However, Dr Anna-Sofia Berglund said that since these were relatively rare cancers and the increase in the risk was small, women should not be alarmed by her findings. Furthermore, endometriosis appeared to reduce the risk of cervical cancer in comparison to the general population, and there was no increased risk of ovarian cancer in women who had had a hysterectomy.
Dr Berglund, a resident obstetrics and gynaecology doctor at Huddinge University Hospital, Stockholm, Sweden, looked at the Nationwide Swedish Inpatient Register and identified 64,492 women who had been discharged from hospital with endometriosis between 1969 and 2000. The data were linked with the National Swedish Cancer Register to identify all cases of cancer.
She found that there was no overall risk of cancer amongst these women, but that there was a slightly increased risk of certain types of cancer. A woman's risk of developing ovarian cancer increased by just under a half during her lifetime, for endocrine tumours it increased by about a third, for non-Hodgkin's lymphoma it increased by about a quarter and for brain tumours the increase was just over a fifth. The risk of developing cervical cancer was reduced by roughly a third.
Dr Berglund said: "It is very important to keep these findings in perspective. The overall risk of cancer does not increase after endometriosis, and where there are slightly increased risks they are in some of the less common cancers. For instance, in Sweden just under 20 women in every 100,000 develop ovarian cancer each year – there are about 855 cases a year. My study shows that for women with endometriosis, another eight women in 100,000 could develop ovarian cancer a year. However, it may be even fewer than that, because we do not know how many of the 855 women with ovarian cancer also had endometriosis. Similarly, for endocrine tumours there are 13 cases per 100,000 women in Sweden and another five women with endometriosis might develop these tumours each year; for non-Hodgkins's lymphoma (12 cases a year amongst Swedish women) there would be another three cases, and for brain tumours (18 women a year) there would be another four cases."
Women who had had a hysterectomy before or at the time that endometriosis was diagnosed did not show an increased risk of ovarian cancer. "Previous studies have shown that hysterectomy protects against ovarian cancer and we found that amongst the 7,811 women in our study who had had a hysterectomy there was a lower risk of ovarian cancer. This suggests that hysterectomy in endometriosis patients may have a preventive effect against ovarian cancer. However 80% of these women had a form of endometriosis called adenomyosis (that is endometriosis in the muscle wall of the uterus) and our study showed that they had a lower risk of ovarian cancer from the beginning. Therefore we can't say for sure that it is the hysterectomy that is protective."
The study also showed that younger women who developed endometriosis between the ages of 20 and 40 had a higher risk than other age groups of getting ovarian cancer and that the cancer developed earlier in their lives than in the general population. However she said screening was not the answer. "It is too early to recommend that young fertile women with endometriosis should have extra screening or ovarian surgery, because there is no reliable screening method for ovarian cancer. We need to study this further in order to identify the patients that are at risk of getting cancer."
Dr Berglund warned that the study did not show that endometriosis causes cancer. "We cannot say that endometriosis causes cancer, only that patients with endometriosis have a slightly higher risk of some kinds of cancers than the general female Swedish population. The true connection between cancer and endometriosis is not known."
She said the findings required further investigation. "We are trying to discover the specific characteristics of endometriosis that might lead to cancer. For instance, if we could find a marker for malignancy in endometriosis then it would be possible to develop a blood test that would tell us if a patient had a risk of developing cancer later in life."
Abstract no: O-233 (Wednesday, 10.00hrs CET, Roma room)
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