Collision Tumor of Dermatofibroma and Basal Cell Carcinoma: Clinical Insights and Diagnostic Challenges

Dear Editor,

Dermatofibroma (DF) is a common benign dermal tumor, predominantly affecting females, and is typically found on the limbs.[1] The lesion is often associated with hyperplasia of the overlying epidermis, which is induced by growth factors derived from the DF. The epidermal hyperplasia can vary in type, with proliferation of germinative cells of basaloid type has been of particular concern due to its close resemblance to basal cell carcinoma (BCC).[2] Many attempts have been made to determine the nature of these lesions. Immunohistochemistry and molecular genetic methods have been applied with conflicting or equivocal results. While most basaloid proliferations are considered reactive, BCCs can develop within longstanding DFs.[3]

Here, we report a rare case of a collision tumor involving both DF and BCC, highlighting the importance of recognizing such entities in clinical practice.

A 52-year-old woman, categorized as phototype III, with a history of pulmonary sarcoidosis, presented with a hard, well-defined erythematous papulonodular lesion, measuring 9 × 10 mm, on her right forearm. The lesion was asymptomatic but had exhibited overgrowth over the past six months [Figure 1a]. Dermoscopy revealed a pale pink amorphous area with blue-gray ovoid nests, specks of pigment, and spoke-wheel pigmentation. A biopsy was performed for further evaluation, revealing two distinct components [Figure 1b-d]. One component, located beneath the overlying acantholytic epidermis, consisted of atypical basaloid cells forming a palisading pattern, surrounded by a cleft at the periphery, indicative of a superficial BCC. This was consistent with its confinement to the superficial dermis and presence of isolated basaloid lobules projecting from the lower epidermal margin [Figure 1c]. Multiple serial sections confirmed the absence of features suggesting an infiltrative component. The other component, located in the dermis, comprised of a mixture of fibroblasts and histiocytes arranged between collagen fibers, consistent with a dermatofibroma (DF) [Figure 1d]. Based on these findings, a diagnosis of a collision tumor comprising a superficial BCC and DF was made. Surgery was performed to remove the lesion, and annual follow-up was advised for new lesions.

Figure 1.

(a) Papulonodular lesion. (b) Atypical basaloid cells in a palisading pattern and a mixture of fibroblasts and histiocytes arranged between collagen fibers in the dermis (H and E, 40x). (c) Basaloid cells in a palisading pattern and clefting in magnification (H and E, 100x). (d) Fusiform cells in the dermis in magnification (H and E, 400x)

The first documented report of a BCC overlying a DF was described in 1964. By 1992, only 15 cases had been reported in the literature, with a few additional cases reported since then. Its incidence is not well-known, but it is considered a rare occurrence. Adding complexity to such cases, not only BCC but also BCC-like changes have been observed as part of a range of epidermal changes overlying a dermatofibroma, with a frequency of 2-23% and a male predominance.[4] Both BCC and reactive basaloid cell hyperplasia (BCH) in the context of dermatofibromas are thought to represent the proliferation of germinative basaloid cells due to the inductive influence of DF on the epithelial cells of the hair follicle.[1,2,3,4]

Some authors speculate that these basaloid cell proliferations are true BCCs, while others argue that BCC-like changes in dermatofibromas are benign reactive processes.[1] This distinction can be challenging to make, as BCHs closely resemble or are identical with BCCs histologically and cytologically.[3] As there are some reports of BCC associated with DF, these lesions should not be regarded as reactive BCHs alone. Supporting the latter view are factors such as superficial location, absence of atypia or mitoses, follicular differentiation, and association with epidermal hyperplasia.[1,3]

In favor of a true BCC diagnosis, we must consider factors such as the patient’s age, lesion location, the presence of basaloid nests deep within the dermis with a peripheral palisade arrangement of nuclei, a mucinous stroma, retraction artifacts (extensive clefting), mitoses, and pigment, all of which suggest a neoplastic origin.[1,3] These factors are evident in our case, indicating that the infiltration of the basaloid proliferation into the dermis leans more towards a diagnosis of neoplasia rather than a mere reactive effect, as there are no definitive diagnostic criteria favoring benign nature of the lesion.

Immunohistochemical analyses show varied outcomes, complicating the distinction between basaloid proliferations and BCC. Ki-67, Bcl-2 protein, 2-microglobulin and Ber-Ep4 are similarly expressed in both conditions. Metallothionein, is notably diminished or absent in basaloid hyperplasia as well as BCCs. The role of p53 in distinguishing BCC from BCH shows inconsistent findings, with some studies suggesting identical expression in both lesions.[1,3] Furthermore, CK20-positive Merkel cells in BCH and their reduced or absent expression in BCC may offer future diagnostic utility. However, these distinctions are subtle, necessitating further research to assess their clinical significance.[3]

Consideration of these factors is crucial in patient management due to the contrasting clinical behavior of BCH, which is generally non-aggressive, and BCC, known for its local aggressiveness. Treatment decisions should be based on recognizing their atypical features and suspected malignancy. When a BCC diagnosis within a dermatofibroma is suspected, surgical excision with direct closure is recommended to confirm the diagnosis and achieve complete removal with clear margins in a single procedure.[5]

Further research is required to better understand the underlying nature of BCH and to improve the differentiation between BCC and BCH, thereby enhancing patient care. Meanwhile, integrating clinical, morphological, and immunohistochemical features remains essential in challenging cases.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.