The cancer in question is an aggressive form of non-Hodgkin lymphoma called diffuse large B-cell lymphoma (DLBCL). Non-Hodgkin lymphomas include a large group of cancers that affect the lymphatic system, a part of the immune system.

Some research has linked obesity to a greater risk of developing DLBCL. And in certain other cancers, like breast and colon cancers, obese patients may have a worse prognosis than those who are thinner when they're diagnosed.

"We expected something similar when we started this project," lead researcher Dr. Kenneth Carson, of Washington University School of Medicine in St. Louis, said in an email.

So what the researchers actually found came as a surprise, said Carson, whose findings appear in the Journal of Clinical Oncology.

Of more than 2,500 U.S. veterans with the cancer, men who were obese at diagnosis had only about two-thirds the risk of dying during the study period of normal-weight men -- after considering other factors, like age and overall health.

Out of 625 obese men, 294 -- or 47 percent -- died over the next five years. That compared with 64 percent of 849 men who were normal-weight when they were diagnosed.

Men who were not obese, but overweight, also had a better prognosis than their normal-weight counterparts: a 27 percent lower death risk during the study period.

It's not clear exactly why heavy men with the lymphoma would survive longer.

And Carson stressed that the findings do not mean that men should allow themselves to gain or hang on to excess fat -- especially since obesity may be a risk factor for developing DLBCL in the first place.

"It is important to note that by no means are we trying to discourage weight loss in overweight or obese persons," Carson said.

But figuring out why heavier men with DLBCL have a better outlook could lead to a better understanding of the cancer -- and possibly better treatment for all, according to Carson.

"In my mind," Carson said, "there are two main factors that might explain why overweight and obese patients have improved survival: differences in disease biology or differences in patient responses to chemotherapy."

If obese patients have a tendency to develop DLBCL tumors with a more "favorable" biology, Carson explained, that could give clues to the causes of the lymphoma.

On the other hand, if heavier people respond better to chemotherapy, figuring out why could help improve treatment for patients of all sizes.

It's possible that bigger patients are able to tolerate more-intense chemotherapy before having severe side effects. But, Carson's team notes, other research suggests that in the real world, obese chemotherapy patients may tend to get lower doses relative to their body size.

The American Cancer Society estimates that just over 70,100 Americans will develop some type of non-Hodgkin lymphoma in 2012. DLBCL is the most common form.

SOURCE: bit.ly/Kz3rF1 Journal of Clinical Oncology, online May 29, 2012.

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