A substantial minority of patients with lung or colon cancer continue to smoke after they have received their diagnosis, a new study finds.

Quitting smoking is important for cancer patients, because smoking can interfere with treatment, CNN reports.

The study included 5,300 patients with lung or colon cancer. At the time they were diagnosed, 39 percent of lung cancer patients, and 14 percent of colon cancer patients were smokers. Five months after diagnosis, 14 percent of lung cancer patients were still smoking, as were 9 percent of colon cancer patients.

The lung cancer patients who still smoked tended to be on Medicare, had received little treatment for their cancer, and were heavy smokers before they were diagnosed. The colon cancer patients who continued to smoke tended to be under-educated males who were uninsured and were heavy smokers before their diagnosis.

“These findings can help cancer clinicians identify patients who are at risk for smoking and guide tobacco counseling treatment development for cancer patients,” said lead researcher Dr. Elyse R. Park, of the Massachusetts General Hospital/Harvard Medical School in Boston.

The findings appear in the journal Cancer.