Clinical characteristics and outcomes of patients with intrathyroidal thymic carcinoma (ITC): a retrospective cohort study.

Background: Intrathyroidal thymic carcinoma (ITC) is an extremely rare type of thyroid cancer with very limited knowledge of its clinical characteristics and optimal treatment modalities. We aimed to assemble the largest ITC cohort to date for a better understanding.

Methods: We collected 43 surgically-treated patients with ITC in a single Chinese tertiary center. Kaplan-Meier analysis and multivariate Cox regression analysis is performed to explore prognostic factors. Hazard ratio (HR) and 95% confidence interval (95% CI) were used to indicate the risk associated with the risk factors.

Results: A total of 43 cases with ITC were included. Eighteen patients (41.9%) had cervical or mediastinal lymph node metastases, while 16 (37.2%) had distant metastasis at presentation. Ten patients with locally advanced disease received palliative resection and following adjuvant radiotherapy, but none of them showed local progression. After a median follow-up of 69 months, we observed a favorable prognosis with a 3- and 5-year progression-free survival (PFS) of 92.1% and 59.8%, and a 3- and 5-year disease-specific survival (DSS) of 97.0% and 93.1%, respectively. Multivariate analysis showed that palliative resection (HR =5.68, 95% CI: 1.48-21.76, P=0.01) and lateral lymph node metastasis (HR =12.27, 95% CI: 2.40-62.81, P=0.003) were independent risk factors of tumor progression. Partial response was achieved in two patients who received immunotherapy combined with targeted therapy or chemotherapy.

Conclusions: Despite with an overall favorable survival, ITC is still prone to lymph node and distant metastasis. Lateral neck metastasis and incomplete tumor resection predicted a poorer outcome. Once achievable, radical surgery combined with radiotherapy should be performed to improve local control, while immunotherapy and targeted therapies are potentially effective in advanced disease.