcoma |
| Vimentin | R | R | N | R | R | R |
| Cytokeratin | N | N | R | N | N | N |
| S-100 | N | N | N | N | R | N |
| Desmin or smooth muscle actin | N | N | N | N | N | R |
| *Spindle cell malignant melanoma | ||||||
New published data indicate that LN-2 antibody (CD74) and p53 immunoreactivity may help differentiate between malignant lesions in the fibrohistiocytic series, but further confirmation is needed. SCC, spindle cell MM, and leiomyosarcoma usually are differentiated using immunocytochemistry. No reliable consistent immunocytochemistry method is specific for AFX, and the diagnosis is based on typical histologic findings and the absence of immunomarker positivity for melanocytes, keratin, and smooth muscle actin.
DNA content quantification
Diploid or euploid cells have pairs or multiple pairs of chromosomes and usually are considered benign. Aneuploid cells have single or multiple single sets of chromosomes and are more common in malignant neoplasms.
Attempts to evaluate chromatin content in AFX have resulted in much confusion because of the methods of evaluation. The average picture produced with flow cytometry suggests that AFX is diploid. Using individual cell analysis, aneuploidy was found in giant cells, while diploidy has been found in smaller spindle-shaped cells. This picture is similar to MFH and does not allow nuclear cytometry to differentiate between MFH and AFX.
Dermoscopic features
A diagnosis of AFX is often not suspected clinically. The histologic findings of the biopsy specimen may cause great consternation, bringing to mind serious and possibly life-threatening diagnoses. Only by combining the clinical presentation with specific staining panels to eliminate other suspected diagnoses is the correct one determined. Although dermatoscopic features of the tumor have been reported, they are not specific. Bugatti and Filosa found white areas and an atypical polymorphous vascular pattern cauterized by linear, dotted, hairpin and arborescent vessels irregularly distributed over the surface of the tumor.[21]
Histologic variants
Nonpleomorphic AFX (spindle cell AFX) was reported by Wilson-Jones et al in a series of cases of sun-damaged skin of the head and neck in older individuals. The immunocytochemistry findings and the tumors' benign clinical course supported the diagnosis of AFX. Pathologically, lesions were monomorphic, spindle-celled, fascicular variants without pleomorphic cells. All lesions were vimentin positive with approximately 50% showing focal actin activity. Desmin, keratin, and S-100 protein were negative in all cases.
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| Antibody | AFX | MFH | SCC | DFSP | Spindle MM* | Leiomyosarcoma |
| Vimentin | R | R | N | R | R | R |
| Cytokeratin | N | N | R | N | N | N |
| S-100 | N | N | N | N | R | N |
| Desmin or smooth muscle actin | N | N | N | N | N | R |
| *Spindle cell malignant melanoma | ||||||
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