Abstract
The American Joint Committee on Cancer (AJCC) 8th TNM staging system of differentiated thyroid cancer defines gross strap muscle invasion as T3b stage. However, the impact of strap muscle invasion on disease-specific survival (DSS) remains controversial. To elucidate the survival impact of strap muscle invasion of any degree in thyroid cancers, the Surveillance, Epidemiology, and End Results (SEER) database (1973–2018) was queried for thyroid cancer only patients on July 2019 (n = 19,914). The Cox proportional hazard analysis with multivariable adjustment revealed that strap muscle invasion was not a significant factor for DSS in tumors equal to or smaller than 40 mm (hazard ratio (HR) = 1.620 [confidence interval (CI) 0.917 – 2.860]; p = 0.097). The competing risk analysis with multivariable adjustment showed that strap muscle invasion did not significantly impact DSS regardless of tumor size or cause of death (cancer-caused death (Subdistribution HR (SDHR) = 1.567 [CI 0.984 – 2.495]; p = 0.059); deaths to other causes (SDHR = 1.155 [CI 0.842 – 1.585]; p = 0.370). A “modified” staging schema discarding strap muscle invasion as a T stage criterion showed better 10-year DSS distinction between T stages. The modified staging schema may better reflect cancer-caused death risk and may prevent potential overstaging.
Highlights
The American Joint Committee on Cancer (AJCC) 8th TNM staging system of differentiated thyroid cancer defines gross strap muscle invasion as T3b stage
A total of 18,085 (90.8%) patients were diagnosed with papillary carcinoma and its variants, while 1406 (7.1%) patients were diagnosed with follicular carcinoma and its variants
This study suggested that strap muscle invasion alone of any degree was not a statistically significant prognostic factor of disease-specific survival (DSS) in all tumor sizes of differentiated thyroid cancer (DTC)
Summary
The American Joint Committee on Cancer (AJCC) 8th TNM staging system of differentiated thyroid cancer defines gross strap muscle invasion as T3b stage. To elucidate the survival impact of strap muscle invasion of any degree in thyroid cancers, the Surveillance, Epidemiology, and End Results (SEER) database (1973–2018) was queried for thyroid cancer only patients on July 2019 (n = 19,914). A recent study has suggested similar disease-free survival rate between the minimal ETE and gross strap muscle invasion alone groups in papillary thyroid carcinomas with tumor size 10- to 40-mm[14]. The impact of strap muscle invasion alone, both minimal and gross, on the survival of thyroid cancer patients needs to be elucidated to accurately allocate patients into appropriate staging groups according to individual risk, which will ensure longer disease-free survival and prevent overtreatment. Patient characteristics Age at diagnosisa < 55 years old ≥ 55 years old Sex Male Female Race White Black Other Tumor size (mm)a Pathology according to ICD-O-3 Papillary carcinoma Follicular carcinoma Other Multifocal Extrathyroidal extension (ETE) No or minimal invasion Strap muscle invasion alone Major organ invasion Major vessel invasion LN metastasis Distant metastasis Total thyroidectomy Radiation therapy Chemotherapy Follow-up monthsa
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