The correct answer is to proceed with Mohs surgery for residual metatypical basal cell carcinoma.
Differentiating basal cell carcinoma (BCC) from other tumors is usually not difficult given the characteristic presence of basaloid islands with retraction artifact and peripheral palisading present in the dermis and budding off of the epidermis.
In this case, the metatypical nature of the BCC makes it difficult to differentiate from squamous cell carcinoma (SCC), malignant melanoma (MM), and merkel cell carcinoma (MCC), particularly on the ear. However, the presence of elongated and hyperchromatic nuclei, visible cytoplasm, scattered mitotic activity, and necrosis surrounding dissecting keloidal collagen points more toward BCC (Jones et al. Br J Dermatol 2009
Ber-EP4 is a monoclonal antibody which detects specific epithelial-glycoprotein-adhesion-molecules (EpCAM) on BCC cells. Ber-EP4 is absent in SCC and MM, but can stain MCC, although rarely.
Architecturally, MCC typically shows a large dermal “blue” tumor nodule with sheet-like or trabecular growth. However, when a nested or infiltrative pattern is present, this may mimic BCC. While MCC may have stromal fibrosis, the classic myxoinflammatory stroma of BCC is not observed in MCC.
Cytologically, BCC tumor cells are elongated with hyperchromatic nuclei, visible cytoplasm, and scattered mitotic activity and necrosis whereas tumor cells of MCC have a homogenous salt and-pepper chromatin pattern, nuclear molding, scant cytoplasm, and abundant mitotic activity and necrosis.
At times, immunostaining may be necessary to differentiated BCC and MCC. MCC expresses epithelial markers including low-molecular-weight keratins, EMA, and neuroendocrine markers. (chromogranin and synaptophysin)
Most MCCs express CK20, whereas that stain is negative in BCC. Chromogranin or synaptophysin are not useful for differentiating between MCC and BCC because some BCCs may also express neuroendocrine markers.
BCC and MCC may both be treated with MMS; however this treatment is more established for BCC. MCC oftentimes requires concomitant sentinel lymph node biopsy and adjuvant radiation therapy. The National Comprehensive Cancer Network (NCCN) Guidelines recommend 1-2 centimeter margins to investing fascia of muscle or pericranium when clinically feasible. More exhaustive histologic margin assessment (Mohs surgery) may be considered, provided it does not interfere with sentinel lymph node biopsy when indicated.
The S-100/Melan-A and cytokeratin immunostaining negativity would essentially rule out concomitant MM and SCC, respectively.
Clinical pearl: Metatypical BCC on Mohs histology can make it difficult to differentiate from other malignancies, including MCC, MM, and SCC. ber-EP4 immunostaining is useful to help differentiate BCC from other tumors. Keloidal collagen dissecting through elongated tumor cells with hyperchromatic nuclei, visible cytoplasm, and scattered mitotic activity and necrosis from the ear is seen more commonly with basal cell carcinoma than other cutaneous malignancies.