Ava Kwong, chief of breast surgery in Tung Wah Hospital, is keen to make the warning after finding out that many patients discontinue their follow-up checks after five years, thinking they are completely safe from the disease recurring.

Kwong warns there is a 5.1 percent risk of a relapse in the seventh year, which is higher than the 4.5 percent risk in the fifth year.

The highest risks are in the first two years, 13.3 percent and 11.9 percent.

These figures have been established by the Hong Kong medical sector.

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One of the culprits blamed for relapses is an estrogen- producing enzyme known as aromatase. The enzyme can boost the level of the female hormone estrogen, increasing the risk of a recurrence. Aromatase is found in estrogen-producing cells - the adrenal glands, ovaries, placenta, fat tissue and the brain.

Scientists have invented a new generation of anti-hormone treatment, known as Letrozole, to block the action of aromatase and control the levels of estrogen.

The latest findings, published in The New England Journal of Medicine, show Letrozole can reduce the risk of a recurrence in postmenopausal patients by 19 percent, compared with the conventional treatment, Tamoxifen.

The six-year follow-up also shows Letrozole can boost overall survival chances by 19 percent.

However, Kwong said that instead of simply relying on the new treatment, patients should continue with their regular medical checks, no matter how long after they have recovered.

According to research, the rate of recurrences is 2.1 percent in the 10th year and 1.6 percent in the 11th year.

"When patients ask me for suggestions about their lifestyle and diet to control the risk of recurrence, I tell them to continue the follow-up medical checks at least once a year and to cut down on high-fat diets. High fat stimulates the production of estrogen, and increases the risk of a relapse."

Kwong is particularly concerned about the drastic rise in breast cancer cases in Hong Kong.

The number of patients has jumped 60 percent from 1,608 in 1997 to 2,584 in 2006.