We present the case of a 73-year-old man with a history of non-insulin-dependent diabetes mellitus, arterial hypertension, abdominal aortic aneurysm, and hypercholesterolemia. He was referred to our department for evaluation of a tumor on the scalp that had appeared 6 weeks earlier. The tumor was pink with some reddish and violaceous areas, had a maximum diameter of 18mm and distinct borders, was nonulcerated, and displayed mild scaling in the center (Fig. 1).

Figure 1.

Tumor on the scalp and nearby actinic keratoses.

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Dermoscopic examination was performed with a polarized light contact dermoscope (DermLite Foto, 3Gen LLC) using ultrasound gel as the liquid interface. The dermoscopic images showed a round, symmetrical lesion with a reddish peripheral area from which atypical, irregularly distributed, out-of-focus blood vessels—mostly linear and unbranched—extended in a vaguely radial pattern (Fig. 2). Most of the tumor surface displayed rainbow-patterned areas, often arranged in parallel to the linear, irregular blood vessels. None of the criteria specific to melanocytic lesions were observed. Shiny whitish areas were observed between the rainbow-patterned structures, and scales were visible on the surface.

Figure 2.

Dermoscopic image, obtained using a polarized light contact dermoscope, showing irregular blood vessels in the peripheral area and a rainbow pattern in the center.

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Complete surgical excision of the lesion was performed. Histologic examination revealed a nodular cell proliferation in the dermis comprising aberrant spindle-shaped cells, epithelioid cells, multinucleated giant cells, and abundant mitotic figures. Hemorrhagic zones were observed in some areas. Immunohistochemistry was positive for vimentin, CD68, and CD10 and negative for CD31, CD34, FVIII, S100, cytokeratins, epithelial membrane antigen, desmin, and smooth muscle actin. The proliferative index, measured using Ki-67, was between 10% and 20%. A diagnosis of atypical fibroxanthoma was established on the basis of the histopathologic findings.

Atypical fibroxanthoma is a rare low-grade tumor. It usually develops on chronically sun-damaged skin, especially on the head or neck of elderly patients.1 Clinically, it presents as a single, rapidly growing nodule. Differential diagnosis should include basal cell carcinoma, squamous cell carcinoma, Merkel cell carcinoma, and melanoma. Lymph node metastases are rare and distant metastases are very rare.

Only 4 dermoscopic observations of this rare tumor have been described in the literature. An atypical vascular pattern with irregularly distributed polymorphic vessels—including linear, punctate, globular, tortuous, and arborizing vessels—was observed in 3 cases.2 Whitish areas were observed in 2 of these patients and heterogeneous hyperpigmentation in the other. The remaining case was a collision tumor associated with a basal cell carcinoma, in which the atypical fibroxanthoma component appeared as a hemorrhagic, ulcerated lesion with small telangiectasias.3

In our patient, the most striking dermoscopic finding was the rainbow pattern in the center of the tumor. Although the rainbow pattern was initially considered to be a characteristic feature of Kaposi sarcoma, it has since been observed in various conditions, including melanoma, stasis dermatitis, lichen planus, hemosiderotic dermatofibroma, and basal cell carcinoma.4-7 The rainbow pattern is an optical phenomenon that can only be observed with a polarized light dermoscope (either contact or noncontact). It occurs when light in different states of polarization interacts with the structures of the lesion.6 Bugatti et al.2 specified that a contact dermoscope was used in the 3 cases they reported but did not state whether or not the device used polarized light, without which the rainbow pattern cannot be observed.

The shiny white structures can take the form of shiny white streaks (also known as chrysalis structures), shiny white areas, or rosettes. These structures can be observed much more clearly with a polarized light dermoscope. The presence of shiny white structures has been described in various benign and malignant tumors—including melanoma, Spitz nevus, dermatofibroma, actinic keratosis, squamous cell carcinoma, and basal cell carcinoma—and is indicative of an increase in dermal collagen.8,9

All dermoscopic observations of atypical fibroxanthoma reported to date have described an atypical vascular pattern characterized by polymorphic vessels interspersed with whitish areas. This dermoscopic pattern is nonspecific and indicative of chaotic neoangiogenesis during growth. It is also seen in other tumors that are clinically similar to atypical fibroxanthoma, such as squamous cell carcinoma, Merkel cell carcinoma, and amelanotic melanoma.10 Since the rainbow pattern is also found in a number of these tumors, this finding alone is not sufficient to establish a dermoscopic diagnosis.

In summary, we present the first dermoscopic observation of the rainbow pattern in an atypical fibroxanthoma, thereby adding atypical fibroxanthoma to the list of entities that can present this finding. A vascular pattern characterized by polymorphic vessels is a constant finding in all reported cases of this tumor.