Despite Canada's universal healthcare system, poorer Canadians with cancer are more likely to die early than their wealthier peers, suggests a new study of almost 100,000 patients from Ontario.

But unlike studies conducted in the United States, most of the difference in survival rates could not be explained by how early doctors caught the cancer.

"It is reassuring that stage of cancer (at diagnosis) does not vary across social groups in Ontario," Dr. Christopher Booth of the Queen's University Cancer Research Institute, the study's lead author, told Reuters Health in an email. But, he continued, "we need to better understand why survival does vary across socioeconomic groups."

The authors identified patients diagnosed with six different types of cancer in Ontario between 2003 and 2007. They used records of patients' home addresses to group the patients based on income levels in their communities. Then, the researchers calculated how likely patients in each group were to die of cancer in the three years after they were diagnosed or to die from any cause in the five years after.

They also looked at how early the cancer was caught for about 40,000 of those patients.

Poorer patients with cancer of the breast, colon, or larynx (voice box) were significantly more likely to die from cancer within three years than wealthier patients, while there was no significant difference for rectal, cervical, or nonsmall cell lung cancer.

For all cancers, wealthier patients had a better chance of being alive in five years than poorer patients. The biggest difference was in women with cervical cancer: 63 percent of the poorest group of patients survived for at least five years, compared to about 79 percent of the wealthiest group.

For the most part, differences in survival rates weren't a reflection of poorer patients getting diagnosed later, the authors report in the journal Cancer. On average, only poorer patients with breast and rectal cancer had more advanced disease when they were diagnosed compared with wealthier patients. And even these differences were only big enough to explain a "modest" fraction of the gap in survival rates at most, the authors say.

Previous research has shown that a patient's social class is a key factor in determining how likely that person is to survive a cancer diagnosis.

In studies done in the United States, researchers have found that wealthier cancer patients are significantly more likely to survive - largely because they have better access to screening and are more likely to have the cancer caught early.

Canada's universal healthcare system could be helping to close this gap in early diagnosis, Booth said. But it hasn't eliminated other factors that put the poor more at risk than the wealthy. "Factors such as nutrition, smoking and exercise vary widely across social groups and of course have a major impact on the health of an individual," including impacting cancer survival, Booth said.

Dr. Brenda Edwards, the associate director of the U.S. National Cancer Institute's Surveillance Research Program, said that the relationship between social class and cancer survival is a complicated one.

Attributing country-wide differences in how early people are diagnosed - or any other aspect of cancer treatment and survival rates - to Canada's universal healthcare system would miss some of the point, she said.

"I think that oversimplifies the delivery of care, the access to care, the utilization of care, and the quality of care," Edwards told Reuters Health. Social, personal, and community factors affect who gets screened for cancer and the kind of treatment they want and can get, she said.

"It takes more than just a good medical care system for us to address (differences) in outcomes," she said.