Abstract P4-03-38: Rare Breast Cancer Histologic Subtypes: 30-year experience in a Mexican Cancer Center
Authors: Valdez-Sandoval Paola , Martinez-Cannon Bertha Alejandra , Alvarez Sandra Ileana Perez , Leon-Rodriguez Eucario 
Published: March 2023

Abstract
Introduction Breast cancer (BC) is a heterogeneous disease composed of multiple histologic subtypes. Invasive ductal carcinoma is the most common subtype, accounting for 75% of all BC cases and is followed by lobular carcinoma which represents another 15%. Other rare histologic subtypes make up the remaining 10%. To date, limited evidence exists regarding about the frequency, clinicopathologic characteristics, and prognoses of these uncommon variants in Mexico.
Methods A single center retrospective cohort including women diagnosed with invasive BC between January 1990 and December 2019 was conducted. Only patients with rare histologic subtypes, excluding not otherwise specified ductal and lobular carcinoma, were included. The main objective of this study was to describe the frequency, clinicopathological characteristics, and outcomes of these histologic subtypes. Descriptive statistics including means, medians, and standard deviations, were used to analyze clinicopathological characteristics. Recurrence-free survival (RFS) and overall survival (OS) were calculated using the Kaplan-Meier method and compared by the log-rank test. A p value of &lt; 0.05 was considered statistically significant.
Results Out of 1744 women diagnosed with invasive BC, 106 patients (6.1%) had a rare histologic subtype. Among the most frequent subtypes, 39 (36.8%) were mucinous, 12 (11.3%) metaplastic, 11 (10.4%) tubulolobular, 11 (10.4%) papillary, 10 (9.4%) tubular, and 8 (7.5%) medullary carcinomas. In the less common histologic subtypes (referred as “others”), 7 (6.6%) were apocrine, 4 (3.8%) neuroendocrine, 1 (0.9%) micropapillary, 1 (0.9%) signet cell, 1 (0.9%) acinic cell, and 1 (0.9%) histiocytoid carcinoma.
In the overall population, the median age at diagnosis was 58 years, 76.4% were postmenopausal at diagnosis, 29.2% had a family history of BC, and 7.5% had personal history of BC. Seventy percent of the patients were overweight or obese at time of diagnosis and 82.1% had diabetes mellitus type 2 diagnosis. An important proportion of women were diagnosed by self-detected tumors (61.3%), as opposed to less than a quarter (24.5%) detected by screening mammography. The median time of diagnosis interval was 3 months, 71.7% were diagnosed with early (I – IIA), 22.6% with locally advanced (IIB – IIIC) and 5.7% with metastatic (IV) disease. Most tumors (58.3%) expressed hormone receptors (HR), 7.2% were HER2 enriched, 8.3% expressed both HR and HER2, whereas 26% were triple negative. Clinicopathological characteristics according to the most frequent histologic subtypes are listed in Table 1.
With a median follow-up of 108 months, 12 recurrences were identified (5 mucinous, 3 metaplastic, 2 tubulolobular and 2 apocrine), 41% were local and 59% were distant recurrences. Globally, 15-year RFS and OS were 83% and 61%, respectively. Differences in RFS and OS were not statistically different according to histologic subtypes. The 15-year RFS and OS was 83% and 80% for mucinous, 100% and 78% for tubular, 100% and 58% for medullary, 65% and 52% for metaplastic, 77% and 86% for tubulolobular, and 100% and 49% for papillary carcinoma, respectively.
Conclusions To our knowledge, this is the first study describing the frequency, clinicopathological characteristics, and outcomes of rare histologic BC subtypes in a large Mexican cohort. Our results show that these subtypes represent 6% of all invasive BC cases, that they are commonly diagnosed at an early stage, and present long RFS and OS.
Table 1: Clinicopathological characteristics according to the most frequent histologic subtypes
Citation Format: Paola Valdez-Sandoval, Bertha Alejandra Martinez-Cannon, Sandra Ileana Perez Alvarez, Eucario Leon-Rodriguez. Rare Breast Cancer Histologic Subtypes: 30-year experience in a Mexican Cancer Center [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-03-38.