Washington: A large number of people, believe it or not, continue to puff away merrily even after being diagnosed with lung and colorectal (large intestine) cancer.

Putting a halt to smoking after a cancer diagnosis is important because it can negatively affect patients' responses to treatments, their subsequent cancer risk and, potentially, their survival.

Elyse R. Park from the Massachusetts General Hospital - Harvard Medical School in Boston led a team that looked to see how many patients quit smoking around the time of a cancer diagnosis, and which smokers were most likely to quit.

The investigators determined smoking rates around the time of diagnosis and five months after diagnosis in 5,338 lung and colorectal cancer patients, the journal Cancer reports.

At diagnosis, 39 percent of lung cancer patients and 14 percent of colorectal cancer patients were smoking. Five months later, 14 percent of lung cancer patients and nine percent of colorectal cancer patients were still smoking, according to a Harvard statement.

These results indicate that a substantial minority of cancer patients continue to smoke after being diagnosed. Also, although lung cancer patients have higher rates of smoking at diagnosis and following diagnosis, colorectal cancer patients are less likely to quit smoking following diagnosis.

Colorectal cancer patients who continued to smoke tended to be male, completed less education, uninsured, not have had surgery, and once smoked a high number of cigarettes per day.

"These findings can help cancer clinicians identify patients who are at risk for smoking and guide tobacco counselling treatment development for cancer patients," said Park.