Case Report: Evolution of a BRAF V600E-mutant papillary thyroid carcinoma from latissimus dorsi metastasis and squamous transformation to PD-L1 upregulation and effective immunotherapy.

This case report describes an unusual clinical course of a patient with papillary thyroid carcinoma (PTC). The patient sequentially developed metastases to the cervical lymph nodes, lungs, pleura, and brain. Following tumor invasion into the latissimus dorsi muscle, a biopsy of this metastatic lesion revealed a BRAF V600E-mutant squamous cell carcinoma (SCC) phenotype indicating a high-grade dedifferentiation from the primary PTC. This metastatic focus also demonstrated upregulation of programmed cell death ligand 1 (PD-L1) expression. Subsequently, the patient exhibited a robust sensitivity to treatment with a programmed cell death protein 1 (PD-1) inhibitor and chemotherapy. This case highlights a highly aggressive evolutionary pattern of PTC and its associated potential for immunotherapy, providing valuable insights for the clinical management of thyroid cancer.