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Patients surviving certain types of cancer may be at an increased risk of a second cancer of the same type

Nov. 28, 2011

By Maria Serraino

Cancer survivors have more than double the risk of a second primary cancer of the same type as the first, but their risk of developing a different type of cancer is only slightly higher than the rest of the population, according to a study in the CMAJ.

A second primary cancer is a cancer occurring in someone who has already had cancer at any time in the past and occurs when the first cancer has spread to another area. These second cancers can be the same type as the first, or can be of a different type, but are not relapses of the first. According to the study, second cancers are seen in 15 per cent of cancer survivors worldwide.  

Danish researchers conducted a study using databases covering the entire population of Denmark (7,493,705 people) from 1980 to 2007 to determine whether the risk of second cancers is associated with the first type of cancer. They compared incidence of second cancer in cancer survivors to the incidence of cancer in a control group representing the general population. Overall, they observed 74 significant associations among 27 types of first cancer and 27 possible types of second primary cancer.

They found that 765,255 people – about 10 per cent of the population – had one or more primary cancer diagnoses. The researchers found that the risk of developing any future case of cancer was 25 per cent higher in cancer survivors, and the risk varied depending on the type of cancer. Most of the risk was related to having the same type of cancer they had before.

Survivors had more than double the risk of a second cancer of the same type as their first diagnosis. This risk was reduced after prostate cancer and increased after larynx cancer.

The risk of a different type of second cancer was found to be 1.1-fold, meaning that the risk was only about 10 per cent higher. It, too, was reduced after prostate cancer and greatest after larynx cancer.

The researchers explain that risks of second cancers vary by cancer type, and seem to be associated with a patient’s genetic and lifestyle factors, possibly predisposing the person to both cancer types.  

Also, the diagnostic or monitoring exams for one type of cancer may influence the likelihood of detecting another type of cancer. Further, the treatment of the first type of cancer may affect the chances of secondary cancer. For example, surgically removing much or all of the affected area associated with the first cancer (such as a mastectomy in the case of breast cancer) may reduce risk of the same type of cancer, while chemotherapy or radiation may increase risk of second cancer of the same or different type.

The researchers also looked at smoking as a risk factor for second cancers, because it is known to increase the risks of many types of cancer. They found that in the individual cancer survivor, the increased risk of a new cancer is mainly confined to the same type of cancer as the first. The same held true for people who smoke. 

They suggest that future studies should clarify whether the second cancers are due to shared genetic or lifestyle risk factors, treatment of the first cancer, or the timing of diagnosis of the first cancer. The findings may be helpful to clinicians in developing programs for their patients who have cancer, by focusing on second cancers with the highest risks.

The study was published in the Canadian Medical Association Journal on Nov. 28, 2011. 

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  • Women's College Hospital