Solitary trichoepithelioma in an 8-year-old child: clinical, dermoscopic and histopathologic findings

Solitary trichoepithelioma (TE) is a rare, benign tumor of follicular origin that in certain cases is difficult to differentiate from basal cell carcinoma (BCC). We report the case of an 8-year-old girl with a pale pink, soft lesion on the neck. The clinical image of the lesion was equivocal, while some dermoscopic findings—blue-gray globules and arborizing vessels—could not exclude the presence of BCC from the differential diagnosis, although that would have been a very unlikely case considering the age of the patient. The histopathologic examination established the diagnosis of TE. Given the occasion of this challenging case we try to list the key clinical, dermoscopic and histopathological characteristics of TE and BCC in order to elucidate the differential diagnosis of these two entities.

Solitary trichoepithelioma (TE) is a rare, benign tumor of follicular origin that in certain cases is difficult to differentiate from basal cell carcinoma (BCC). We report the case of an 8-year-old girl with a pale pink, soft lesion on the neck. The clinical image of the lesion was equivocal, while some dermoscopic findings-blue-gray globules and arborizing vessels-could not exclude the presence of BCC from the differential diagnosis, although that would have been a very unlikely case considering the age of the patient. The histopathologic examination established the diagnosis of TE. Given the occasion of this challenging case we try to list the key clinical, dermoscopic and histopathological characteristics of TE and BCC in order to elucidate the differential diagnosis of these two entities.

Discussion
TE is a rare, benign dermal tumor of follicular origin [1].
Three major variants have been described in the literature, namely solitary, multiple and desmoplastic TE [1] The solitary subtype is commonly found in young adults. It is usually located on the central face and on the perinasal area in particular. This site predilection could be attributed to the high concentration of pilosebaceous units in this area. However, on rare occasions TE can acquire a diameter of ≥1 cm and can be situated on the neck, scalp or trunk [2]. Sometimes, TE can closely resemble BCC. The occurrence of TE and/or BCC in childhood, although it has been reported in anecdotal cases, is very uncommon [3]. The key clinical, dermoscopic and histopathologic characteristics of TE and BCC are summarized in Table 1.      In conclusion, histopathology remains the gold standard method for the differential diagnosis between TE and BCC [3,4]. The dermatoscopic criteria for TE need further investigation since only a small case series has been published until now [2]. Moreover, dermatoscopic findings must always be interpreted in light of important clinical information, such as the age of the patient and the natural history of the lesion. In that sense, our case shows that dermatoscopic criteria suggestive of the diagnosis of BCC, such as blue-gray globules and fine arborizing vessels, can also be seen in some cases of TE.
In this case the clinical presentation of the lesion was in favor of TE mainly due to its soft surface and to the absence of central erosion or ulceration, which usually develops in enlarging BCCs. Moreover, the young age of the patient almost excludes the diagnosis of BCC, although the relatively rapid enlargement of the lesion (6 months to 1 year) is not characteristic for TE. The dermatoscopic examination revealed several findings that could be attributed to BCC, such as some blue-gray globules and fine arborizing vessels.
However these vessels, in contrast to the arborizing vessels commonly found in a nodular BCC, were very thin, few in number and were not "in focus" under dermatoscopy. The presence of blue-gray globules, which were not very evident to the clinical eye, probably corresponds on histopathology to presence of melanin within the neoplastic aggregates found in the dermis. All the above criteria taken together with the absence of other well-known dermatoscopic clues for the diagnosis of BCC, such as leaf-like structures and spokewheel areas, do not coincide with the typical findings in BCC.
Finally, the histopathologic examination undoubtedly proved