Long-term control with immunotherapy and aggressive re-irradiation for basal cell carcinoma metastatic to the vertebra: A case report and literature review of rare osseous metastasis

<h2>Abstract</h2><h3>Background</h3> Cutaneous basal cell carcinoma (BCC) is a prevalent malignancy with a rising incidence, though it rarely metastasizes, with an incidence of 0.0028–0.5 %. Hematogenous metastatic BCC (mBCC) is particularly rare and associated with a median survival of 8–14 months. <h3>Case details</h3> This case report details a 59-year-old male with a primary BCC on his left shoulder metastasizing to the lungs and spine. Despite aggressive multimodal treatment, including surgery, radiation, and systemic therapy with hedgehog inhibitors (vismodegib, sonidegib), the disease progressed. At 55 months post-diagnosis, pembrolizumab was introduced due to the high tumor mutation burden (TMB-H), resulting in a notable therapeutic response and prolonged survival. The patient underwent extensive follow-up, including re-irradiation of spinal metastases and immunotherapy, maintaining stable disease for over 101 months. <h3>Results/literature review</h3> A systematic literature review identified only 26 cases of osseous mBCC, primarily treated with surgery and radiation, with variable survival outcomes. This case stands out due to its prolonged survival and novel use of pembrolizumab, highlighting the potential role of immunotherapy in TMB-H mBCC. The patient's TMB-H status likely contributed to the favorable response to immunotherapy. <h3>Conclusion</h3> This report underscores the importance of genetic profiling in mBCC to identify candidates for emerging immunotherapy treatments. The findings suggest that patients with high mutational burden mBCC may benefit from pembrolizumab, offering a new therapeutic avenue for this rare osseous metastasis.