Study: Smoking before or after a breast cancer diagnosis associated with poorer breast cancer survival

IN DEPTH REVIEW OF RESEARCH

Study background:

Previous research linked cigarette smoking before breast cancer diagnosis to lower breast cancer survival but little is known about how smoking after a breast cancer diagnosis affects breast cancer survival.

In January 2016, Michael Passarelli and colleagues from the University of California, San Francisco and other institutions published a study in the Journal of Clinical Oncology about how smoking before and after a breast cancer diagnosis affects breast cancer survival.

Researchers of this study wanted to know:

Whether an association exists between breast cancer survival and smoking before or after a diagnosis of breast cancer.    

Population(s) looked at in the study:

About 21,000 women from the Collaborative Breast Cancer Study (CBCS) participated in this study and:

• had invasive breast cancer.
• were between the ages of 20-79.
• were from Wisconsin, New Hampshire, and Massachusetts.

The women were asked several key questions about smoking: whether they had smoked at least 100 cigarettes during their lifetime, when they started, how long they had been smoking, the average number of cigarettes they smoked each day, whether they had smoked one year before they were diagnosed with breast cancer, if they currently smoked, and (for the women who had quit) the age they quit.

About 15,000 women from the CBCS study were invited to participate in the Collaborative Women’s Longevity Study (CWLS).  The women who completed the survey were all breast cancer survivors and were an average of 6 years post-breast cancer diagnosis. This questionnaire asked the women for information about their post-diagnosis exposures and health events. 

Study findings: 

1. Women who smoked one year before their breast cancer diagnosis had a 25% increased risk of dying from their breast cancer than women who never smoked. About 14% of women who had never smoked died from breast cancer compared to about 17% of women who smoked one year before they were diagnosed.
   • Women who smoked one year before their breast cancer diagnosis were:
     • 14 times more likely to die of respiratory cancer than women who had never smoked. Less than .5% of women who had never smoked died from respiratory cancer compared to about 4% of women who smoked one year before they were diagnosed with breast cancer.
     • 2 times more likely to die of cardiovascular disease than women who had never smoked. About 6.5% of women who had never smoked died from cardiovascular disease compared to about 8% of women who smoked one year before they were diagnosed with breast cancer.
2. Women who quit smoking after their breast cancer diagnosis had a 33% decreased risk of dying from breast cancer than women who continued to smoke after diagnosis. This means that women who quit smoking post breast cancer diagnosis were more likely to survive than the women who kept smoking.
3. 10% of women in the study reported that they kept smoking after their diagnosis. These women had a 72% increased risk of dying from breast cancer than women who never smoked. Using the data from the CWLS population, about 5% of women who had never smoked died from breast cancer compared to about 8% of women who were still smoking.

Limitations:

Conducting a study based on responses from a self-reported questionnaire has limitations. It is possible that some women may not have accurately presented their smoking status. The study design allowed only one opportunity to follow up with the smoking status of women after diagnosis; because the study represents only a snapshot, it may not have been entirely accurate.

Nor did this study take into account the hormone receptor status of breast tumors, or whether or not women had a mutation in BRCA or another gene that increases cancer risk; whether these factors combined with smoking affect breast cancer survival—and to what extent—is unknown.

Conclusions:

This research provides evidence that smoking affects breast cancer survival before and after a breast cancer diagnosis, and showing that quitting after breast cancer diagnosis has benefits. However, according to the study authors, “Regardless of a diagnosis of breast cancer, smokers should undergo recommended respiratory and cardiovascular disease surveillance to reduce smoking-related survival.”