Study finds minimal risk of second cancer in breast cancer survivors

For women diagnosed with early breast cancer, the long-term risk of developing a second primary cancer is low (around 2-3% greater than women in the general population), finds a study published by The BMJ today.

The researchers say this information can help reassure many breast cancer survivors who believe their risk of a second primary cancer is much higher than these results suggest.

Breast cancer survivors are at increased risk of developing second primary cancers, but risk estimates from previous studies are inconsistent. The risk and type of a second cancer can also be affected by the initial treatment received, as well as social, lifestyle and genetic factors.

To address this uncertainty, researchers used data from the National Cancer Registration and Analysis Service for England to estimate the long-term risks of second primary cancers compared with the general population, and the various factors associated with these risks.

Their findings are based on 476,373 women diagnosed in England from 1993 to 2016 with early invasive breast cancer at ages 20-75 years who underwent surgery.

Over a follow-up period of up to 20 years, 64,747 women developed a second primary cancer, but the absolute excess risks compared with risks in the general population were small. 

By 20 years, 13.6% of women had developed a non-breast cancer (mainly womb, lung or bowel cancer), 2.1% more than expected in the general population, and 5.6% had developed a contralateral breast cancer (on the other side of the body), 3.1% more than expected.

When patients were grouped by age at first breast cancer diagnosis, the excess risk of a second non-breast cancer varied little across age groups. However, the excess risk of a contralateral breast cancer was greater in younger than in older women.

For example, for a woman whose first breast cancer was diagnosed when aged 60, her estimated risks of developing a new cancer by the age of 80 are 17% for non-breast cancer and 5% for contralateral breast cancer compared to risks of 15% and 3% respectively for women of the same age in the general population.

For a woman diagnosed when aged 40, her estimated risks of developing a second cancer by the age of 60 are 6% for both non-breast and contralateral breast cancer compared to 4% and 2% respectively for women in the general population.

When patients were grouped according to other (adjuvant) treatments they received after surgery, radiotherapy was associated with higher rates of contralateral breast and lung cancers, endocrine therapy with uterine cancer (along with reduced contralateral breast cancer), and chemotherapy with acute leukaemia.

Based on these results, the researchers estimate that around 7% of excess second cancers may be due to the use of adjuvant therapies, but note that their benefits outweigh this small risk in almost all circumstances where these treatments are recommended.

The authors acknowledge that their findings may be affected by incomplete cancer registry data for some variables. What's more, they did not have information on family history, genetic predisposition and lifestyle choices such as smoking.

However, this long-term study describes the development of second cancers according to multiple patient, tumour and treatment characteristics amongst all women with early invasive breast cancer so the findings can reliably inform breast cancer patients and the clinicians who treat and support them.

They are also relevant to clinical practice and policy, and may help inform the design of future studies to assess cancer risk.

These results are reassuring and should be shared widely, say patients in a linked opinion article, who note that finding detailed information on the risks of second cancers after breast cancer was particularly difficult.

In general, the benefits of treatments in protecting patients from recurrence of breast cancer far outweigh the potential downsides, the patients write. Such information should be available and offered by clinicians at the time that adjuvant therapies are discussed.

They acknowledge that not everyone will want all the details at diagnosis, but say it should be there for those who do want it and for those who seek it later. "Information on risks should be readily available. It helps us to plan our lives and think ahead to the future," they conclude.