Breast cancer can happen in both breasts simultaneously or at separate times, and it can also be a clone of the original cancer (metastatic) or it can be a separate primary tumour altogether.
Researchers and clinicians have been unsure about how to classify cancer in the opposite breast (called contralateral breast cancer), but it is an issue with important implications for patient treatment and prognosis.
Without the means to know what kind of cancer it is, typically cancer in the second breast is treated as though it were a new primary tumour, independent of the first cancer.
However, women with metastatic cancer generally have a poorer prognosis and different treatment options than those with new primary tumours. That means if the cancer is a clone, or metastases, of the original primary tumour these treatments may not be effective.
A recent study has taken advantage of advances in genomic sequencing technology to look at the ‘genetic barcode’ of tumours in each breast to determine what relationship, if any, they have to each other at a chromosomal level.
This small study for the first time has shown that cancer in the opposite breast can be a metastatic spread of the first tumour.
Given the poor prognosis of a metastasising disease compared with a new primary tumour, this is a promising method for identifying these women at diagnosis so they can be given appropriate treatment, care and survival outlook.
It’s important to keep in mind that when breast cancer metastasises, it usually spreads to the brain, lungs and bones. It rarely spreads to the other breast.
There is one type of cancer that begins in the lobes or lobules, the glands that make milk (called lobular carcinoma), that is more often found in both breasts than other types of breast cancer.