A Rare Collision in Dermatopathology: Basal Cell Carcinoma and Atypical Fibroxanthoma.

To the Editor: We present the second report of a rare collision tumor comprised of a basal cell carcinoma (BCC) and an atypical fibroxanthoma (AFX) in a 64-year-old white man, who presented for evaluation of a bleeding growth on his right temple. The lesion had been present for several years but was becoming larger and increasingly pruritic over the past 3 months. On physical examination, a 3.5-cm pink, pearly irregularly shaped plaque was noted over the right temple, with light brown crust along the edges. A hyperpigmented focus was noted at the inferior aspect of the plaque with overlying erosions (Fig. ​(Fig.11). FIGURE 1 A 3.5-cm pink, pearly irregularly shaped plaque was noted over the right temple, with light brown crust along the edges and a hyperpigmented focus at the inferior aspect. Histologic examination revealed a mass with 2 different histologic patterns (Fig. ​(Fig.2).2). One part of the tumor exhibited features consistent with a classic BCC, comprised of islands and nests of basaloid cells, originating from the basal layer of the epidermis and extending into the dermis, with peripheral palisading and focal epidermal-stromal clefting, surrounded by a mucinous stroma (Fig. ​(Fig.3).3). Adjacent to and intermixed with the first tumor was an infiltrating spindle cell proliferation in a haphazard arrangement that abutted the epidermis but did not appear to be connected to it (Figs. ​(Figs.4,4, ​,5).5). The cells demonstrated marked pleomorphism with hyperchromasia, abundant eosinophilic cytoplasm, and prominent nucleoli. The tumor cells contained both typical and atypical mitoses with an index of 1–4 mitoses per high-power field. Multiple giant cells, scattered acute and chronic inflammation, ectatic capillary vessels, and focal hemorrhage were present. Perineural infiltration, lymphovascular invasion, and necrosis were not identified in the planes of section examined. Both tumors were transected laterally and at the base. The classic BCC tumor cells were diffusely and strongly positive for Ber-EP4 (Fig. ​(Fig.6),6), whereas the spindle cells were diffusely and strongly immunoreactive for CD10 (Fig. ​(Fig.7).7). Based on the histologic and immunohistochemical findings, the case was diagnosed as a collision tumor comprised of AFX and BCC. Because of the fact that the tumor was broadly transected at the base, the more aggressive undifferentiated pleomorphic sarcoma (uPS) could not be excluded. FIGURE 2 A shave biopsy of skin consisting of 2 histologically distinct tumors. There is a cystic basaloid proliferation on the right and a spindle cell proliferation on the left. The area above the dashed line represents what would be viewed on a more superficial ... FIGURE 3 There is a basaloid proliferation of cells with nodular and cystic morphology, surrounded by a mucinous stroma (×10 objective). FIGURE 4 Adjacent to and abutting this component is a pleomorphic and mitotically active spindle cell proliferation (×10 objective). FIGURE 5 There is an infiltrating spindle cell proliferation in a haphazard arrangement that abuts but is not connected to the overlying epidermis. Marked pleomorphism and multiple atypical mitoses are present (×10 objective). FIGURE 6 Strong and diffuse Ber-EP4 positivity in the basaloid tumor component (×2 objective). FIGURE 7 Strong and diffuse CD10 positivity in the spindle cell tumor component (×2 objective). Because of the location of the lesion, morphology, and ill-defined margins, the decision was made to proceed with a Mohs excision. The residual BCC component of the tumor extended into the subcutis and was completely excised with no complications. There was no residual AFX component identified in the Mohs sections. Given the patient's complicated dermatologic history, he will require at least annual follow-up. A collision tumor is defined as 2 or more histologically distinct neoplasms coexisting in the same anatomical location with clearly defined boundaries, and can pose clinical and histologic diagnostic challenges. This is especially true when they involve a combination of 2 malignant tumors, each with its own prognosis, treatment, and ability to metastasize, which cause additional risk to the patient if misdiagnosed. Although the exact etiology of collision tumors is unknown, most authors believe that these combinations occur serendipitously, either because of the high biopsy rate of the 2 tumors or the high incidence of both tumors in sun-exposed skin. However, others suggest that epithelial or stromal changes in 1 tumor can induce the formation of the other,1,2 or that both are derived from similar cell lineages.3 AFX is a rare component of collision tumors, with combinations including primarily malignant entities, such as Merkel cell carcinoma,4 squamous cell carcinoma in situ,5 and invasive melanoma.6,7 Although it is generally classified as a low-grade sarcoma, its clinical behavior remains controversial, with outcomes ranging from spontaneous regression8 to recurrence and distant metastatic disease,9,10 especially after incomplete excision.11 Because of the high rate of local recurrence and ability for metastatic disease, wide local excision with 1-cm margins or Mohs micrographic surgery are recommended.12,13 Many believe that AFX is a more superficial and less aggressive variant of uPS, for which treatment requires margins up to 4 cm in some reports.14 BCCs are common components of cutaneous collision tumors, with the most common combinations, in contrast to AFX, being with benign entities, such as melanocytic nevus, seborrheic keratosis, and neurofibroma.15 Because of the indolent course and extremely low incidence of metastases (0.0028%–0.55%)16 in BCC, they are often effectively managed by electrodessication and curettage or local excision with 4-mm margins. We present this case because the combination of BCC and AFX is a rare finding in collision tumors, with only 1 other case reported in the literature without discussion of possible therapeutic differences. Although the AFX component in our case represented a large portion of the biopsy, it is important that dermatopathologists be aware of this potential combination. If a more superficial shave biopsy had been performed on this patient (demonstrated by area above dashed line in Fig. ​Fig.2),2), it could have potentially lead to misdiagnosis of a focus of invasive BCC. The higher incidence of local recurrence and possibility of distant metastases for AFX requires a wider surgical margin and more attentive clinical follow-up. Additionally, a focus of AFX may represent the superficial portion of the more aggressive uPS, which would significantly alter the patient's treatment and prognosis. We report this collision tumor as an interesting twist on a “routine” case of BCC.