Basomelanocytic neoplasms refer to tumors that contain elements of both melanoma and basal cell carcinoma (BCC). Tumors of this type containing 2 malignant components have been referred to by a variety of names, such as collision, colonization, combined, biphasic, biphenotypic, or intermingled neoplasms.1,2 The nomenclature used in the past has been confusing and inconsistently applied; however, in 2009 Satter et al1 proposed 4 distinct categories by which to classify and describe basomelanocytic neoplasms (Table 1).
Basomelanocytic neoplasms are exceedingly rare with approximately 30 reports in the literature describing 49 individual tumors.2 The largest series to date found only 11 cases of contiguous melanoma and BCC when examining more than 78,000 cases of cutaneous cancers.3 These basomelanocytic neoplasms commonly appear on the head and neck (47%), the trunk (35%), and the extremities (16%),2 and favor men to women in an approximately 3:1 ratio.1 The problem that inevitably arises with these neoplasms is determining whether they should be classified and treated as invasive melanoma or melanoma in situ.
A 75-year-old man with a history of multiple nonmelanoma skin cancers presented to the dermatology clinic with a lesion on his right cheek that had been “present for years” but had shown recent evolution. The lesion had been noted 10 months prior (Fig. 1A) at which time the decision for close monitoring was made. Clinical examination showed a 1.2 × 1.1 cm brown patch with a central 2-mm comedone-like papule on the right zygoma (Fig. 1B); a shave biopsy was performed. Histopathological analysis revealed a lentigo maligna overlying an infiltrative nodular proliferation of atypical basaloid and pigmented plump spindled cells emanating from the invaginating epidermis (Figs. 2A, B). Abundant, heavily pigmented, dermal melanophages were closely associated with the invasive component. Peripheral palisading of basaloid cells and retraction artifact were focally identified. The immunohistochemical panel revealed the infiltrative neoplasm to be composed of 2 distinct but intermixed cell populations. AE1/3 and Ber-EP4 strongly stained the majority of cells composing the infiltrative malignant component—consistent with a BCC (Fig. 2C). However, microphthalmia transcription factor immunohistochemical stain highlighted a lentiginous proliferation of atypical melanocytes abutting the dermal tumor on both sides with an extension from the overlying invaginating epithelium (Fig. 2D). Microphthalmia transcription factor–positive cells also extended into the dermal malignancy and were intermixed within and throughout the dermal tumor nests. The melanocytes appeared to be limited to within the epithelial tumor islands. If it were to be classified as invasive melanoma, the Breslow depth would be at least 1.2 mm, with no evidence of ulceration, perineural, or perivascular invasion. The dermatopathologists working on the case felt that this tumor was most consistent with a colonization tumor because no definitive dermal atypical melanocytes were seen infiltrating the dermis. The decision was made to treat this tumor as melanoma in situ, and the patient had the lesion fully excised in a single-stage Mohs procedure followed by eventual closure using a rotational cheek flap. There was no evidence of recurrence or metastasis at 4–10 months; however, long-term follow-up and surveillance will continue.
A 70-year-old woman presented to a head and neck surgeon for re-excision of malignant melanoma on the right nasal sidewall after initial biopsy and subsequent wide local excision (WLE) at an outside institution. An initial partial biopsy was interpreted as malignant melanoma with a Breslow depth of 0.83 mm. The subsequent WLE specimen was diagnosed as malignant melanoma with a Breslow depth of 2.05 mm, resulting in a recommendation for repeat excision with sentinel lymph node biopsy (SLNB). Neither specimen was initially noted to contain carcinoma. Before subsequent re-excision and SLNB, the consulting surgeon requested consultative interpretation of the outside material. On review, the dermatopathologist identified a proliferation of atypical basaloid keratinocytes with peripheral palisading and retraction artifact associated with melanophages. In addition, there was a broad lentiginous proliferation of atypical single and nested melanocytes, which populated the basaloid keratinocyte proliferation, resulting in a diagnosis of basomelanocytic tumor with melanoma in situ (Fig. 3). Immunohistochemical stains for keratinocyte and melanocyte lineages confirmed the light microscopic impression and did not identify definitive invasive malignant melanoma. After discussion with the patient, given the unusual nature of the lesion, the surgeon elected to proceed with repeat WLE with SLNB. No melanoma was identified in either of the subsequent skin or lymph node specimens, and the patient had no evidence of recurrence at 8 months.
Breslow thickness is the most important prognostic factor for melanoma and guides treatment options based on the depth of the tumor. Basomelanocytic neoplasms present an interesting conundrum because the presence of a BCC component can increase the Breslow depth. This becomes particularly problematic in colliding and colonizing tumors in which melanoma tracks along the BCC component as part of its horizontal growth phase. This creates a diagnostic dilemma as to whether to classify the melanoma component as in situ or invasive and leads to uncertainty regarding treatment options. Several authors have suggested that clear cases of colonizing or colliding tumors may be better classified as melanoma in situ, similar to how the extension of neoplastic melanocytes into appendageal structures is not considered invasion and is not included in the Breslow depth.2,4–10 Other authors have advocated for inclusion of the deepest BCC component containing melanoma when determining the Breslow depth.11 However, strong or generalized recommendations cannot be made until long-term follow-up and a greater number of cases are available.
Our 2 contrasting cases highlight the drastically different treatment modalities available for these types of tumors. In case 1, the tumor was classified as melanoma in situ, and a Breslow depth was not used to guide treatment. The decision for excision with Mohs was less invasive and spared tissue compared with the WLE and SLNB that would have occurred had a Breslow depth been reported and used for treatment. Case 2 provides a contrast and shows the treatment course that occurred based on the Breslow depth reported by the original pathologist. This ultimately led to wide re-excision and a lymph node biopsy that did not show evidence of disease. Although WLE with SLNB has a low overall morbidity and complication rate (∼10%),12 the procedure still adds additional risk, cost, and invasiveness that may not provide better patient outcomes or be necessary in cases like this. The literature suggests that aggressive treatment (SLNB) is not necessarily indicated in the absence of definite invasive disease; however, the issue of diagnosis and treatment of basomelanocytic neoplasms remains complex, and most authors advocate for a personalized, case-by-case approach in treating these tumors that considers the individualized patient circumstances and tumor’s specifics.
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