Metastatic basal cell carcinoma with diffuse bone marrow infiltration: A rare case with exceptional response to vismodegib

Background Basal cell carcinoma (BCC) is the most common non-melanoma skin cancer, comprising approximately 80% of cases. Despite its locally invasive behaviour, metastatic disease is exceptionally rare, occurring in 0.0028-0.55% of cases. Diffuse bone marrow infiltration is an extremely rare metastatic manifestation associated with poor prognosis. Case Presentation We report a case of a 75-year-old woman with recurrent, aggressive infiltrative micronodular BCC of the right cheek requiring multiple surgical excisions and adjuvant radiotherapy (completed July 2024). In September 2024, she presented with pancytopenia and a leukoerythroblastic blood picture. Bone marrow biopsy revealed extensive metastatic carcinoma. Immunohistochemistry showed positivity for BerEP4, B63, CK 5/6, chromogranin, and GATA3, consistent with the primary BCC and suggesting squamous and neuroendocrine differentiation. CT and PET-CT imaging excluded alternative primary sites and confirmed metabolically active malignant infiltration throughout the axial and appendicular skeleton. Treatment with Vismodegib (150 mg daily) was initiated in September 2024. Re-staging PET-CT in February 2025 demonstrated marked partial metabolic response with significant recovery of bone marrow function and normalization of blood counts. Following subsequent disease progression on vismodegib, the patient received second-line cemiplimab, with a mixed metabolic response before eventual clinical deterioration related to progressive marrow involvement. Conclusion This case represents an exceptionally rare presentation of metastatic BCC with diffuse bone marrow involvement. It demonstrates rapid and clinically meaningful response to Hedgehog pathway inhibition with vismodegib, reinforcing its critical role as first-line systemic therapy for advanced and metastatic BCC.