Prognostic factors in thyroid cancer with lung-only metastasis: SEER-based retrospective study

Lung-only metastasis in thyroid cancer is uncommon, and its prognostic determinants remain poorly defined. This study aimed to investigate the clinicopathological features and prognostic factors associated with survival in patients with primary thyroid cancer with lung-only metastases (PTCSLMs).

Patients diagnosed with PTCSLMs from 2010 to 2015 were screened from the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) database. Inclusion criteria comprised histologically confirmed thyroid cancer, first primary malignancy, and lung‑only metastasis at initial diagnosis, while patients with bone/liver/brain metastases, unknown race, unknown T stage/T₀, or missing survival data were excluded. Variables analyzed included demographics (age, sex, marital status, race), clinicopathological features (histological subtype, T stage, N stage, grade stage), and treatment modalities {surgery, radiotherapy [external beam radiation therapy (EBRT)/radioactive iodine (RAI)/others], and chemotherapy}. Overall survival (OS) was defined as time from diagnosis to death from any cause, and cancer-specific survival (CSS) as time to thyroid cancer‑specific death. Univariate and multivariate Cox proportional hazards model were used to identify prognostic factors of PTCSLMs. The Kaplan-Meier method was used to generate survival curves, and the difference was compared using the log-rank test.

A total of 592 patients diagnosed with PTCSLMs from 2010 to 2015 were extracted from the SEER database based on the specified criteria for inclusion and exclusion. Univariate analysis demonstrated statistically significant associations between OS and age at diagnosis, marital status, histologic subtype, T stage, N stage, grade stage, surgery, radiotherapy and chemotherapy (P<0.001). The results of multivariate survival analyses showed that age at diagnosis [43-80 vs. 6-42 years, hazard ratio (HR) =5.044, 95% confidence interval (CI): 2.290-11.111, P<0.001; 81-99 vs. 6-42 years, HR =7.769, 95% CI: 3.296-18.312, P<0.001], histologic subtype [others vs. papillary adenocarcinoma (PA), HR =1.786, 95% CI: 1.323-2.411, P<0.001], T stage (T₂ vs. T_X, HR =0.480, 95% CI: 0.247-0.935, P=0.03), grade stage (poorly differentiated vs. well differentiated, HR =1.976, 95% CI: 1.123-3.476, P=0.02; undifferentiated vs. well differentiated, HR =2.531, 95% CI: 1.417-4.521, P=0.002), surgery (total thyroidectomy vs. refuse/none, HR =0.387, 95% CI: 0.278-0.538, P<0.001; others vs. refuse/none, HR =0.663, 95% CI: 0.455-0.967, P=0.03), radiotherapy (EBRT vs. refuse/none/unknown, HR =0.734, 95% CI: 0.564-0.955, P=0.02; RAI vs. refuse/none/unknown, HR =0.481, 95% CI: 0.336-0.688, P<0.001; others vs. refuse/none/unknown, HR =0.498, 95% CI: 0.254-0.978, P=0.043) and chemotherapy (yes vs. no/unknown, HR =0.741, 95%CI: 0.560-0.982, P=0.04) were independent prognostic factors of PTCSLMs.

Our study suggested that the higher age at diagnosis and higher pathological grade were correlated with worse prognosis. In contrast, surgery, radiotherapy and chemotherapy showed a favorable prognostic association. Age at diagnosis, histological type, T stage, grade stage, surgery, radiotherapy and chemotherapy are associated with prognosis and they are independent prognostic factors for PTCSLMs.