Using the American Thyroid Association Risk-Stratification System to Refine and Individualize the American Joint Committee on Cancer Eighth Edition Disease-Specific Survival Estimates in Differentiated Thyroid Cancer

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Abstract
Background The 8th edition AJCC staging system has reclassified up to one third of differentiated thyroid cancer patients into one of the younger prognostic stage groups (<55 years of age at diagnosis, stage I or stage II). This reclassification widens the spectrum of disease in these lower stages without significantly impacting overall disease specific survival (DSS) for the entire stage group. However, the optimistic DSS estimates in the < 55 year old stage groups may not accurately reflect the prognosis of individual patients with American Thyroid Association (ATA) high risk features. Therefore, the aim of this study was to integrate the ATA risk classification system into the 8th edition AJCC staging system to refine and individualize DSS estimates for differentiated thyroid cancer patients < 55 years of age at diagnosis. Methods Using the Memorial Sloan Kettering Cancer Center (MSKCC) tumor registry, we retrospectively analyzed 4881 adult DTC patients < 55 years old at diagnosis receiving initial therapy between 1980 and 2016. Using MSKCC registry coded data, we assigned all patients an 8th edition AJCC stage (I or II), ATA risk of recurrence (low, intermediate, high), and age group at diagnosis (younger patients defined as ≤ 45 yrs old, older patients defined as 45-55 years old). The primary outcome was 10 year DSS. Results We observed 122 disease-related deaths (2.5%) in our cohort of 4881 patients, during a median follow up of 6.6 years. Integration of the AJCC stage, ATA risk, and age groups identified six subgroups with differing outcomes: (1) stage I/ATA low risk, younger and older, 100% DSS, (2) stage I/ATA intermediate risk, younger and older, 98% DSS, (3) stage I/ATA high risk, younger, 95% DSS, (4) stage I/ATA high risk, older, 89% DSS, (5) stage II/ATA high risk, younger, 78% DSS, and (6) stage II/ATA high risk, older, 61% DSS. Conclusions: Integration of AJCC stage, ATA risk, and age group (a) identifies six subgroups of patients with progressively worse DSS as AJCC stage, ATA risk, and age increases and (b) provides a more individualized estimate of disease specific survival, especially in ATA high risk patients.

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