Cancer has many vicious faces, and if you’ve had cancer once, you are more inclined to develop a new cancer.
Recovered cancer patients are twice as likely to develop the exact same cancer again as people with no cancer record are to develop any specific cancer. On the other hand, the risk of developing other types of cancer than the one previously diagnosed is only slightly higher.
These are the main conclusions of an extensive new Danish study, published in the Canadian Medical Association Journal.
"Our main finding is that there is a greater risk of developing cancer if you have a previous cancer history than if you don't – and the risk of developing the very same disease is the greatest,” says co-author Dr Stig E. Bojesen of the Department of Clinical Chemistry at Herlev Hospital, Copenhagen.
Several studies have shown that people who develop one type of cancer are also prone to develop a second cancer later on in life.
Up to now the most common method of determining which cancers are related has been to study different cancers in pairs. Researchers have for example estimated the risk of women with breast cancer later developing cancer of the ovaries or the lungs. According to Bojesen, the links that have been detected with these methods are based on studies of a small number of selected patients.
"We believe our method is significantly more relevant than in previous studies," he says. "For every individual in the Danish population, we have studied the correlations among all the 27 types of cancer at the same time." (see factbox)
The calculations indicated that patients who had survived one type of cancer were 2.2 times more likely to develop the very same type of cancer later in life than people with no previous record of cancer.
The risk of developing some other type of cancer later in life, however, was only 1.1 times greater than for people with no cancer history.
The study also revealed cancer types, including prostate cancer, where patients displayed a lower risk of developing a new type of cancer, compared to healthy people. This finding came as a big surprise to the researchers.
"The striking contrast between the two situations indicates that the patient's destiny is closely linked to genetic and environmental factors which are characteristic for just that person and that disease," says Bojesen.
One of the environmental factors known to increase the risk of developing a variety of cancers is smoking. It is well-documented that smokers face an increased risk of developing various cancers such as bladder, lung and throat cancer.
The researchers have created a gigantic chart with 27 x 27 rows and columns outlining all possible combinations of first and second primary cancer instances.
Each square in the chart indicates the risk of developing one specific cancer if you have previously been diagnosed with another specific cancer.
The risk factors are arrived at by means of statistical tools that calculate data about every single individual in the Danish population register and cancer register in the period 1980-2007. That’s almost 7.5 million people.
It turned out that around 10 percent of the population had one or more diagnoses of so-called primary cancer – that is cancer which is not a recurrence of a previous cancer.
The researchers wondered whether one specific cancer that is provoked by smoking would increase the risk of developing other smoking-related cancers.
"Much to our surprise, our study showed that the increased risk of developing other smoking-related cancers for patients with a previous smoking-related cancer history was only 1.2."
This means that the risk of developing lung cancer, for instance, is only marginally increased if you have previously been diagnosed with for instance bladder cancer, as opposed to if you have no previous history of bladder cancer.
The Danish researchers were also on the lookout for 'the absolute cancer risk', which is a measure of the probability of developing a particular form of cancer over the next five years. (see box below the article)
"The take home message is that the absolute risk of developing a new cancer within five years after the first cancer is two percent," says Bojesen. "This means that the absolute risk is not alarmingly high, and that's good news because this is the actual risk that the individual patient experiences."
Cancer researchers distinguish between primary and secondary cancer types. The primary types are those that arise by themselves in the body, whereas the secondary types include relapses into earlier cancers. These relapses usually occur because some cancer cells have been immune to treatment. In cases of relapse, the patient will contract the same cancer type as he or she suffered from before.
When studying the Danish cancer register, the researchers wanted to avoid these relapse cases. They were only interested in finding the primary cancer types and then study the relationship between the various primary cancer types.
The problem is that it can be hard for a doctor to determine whether a cancer type is primary or secondary, which is why there’s a certain risk that many of the entries in the cancer register may be incorrect.
The researchers secured the validity of their conclusions by disregarding patients who had been diagnosed with the same type of cancer within the first ten years of successful cancer treatment. It is within these first ten years that the greatest risk of relapse occurs.
“Even when we disregarded the first five or ten years, we could demonstrate that the risk of developing a new primary cancer type does not decrease over time – which you would otherwise expect if cancer number two was only a relapse. So we believe our findings are reliable,” says Bojesen.