A study published in The BMJ today found that the risk of a woman developing a secondary cancer after a diagnosis of early breast cancer is very low. It’s only around 2-3% higher than the women in the general public.

Researchers say that this information will help to reassure survivors of breast cancer, who may believe that their risk for a secondary primary cancer is higher than the results indicate.

The risk of breast cancer survivors developing a second primary cancer is increased, but estimates from prior studies are inconsistent. Risk and type of second cancers can be affected by initial treatment, social, lifestyle, and genetic factors.

Researchers used data from England’s National Cancer Registration and Analysis Service to estimate long-term risks associated with second primary cancers, compared to the general population. They also looked at the factors that are associated with those risks.

The study was based on the results of 476,373 women who were diagnosed with early invasive cancer in England between 1993 and 2016 at ages 20 to 75 years. These women underwent surgery.

In a 20-year follow-up, 64,747 women developed secondary cancers, but the relative excess risk compared to the risks of the general population was small.

By the age of 20, 13.6% of women developed non-breast (mainly womb or lung cancer) which is 2.1% higher than the expected rate in the general population. 5.6% developed contralateral breast carcinoma (on the opposite side of the body), which was 3.1% higher than the expected rate.

The excess risk for a second cancer other than breast was not different between age groups when patients were grouped according to their age at the time of first diagnosis. The excess risk of contralateral breast cancer is higher in younger women than older women.

Women of the general population have a risk of 15% for breast cancer, and 3% for non-breast.

A woman diagnosed with non-breast or contralateral breast cancer at age 40 is estimated to have a 6% risk of developing another cancer by 60, compared to the 4% and 2 % respectively in the general female population.

Radiotherapy, along with a reduced rate of contralateral lung and breast cancer, and endocrine treatment with uterine carcinoma (alongside reduced contralateral breast tumors), were associated with increased rates of cancer contralateral to the surgery.

The researchers found that the use of adjuvant treatments may have contributed to 7% of second cancers. However, they note that the benefits of these therapies outweigh the small risk almost in all cases where these treatments are prescribed.

The authors admit that incomplete data from the cancer registry may have affected their results. They also did not have any information about family history, genetics, or lifestyle choices like smoking.

This long-term study, however, describes the development and progression of second cancers based on multiple patient, tumour, and treatment characteristics in all women with early-invasive breast cancer. The findings will be useful to breast cancer patients, as well as clinicians and other healthcare professionals who support and treat them.

These studies are relevant for clinical practice, policy and could help to inform future research to assess cancer risks.

Patients in an opinion piece linked to this article say that these results are encouraging and should be widely shared. They note that it was difficult to find detailed information about the risk of second cancers following breast cancer.

Patients write that in general, benefits of treatment to protect patients against a recurrence are far greater than any potential downsides. These information should be made available by the clinicians when adjuvant treatments are discussed.

They admit that not everyone wants all the information at the time of diagnosis, but they say that it should be available for those who want it or later. Information on risk should be easily accessible. “It helps us plan our lives and look ahead,” they conclude.