Basal cell carcinosarcoma with PTCH1 mutations in both epithelial and sarcomatoid primary tumor components and in the sarcomatoid metastasis.

Basal cell carcinosarcoma is a rare biphenotypic malignant skin tumor, in which one tumor component has light microscopic features of basal cell carcinoma, whereas the other has features of sarcoma. Clinical experience with this tumor is limited, and associated molecular genetic alterations are unknown. Herein, we report a unique case of metastatic basal cell carcinosarcoma, in which we analyzed the 2 components of the primary tumor as well as the metastasis by next-generation sequencing. The patient was a 72-year-old man who presented with a 7-year history of a large tumor of the left forearm. The tumor showed mixed features of basal cell carcinoma and undifferentiated sarcoma. The patient underwent a wide local excision and sentinel lymph node biopsy, which revealed microscopic subcapsular deposits of metastatic sarcomatoid tumor. One year later, intra-abdominal metastatic tumor was detected and resected. It had sarcomatoid features by light microscopy and failed to stain for epithelial markers by immunohistochemistry. DNA was extracted separately from the epithelial and sarcomatoid component of the primary tumor, intra-abdominal metastasis, and normal tissue. All exons of 230 cancer-associated genes were sequenced to an average read depth of >500-fold. This revealed multiple identical mutations in epithelial and sarcomatoid tumor compartments. Both compartments harbored 2 identical mutations, a truncating and a missense mutation, in the patched gene (PTCH1). This finding is not only of interest for a shared heritage of different subpopulations in a biphenotypic tumor, but also relevant clinically. It provides a rationale for the clinical use of hedgehog pathway inhibitors for treatment of patients affected by this tumor. Unfortunately, the patient reported herein died of metastatic disease before targeted therapy could be initiated.