Somatic-only SDHD variant with tumor-specific loss of heterozygosity in metastatic carotid body tumor: a case report with review of literature.

Carotid body tumors (CBTs) are rare paragangliomas in which genetic predisposition, particularly pathogenic variants in succinate dehydrogenase (SDHx) genes, plays an important role in tumorigenesis. Previous genetic studies of CBTs have primarily focused on germline SDHx variants, whereas somatic alterations remain poorly characterized. Among SDHx genes, SDHB variants are known to be associated with a higher metastatic risk, while SDHD variants are generally linked to a lower metastatic rate. We report the case of a 41-year-old man who presented with a painless right-sided neck mass. Imaging studies demonstrated a hypervascular tumor located at the carotid bifurcation with circumferential encasement of the carotid artery. During surgery, the tumor was classified as a Shamblin type III CBT, and complete surgical resection with vascular reconstruction was performed. Histopathological examination confirmed paraganglioma with metastasis to a single cervical lymph node. Germline genetic testing did not reveal any pathogenic variants. However, comprehensive tumor genomic profiling identified a somatic SDHD c.304C>G (p.His102Asp) variant accompanied by tumor-specific loss of heterozygosity (LOH) and copy-number loss at the SDHD locus, findings compatible with biallelic SDHD inactivation. The patient remained free of recurrence during follow-up. To our knowledge, this represents the first reported case of metastatic CBT harboring a somatic-only SDHD variant with tumor-specific LOH. This case suggests that reliance on germline testing alone may underestimate the molecular drivers of CBT and highlights the potential clinical value of tumor-based genomic profiling for risk stratification and prognostic assessment.