By Michelle Roberts
BBC News, Health reporter in Lyon
Women with the painful disease endometriosis are prone to certain cancers, mounting evidence suggests.
In endometriosis, uterine tissue is found elsewhere in the body
French researchers told a fertility conference in Lyon how risk of ovarian, kidney and thyroid cancers rises by more than a third with the condition.
Meanwhile, Spanish researchers described a potential new endometriosis drug treatment.
In endometriosis, the type of tissue that lines the inside of the womb is found elsewhere in the body.
When a woman menstruates, the tissue degenerates, causing pain, bloating, heavy bleeding and fatigue.
The condition is known to be linked with certain cancers and with difficulty conceiving, although it has been unclear exactly why.
Dr Anna-Sofia Melin and colleagues at the Karolinska Institute in Stockholm, Sweden, looked at data on 63,630 women with endometriosis.
They identified 3,822 cases of cancer among these women - which is no more than the general population.
However, certain tumour types, including ovary and brain, were far more common with endometriosis.
Dr Melin said various theories for the link between endometriosis and cancer had been put forward.
"It could be that defects in the immune system allow the endometriosis to grow and also might allow cancer cells to grow in different parts of the body.
"Maybe the treatment of endometriosis can influence cancer development. We do not know yet."
The study showed that the cancer risk remained similar regardless of how many children a woman had.
The researchers said this made it unlikely that fertility problems associated with endometriosis were to blame for the elevated cancer risk.
'Not cancer condition'
Dr Edurne Novella-Maestre and colleagues from the Valencia Infertility Institute in Spain told the ESHRE conference how a new treatment might block endometriosis and the associated cancers.
In mice, drugs that mimic the brain chemical dopamine - similar to those used to treat Parkinson's disease - stopped the formation of new blood vessels which feed the abnormal growths.
Professor Jan Brosens, honorary consultant gynaecologist and obstetrician at Imperial College London, said: "Endometriosis is indeed dependent upon the growth of new vessels.
"Furthermore, dopamine agonists are safe drugs, which could be useful in reducing leakage of newly formed vessels.
"However, endometriosis is a slow and progressive disease and whether this class of drugs would really improve the symptoms of affected women remains speculative."
He said endometriosis should not be viewed as a condition that leads to cancer.
But Mary Lou Ballweg, of the Endometriosis Association, said: "There's a danger doctors might feel women don't want to know about the extra cancer risk.
"But what's worse, to be a little afraid and watchful for symptoms or to be dead?
"Women need to be seeing a specialist to ensure they are getting the best possible help."
Dr Joanna Owens, Cancer Research UK's senior science information officer, said: "Previously published research suggests that endometriosis can affect cancer risk and further studies are needed to fully explain this link.
"Our advice to all women is to know your body, contact your doctor promptly if you notice any changes and attend screening when invited."