How do age and type of breast cancer relate to brain metastasis?

This paper analyzes the effect of age and type of cancer on brain metastasis in breast cancer patients. 

Brain metastasis(cancer that spreads to brain) occurs in 3-6% of breast cancer patients. Age and receptor status (absence or presence of particular receptors, proteins on the surface of the cell) are risk factors for brain metastasis. Types of receptors include the estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 (HER2). A younger age, absence of estrogen receptor andover expression of HER2 increase the risk of brain metastasis. The authors studied the relationship between age and brain metastasis among the different breast cancer subtypes.

2,248 female patients with stage I-IV invasive ductal carcinoma (a type of cancer) were studied. Patients were classified into 4 subtypes based on their receptor status. The subtypes are: luminal A (estrogen and/or progesterone receptor present, HER2 absent), luminal B (HER2, estrogen and/or progesterone receptor present), HER2-enriched (HER2 present, estrogen and progesterone receptor absent) and triple negative breast cancer (absence of HER2, estrogen and progesterone receptor). Patients were also divided into 3 groups by age: 35 years and younger, 36-59 years and more than 60 years. All patients received treatment (including hormone treatment, cheotherapy or radiotherapy were indicated) and follow up.

During follow up, 15.6% of patients had recurrent (returning) disease and 10.8% of patients died of breast cancer. After an average follow-up of 54 months, 7.3% of patients developed brain metastases. Overall the estimated 5-year proportion of patients with brain metastasis was 8.2% and the average time from diagnosis to brain metastasis was 35.3 months. Among the subtypes, the proportion of patients who developed brain metastasis was 15.7% in the HER2-enriched group, 15.8% in the triple negative group, 3.7% in the luminal A group and 7.2% in the luminal B group. Among the 3 age groups, the proportion of patients who developed brain metastasis was 20.5% in patients 35 years and younger, 7.8% in patients aged 36–59 and 7.5% in patients 60 and older.

Patients with HER2-enriched patients were 2.53 times more likely than luminal A patients to have brain metastasis. Triple negative breast cancer patients were 4.42 times more likely than luminal A patients to have brain metastasis.

Patients aged 35 or younger were 2.09 times more likely to have brain metastasis overall. Patients aged 36-59 with HER2 enriched disease or triple negative breast cancer had significantly higher risks of brain metastasis compared to with patients with luminal A disease in the same age category. Patients older than 60 with luminal A subtype had significantly lower risk for brain metastasis compared to patients with other subtypes. 

The authors concluded that in younger patients, risk of brain metastasis does not depend on biological subtype, while older patients with luminal A or luminal B disease have significantly lower risk of brain metastasis compared with those with HER-2 enriched and triple negative breast cancer.

There was a relatively small number of patients aged 35 and below in the study which might influence results.