Uncommon Collision Tumors: Dermoscopic and Histopathological Features of Basal Cell Carcinoma Overlying Dermatofibroma

Abstract

Dermatofibromas (DFs) represent prevalent benign fibrohistiocytic tumors, typically manifesting as solitary lesions. In the majority of cases, the clinical presentation and dermoscopic and histopathological features of DFs adhere to a characteristic profile. However, DFs may exhibit atypical clinical presentations and, more commonly, histologic attributes, posing challenges in differential diagnosis. Both DFs and basal cell carcinomas (BCCs) are frequently encountered cutaneous lesions, each characterized by distinct clinical and dermoscopic features and microscopic morphology. The simultaneous occurrence of these two entities within the same lesion is rare. DFs have been documented to form collision tumors in conjunction with a spectrum of benign and malignant lesions, encompassing not only BCC but also balloon cell nevus, squamous cell carcinoma (SCC), and melanoma. Alterations in the epidermis overlaying a DF range from simple hyperplasia to the proliferation of basaloid cells. Accurate diagnosis, leading to the complete excision of the lesion, is contingent upon the recognition of dermoscopic criteria, precluding misinterpretation as a benign lesion. We present two cases of collision tumors comprising DF and BCC. This case report underscores the paramount importance of dermoscopy and adherence to dermoscopic criteria in the assessment of collision lesions and the diagnostic process related to cutaneous malignancies.

Keywords: basal cell carcinoma; collision tumors; dermatofibroma; dermoscopy.

Figure 1

(a) A reddish nodule with a distinctive brownish border. (b) Dermoscopic examination revealed a white patch in the central area and a fine light brown pigmented network at the periphery, gray/blue pigmented blotches, arborizing vessels centrally, and microulceration. (c,d) Images showing 1c (×2)–1d (×10) magnification of histopathology. Stain: hematoxylin and eosin. The histopathological examination confirmed a collision lesion comprising a nodular basal cell carcinoma (BCC) and a dermatofibroma (DF). The basaloid tumor was connected to the epidermis and was nodular and well circumscribed within the dermis. It consisted of large basaloid nodules exhibiting peripheral nuclear palisading, mitotic activity, apoptosis, and central cyst formation. In the surrounding stroma, cleft formation between tumor lobules and stroma was observed, along with fibromyxoid stromal changes. Beneath the basaloid tumor, a dermal lesion composed of spindled fibroblasts or histiocytes was identified, with no cytologic atypia or increased mitotic activity. Additionally, focal multinucleated giant cells, eosinophilic collagen fibers, and scattered blood vessels were noted.

Figure 2

(a) Nodular red/brown lesion on the right shin with central ulceration. (b) Dermoscopic features: a central white structureless patch surrounded by a fine light brown reticulated network and microulceration. Glomerular vessels at the periphery, transitioning to loop vessels at the center. (c,d) Images showing c (×2)–2d (×10) magnification of histopathology. Stain: hematoxylin and eosin. The histopathological examination identified a collision lesion comprised of a dermatofibroma (DF) and an infiltrative basal cell carcinoma (BCC). The ulcerated basaloid tumor was connected to the epidermis and demonstrated infiltration into the dermis. The tumor consisted of irregular basaloid cell nests, exhibiting variability in peripheral nuclear palisading, mitotic figures, and apoptotic activity. Sparse cleft formation between tumor lobules and the surrounding stroma was observed, along with fibromyxoid stromal changes. Beneath the infiltrative basaloid tumor, a relatively well-circumscribed but unencapsulated lesion was identified, composed of spindled fibroblasts and histiocytes, lacking cytologic atypia or increased mitotic activity. Focal multinucleated giant cells, eosinophilic collagen fibers interspersed among fibroblasts, and scattered blood vessels were also present.