Histological Subtype Provides Limited Prognostic Information in Grade 1 Invasive Breast Carcinoma: A Retrospective Cohort Study

Background: Histological grade is one of the strongest prognostic factors in invasive breast cancer. However, it remains unclear whether histological subtype provides additional prognostic information in well-differentiated (grade 1; G1) invasive breast carcinomas. This study investigated the distribution of histological subtypes in G1 breast cancers and evaluated their impact on overall survival. Methods: In this retrospective single-centre study, all patients diagnosed with G1 invasive breast carcinoma in a single institution between 2005 and 2014 were identified. Histopathological and clinical data were retrieved from institutional pathology and clinical databases. Tumours were classified as no special type (NST), pure special type, or mixed subtype. Overall survival was analysed using Kaplan–Meier estimates and compared with the log-rank test. Associations between histological subtype and mortality were evaluated using the chi-squared test. Results: Of 429 initially identified patients, 364 fulfilled the inclusion criteria. NST represented the predominant subtype (67.3%), whereas pure special types and mixed subtypes accounted for 15.1% and 15.7% of cases, respectively. Tubular carcinoma was the most frequent pure special subtype, followed by mucinous, cribriform, papillary, lobular and adenoid cystic carcinoma. After exclusion of patients with simultaneous higher-grade contralateral carcinomas, 348 patients were included in survival analyses. The overall 5- and 10-year survival rates were 94.7% and 85.4%, respectively. Pure special types showed the highest survival rates numerically, whereas mixed subtypes had the lowest. However, overall survival did not differ significantly between NST, pure special types and mixed subtypes (log-rank p = 0.39). Likewise, no significant survival differences were observed among individual pure special subtypes (p = 0.20), and no significant association between histological subtype and mortality was identified (χ2 p = 0.064). Conclusions: Within this cohort of G1 invasive breast carcinomas, no statistically significant association between histological subtype and overall survival was identified. However, clinically relevant differences cannot be excluded given the heterogeneity of the cohort and the small number of events. Larger studies incorporating multivariable analyses and breast cancer-specific outcomes are needed to further clarify the prognostic relevance of histological subtype in G1 invasive breast cancer.