Abstract P6-22-03: Tumor phenotype and concordance in synchronous bilateral breast cancer in young women
Authors: Pak LM , Rosenberg SM , Ruddy KJ , Tamimi RM , Peppercorn J , Schapira L , Borges VF , Come SE , Warner E , Snow C , Collins L , King TA , Partridge AH 
Published: May 2019

Abstract
Background: Synchronous bilateral breast cancer is rare, with reported incidence from 0.3-12%; the incidence and pattern of bilateral breast cancer among younger women is unknown. Here we report the incidence and phenotypes of bilateral breast cancer in women ≤40 years of age enrolled in the Young Women's Study (YWS) cohort.
Methods: The YWS is a multi-center, prospective cohort study that enrolled women with newly diagnosed breast cancer at age ≤40 years from 2006-2016. Those with synchronous bilateral breast cancer (in-situ and/or invasive) formed our study cohort. Disease characteristics and treatment were obtained by medical record review. Central pathology review was performed to capture histologic features and categorize the tumor phenotype as either luminal A (hormone receptor (HR)+, HER2-, grade 1 or 2), luminal B (HR+, HER2+, or HER2- and grade 3), HER2-type (HR-, HER2+), or triple negative (TNC; HR/HER2-). Tumor phenotypes of bilateral breast cancers were compared and evaluated for concordance.
Results: Among 1302 patients enrolled in the YWS, 20 (1.5%) patients presented with bilateral disease, with median age of diagnosis of 38 years (range 18-40). The majority of patients (13 (65%)) presented with unilateral symptoms and contralateral disease was identified on subsequent imaging. 12 (60%) reported a positive family history of breast cancer and 17 (85%) underwent genetic testing; resulting in the identification of 6 mutation carriers (2 BRCA1, 3 BRCA2, 1 TP53). The majority of patients (15 (75%)) underwent bilateral mastectomy, 1 underwent unilateral mastectomy with contralateral lumpectomy, and 4 underwent bilateral lumpectomy. On pathology, 2 patients had bilateral in-situ disease, 5 had unilateral invasive and contralateral in-situ disease, and 13 had bilateral invasive disease. Of those with bilateral invasive disease, all had concordant tumor histology (92% ductal, 8% ductal and lobular), 10 (77%) patients had bilateral luminal tumors and when fully characterized 6 were of the same luminal type. Only one patient had bilateral basal-like breast cancer.
Patient ID ERPRHer2 amplifiedGradePhenotype1Left++-2Luminal A Right++-3Luminal B3Left++-3Luminal B Right++-3Luminal B6Left++-3Luminal B Right++-3Luminal B9Left++-2Luminal A Right++-2Luminal A10Left+++3Luminal B Right++-2Luminal A12Left+--3Luminal B Right+--2Luminal A13Left---NABasal-like Right++-NALuminal A or B14Left+++2Luminal B Right++-3Luminal B15Left++-3Luminal B Right+++3Luminal B16Left+++3Luminal B Right--+NAHEr2-type17Left---3Basal-like Right---3Basal-like19Left++-2Luminal A Right++-3Luminal B20Left++-1Luminal A Right++-2Luminal A
Conclusions: Among a large cohort of young women, only 20 (1.5%) had bilateral disease, and the majority of the invasive tumors were of the luminal phenotype, yet frequently differed by grade or HER2 status; supporting the need for thorough pathologic evaluation of bilateral disease to determine risk and tailor treatment. Overall the low incidence of bilateral disease and preponderance of the luminal phenotype in this population is reassuring.
Citation Format: Pak LM, Rosenberg SM, Ruddy KJ, Tamimi RM, Peppercorn J, Schapira L, Borges VF, Come SE, Warner E, Snow C, Collins L, King TA, Partridge AH. Tumor phenotype and concordance in synchronous bilateral breast cancer in young women [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P6-22-03.