Case Report: Definitive radiotherapy for intrathyroidal thymic carcinoma with neuroendocrine differentiation.

Intrathyroidal thymic carcinoma (ITTC) with neuroendocrine differentiation is rare, and evidence for definitive radiotherapy in nonsurgical patients remains limited. A 70-year-old woman with a 1-year history of a neck mass and hoarseness was diagnosed with ITTC with neuroendocrine differentiation based on histopathological, immunohistochemical, and imaging findings. After declining surgery and systemic chemotherapy, she received definitive radiotherapy: 60 Gy in 30 fractions to the planning target volume, followed by a sequential boost of 6 Gy in 3 fractions to the gross primary tumor and involved lymph nodes, for a cumulative dose of 66 Gy in 33 fractions. No grade ≥3 acute treatment-related toxicities occurred. At approximately 2 months after radiotherapy, the sum of target-lesion diameters decreased from 54 mm to 42 mm, a 22.2% reduction. With no new lesions or unequivocal progression of non-target disease, the overall response was classified as stable disease according to RECIST 1.1. This case documents early radiographic tumor reduction following definitive radiotherapy for ITTC, although longer follow-up is required to assess the durability of disease control and late toxicity.