New research suggests that a treatment with a derivative of vitamin A called retinoic acid may help cut former smokers' risk of lung cancer as it is suspected that lung cells damaged during years of smoking may continue to grow and evolve into cancer even after that person has quit.

Scientists found the therapy reduced growth among those lung cells.

The University of Texas study is published in the Journal of the National Cancer Institute.

Tobacco smoking accounts for 90 per cent of the attributable risk for lung cancer, but the risk of the disease remains elevated for many years after people give up and never decreases to the level of that for non-smokers.

Nearly half of newly-diagnosed lung cancers occur in former smokers.

The researchers, from the university's MD Anderson Cancer Center, work focused on 225 people who were once heavy smokers, but who had quit smoking.

The volunteers either received a three-month treatment combining a form of retinoic acid with vitamin E; a different form of retinoic acid in isolation; or a placebo.

The researchers examined samples of lung tissue taken from all the volunteers before and after treatment. They measured proliferation of the cells by recording levels of a tell-tale chemical "biomarker" called Ki-67.

Both treatments reduced cell proliferation in one layer of the lung cells, the parabasal layer but the researchers were surprised that neither reduced cell growth in a second, the basal layer.

They say more work will be needed to tease out the exact effects of retinoic acid treatment.

Dr Eva Szabo, of the US National Cancer Institute, agreed that more research was needed before the therapy could be tested in more advanced clinical trials.

She said: "We do not have a full understanding of the effects of these agents on [lung cells] or their effects during the full spectrum of carcinogenesis."

Josephine Querido of Cancer Research UK, said: "The effect of vitamin derivatives and supplements on lung cancer is unclear, so giving up smoking is by far the best way for smokers to reduce their risk of the disease."

"These early results are intriguing, but much more work is needed before we know for sure whether these chemicals could prevent, or slow, lung cancer growth."

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