Development of breast cancer after thyroid cancer and trends over time.
Authors: Ahuja Kriti , Shrivastava Trilok , Shah Mihir , Dasgupta Mona , Abate Elias , Ferreira de Araujo Litvin Rafaella , Batra Kunnal , Gupta Shweta 
Published: June 2022

12066 Background: Thyroid cancer occurs early in adulthood and is thrice as common in women. Further, 1 of 8 women in the U.S develop breast cancer, usually later in life. We are studying the development of breast cancer after thyroid cancer in women within the US. Methods: The Incidence – SEER Research Data, 18 registries, Nov 2020 Sub (2000-2018) database was queried with SEER*Stat 8.3.9.2 for thyroid cancer in females and subsequent breast cancer. Further, the MP-SIR function of SEER*Stat 8.3.9.2 was applied to evaluate the Incidence – SEER Research Data, 18 Registries (excl AK), Nov 2020 Sub (2000-2018) for risk of developing breast cancer after thyroid cancer in females in terms of SIR (Standardized Incidence Ratio) and Excess Risk, stratified by age categories. Results: From 2000 to 2018, 151654 cases of thyroid cancer were diagnosed in 150787 women, most of which were papillary adenocarcinoma, NOS (50.2%) and papillary carcinoma, follicular variant (26.8%). About 64% cases occurred in Non-Hispanic Whites (NHW) and 60% of all cases developed from ages 30 to 60 (̃ 20% per decade). There was a considerable rise in incidence over the years, from > 4000 cases in year 2000 to > 9500 cases in 2018, with percentage change (PC) of 122.3%, and Annual Percentage change (APC) of 4.6% (CI 3.6-5.7, p < 0.05) across all age groups, highest in 60-69 age group (APC 7.5; CI 6-9.1, p < 0.05). At follow-up, 3300 cases of primary breast cancer were diagnosed in 3062 women of the above population. Infiltrating duct carcinoma, NOS constituted 70.5% cases, followed by lobular carcinoma, NOS at 10%. About 4 out of 5 breast tumors were ER positive (82.6%). NHW constituted > 70% cases and >50% cases occurred in age group 50-69 (approximately 25% per decade). We found an overall increasing trend in frequency of breast cancer after thyroid cancer with APC 14.1 (CI 11.9-16.3, p < 0.05). MP-SIR function in Incidence – SEER Research Data, 18 Registries (excl AK), Nov 2020 Sub (2000-2018) indicated that 3005 women developed breast cancer after thyroid cancer (consistent with the above data as it has latency criteria of 2 months and excludes AK). Overall, an increased risk of developing breast cancer after thyroid cancer was found, with SIR 1.21 (CI 1.16-1.25, p < 0.05) and Excess risk 4.88 across all age categories, highest at 1 year following diagnosis of thyroid cancer with SIR 1.29 (CI 1.15-1.44, p < 0.05) and excess risk 5.91. Among different age categories, SIR was highest for ages ≥70 at 1.32 (CI 1.2-1.45, p < 0.05) with excess risk 13.45. Conclusions: Women with thyroid cancer are at increased risk for subsequent primary breast cancers (usually ER+), with an overall increasing trend from 2000 to 2018. As we continue to make progress in the diagnosis and treatment of thyroid cancer in women, it is important to be cognizant of their increased risk for developing a subsequent primary and possible significance in terms of breast cancer screening.