Generalized eruptive histiocytoma developing into xanthoma disseminatum with central diabetes insipidus

underwent a skin biopsy of the keratotic lesion at a nearby hospital. The specimen showed thick scaly crusting with erosion and atypical cells with multiple, small and clear vacuoles spanning the full thickness of the epidermis (Fig. 1b). She was referred to our institution with suspected bowenoid actinic keratosis (AK), and dermoscopic imaging was performed. The intact area of the lesion revealed multiple structureless yellowish areas with indistinct borders on an erythematous background. The crusting in the central portion was considered to have arisen as a result of the biopsy procedure. No vascular patterns were apparent (Fig. 1c). We excised the whole lesion. A histopathological examination showed multifocal neoplastic aggregations attached to the underside of the normal-looking epidermis. For the most part, the neoplasm had not invaded the nearby region of solar elastosis. The tumor nests were composed of atypical basaloid and vacuolated cells (Fig. 1d,e). We subjected both the biopsy and excised specimens to immunohistochemical examinations of the expression of adipophilin (a marker of sebaceous differentiation) and cytokeratin 1 (CK1) (a marker of the suprabasal cells of the normal epidermis, AK, Bowen’s disease and squamous cell carcinoma [SCC]). In the biopsy specimen, the atypical clear cells under the region of thick scaly crusting were adipophilin/CK1 (Fig. 1f–i). In the excised specimen, the neoplastic aggregates were adipophilin/CK1 , whereas the normal-looking epidermis was adipophilin /CK1 (Fig. 1j–m). We diagnosed the patient with superficial SC. At present, it is disputed whether SC really arises from AK or whether the precursor lesion of SC is just composed of undifferentiated sebaceous cells with AK-like features. A case of SC accompanied by SCC was also reported, and the pathogenetic mechanism underlying their association has not been fully elucidated. In the present case, no evidence of AK as a source of SC was detected, and it seemed that a pure superficial SC developed without AK. However, there remains a possibility that SC cells arising from AK may replace all of the precursor cells. After a 2-year follow-up period, no local recurrence or metastasis had been detected. The multifocal tumor nests dermoscopically presented as yellowish areas, and the stratum corneum in the excised specimen was generally orthokeratotic. These features provided a clear pathway through which dermoscopic images of the homogenous yellowish areas could be obtained. SC commonly contains polymorphous vessels, but they were absent in our case. Histologically, no dilated vessels were detected (Fig. 1d). However, the possibility that some vascular features had been obfuscated by compression could not be ruled out, which could be a potential pitfall of the dermoscopic diagnosis of rare subtypes of SC.