Local doctors have long known that those with a history of smoking - typically men - run a high risk of getting lung cancer. But they have increasingly noticed an unsettling trend here - a sizeable proportion of their patients are Asian, female and have never touched a cigarette.
"They had never smoked; there was no exposure to cigarette smoke in their homes," said Dr Akhil Chopra, a senior consultant in Tan Tock Seng Hospital's (TTSH) medical oncology department, who had formerly worked in the United States.
"That was in contrast with what you normally see in the West."
What these lung cancer patients tend to have in common is a mutation in a gene associated with cell growth, he and his team found.
7,000 new cases between 2010 and 2014
In Singapore, nearly 7,000 new lung cancer cases were diagnosed between 2010 and 2014, according to figures from the National Registry of Diseases Office.
It ranks among the top three most common cancers in men and women, and is the top cause of cancer death in men.
Among women, it is the second-most common cause of cancer death, after breast cancer.
Between 2008 and 2012, the age-standardised five-year relative survival ratio for men was 11 per cent and that for women was 16 per cent.
This means that after taking into account age differences, less than a fifth of lung cancer patients are alive after five years.
Doctors say that many lung cancers do not show symptoms until the cancer is at an advanced stage. Symptoms include persistent coughing, shortness of breath and weight loss.
The gene - known as the epidermal growth factor receptor, or EGFR, in medical parlance - is involved in making cells grow and multiply. Its mutated version causes cells to multiply excessively.
In lung cancer patients, the result is adenocarcinoma - a type of cancer that starts in the mucus-producing glands in the lungs.
Doctors say that these mutations happen spontaneously and that the trigger is still unknown. They are not passed down from parent to child.
"There is no proof at the moment that these mutations are hereditary," said Dr Chin Tan Min, senior consultant at the National University Cancer Institute, Singapore's (NCIS') department of haematology-oncology.
What is clear is that they occur much more frequently among Asian women, with up to half of Asian adenocarcinoma patients testing positive for them. In contrast, said Dr Tan Chee Seng, also from the NCIS, the incidence rate among Caucasian patients is around 10 per cent.
A 2006 study by the National Cancer Centre Singapore (NCCS) found that three in 10 lung cancer patients are "never-smokers" and the incidence is rising.
The next step, say doctors, is to find a way to stop these mutated proteins from having an effect.
"Drugs that block these proteins have been shown to be effective in controlling cancer cell growth and improving survival in these patients, particularly in advanced-stage lung cancer," said Dr Ang Mei-Kim, senior consultant from the NCCS divison of medical oncology.
Doctors here say that the reason why they seem to be identifying more cases is twofold. First, more molecular tests on lung cancer are being done, said Dr Chin. This means that doctors are likely to identify cases like these more often.
Also, said Dr Aneez Ahmed, chief of TTSH's thoracic surgery division, more cases are being picked up by chance. "The reason may be that people are more health-conscious and go for health screenings," he said.
However, he stressed that people should not randomly sign up for screening tests and should seek follow-up tests only if there is a problem, such as a persistent cough.