Doctors should not reduce chemotherapy doses for obese women with a type of breast cancer, research suggests.
Doses are sometimes reduced for the obese
Some obese patients get lower relative doses of cancer drugs because of fears they may be vulnerable to side effects from the poisonous compounds.
However, researchers found that reduced doses could compromise the survival chances of women with breast cancer not linked to the sex hormone oestrogen.
The study, by the European Institute of Oncology, is published in The Lancet.
Chemotherapy doses generally are based on the body surface area, which is calculated using height and weight.
This means that a bigger person gets a bigger dose of each chemotherapy drug.
But some doctors worry about the possible danger of giving the very high doses that are calculated using an overweight or obese patient's full weight.
There is also concern that obesity can affect the way chemotherapy drugs work.
The researchers found a higher proportion (97 out of 249) of obese patients received less than 85% of the standard recommended dose of drugs during their first course of chemotherapy compared with slimmer women.
Reduced doses of chemotherapy did not seem to affect the outcome of treatment for obese women with hormone sensitive forms of breast cancer.
But it did have a negative impact on those whose cancer was not linked to oestrogen, the team said.
Obese patients in this group whose treatment was not reduced had a significantly higher chance of remaining free of the disease.
The researchers also found that obese patients initially treated with the standard doses of chemotherapy did not show any more sign of toxic side effects than patients who received reduced doses.
Professor John Toy, medical director for Cancer Research UK, said: "There is no complete consensus on how best to balance doses of chemotherapy to achieve a maximum therapeutic benefit against acceptable patient side effects.
"Because it is known that obesity can alter the way certain anti-cancer drugs are handled by the body, doses of such drugs are sometimes reduced.
"It seems from this report that the benefits are reduced for obese, pre-menopausal women with oestrogen receptor negative tumours."
Emma Taggart, of the charity Breakthrough Breast Cancer, said more research was needed to establish exactly how weight impacted on breast cancer treatment.
"In the meantime, women who are concerned should discuss chemotherapy dosage on an individual basis with their consultant."