Incidence of breast cancer subtypes in Belgium: a population-based study

The aim of this study is to provide a reference for the Belgian breast cancer population, offering detailed information on various patient and tumour characteristics for the breast cancer population as a whole, as well as for the different molecular subtypes. Incidence data for primary invasive breast cancer in females diagnosed in 2014 were selected in the Belgian cancer registration database and underwent individual manual reviewing of the pathology protocols. Subsequently, in 95% of the study population a surrogate molecular subtype was successfully derived, using the combined expression of oestrogen receptor, progesterone receptor, human epidermal growth factor receptor-2, and tumour differentiation grade as surrogate for the proliferation marker Ki67, in conformity with the 2011 St Gallen surrogate classification. Ultimately, differences between the molecular subtypes regarding initial presentation and histopathological features were evaluated by means of a Pearson Chi-squared test for independence. Furthermore, relative survival was calculated for the different molecular subtypes. Histologically, the large majority of the Belgian breast cancer population presents with invasive breast carcinoma of no special type (NST), formerly called invasive ductal carcinoma (75.2%), 14.5% with invasive lobular carcinoma and 5.8% with mixed ductal/lobular invasive carcinoma. Less than five percent of the population harbours less frequently occurring histological subtypes. The Belgian breast cancers are predominantly of the luminal A-like subtype (54.4%), followed by the luminal B-like HER2 negative (14.7%) and the luminal B-like HER2 positive subtype (12.2%). The mean age at diagnosis is 62 years, with almost a third of the patients being 70 years or older. One out of five patients is younger than 50 years, and in the triple negative population this group counts for 31.9%, compared to 16.6% in the luminal A-like breast carcinomas. Most patients (69.4%) are diagnosed with early stage breast cancer (clinical stage 0-II); six percent of the breast cancers are clinically metastasised at the time of diagnosis. For 19% of the patients, information on clinical stage was lacking or staging was not applicable. The unadjusted five-year relative survival proportion for the Belgian cohort is 91.4%. Luminal A-like breast cancer opposed to triple negative breast cancer have the best and worst relative survival, with respectively 96.8% and 77.4% five-year relative survival proportions.