Researchers have introduced a hypothesis to explain the "Lance Armstrong effect," or why testicular cancer survivors fare better than people with other advanced forms of cancer.

The seven-time Tour de France winner had a highly treatable form of cancer, despite the disease spreading to his abdomen, lungs and brain.

Heat-sensitive sperm develop in the testicles, which are a few degrees cooler than the rest of the body.

"If we understand how heat may naturally help kill testicular cancer cells, then perhaps we can make it happen in other solid tumours," said Prof. Robert Getzenberg, director of urology research at Johns Hopkins University in Baltimore, Md., and co-author of a commentary on the theory published in Wednesday's issue of the Journal of the American Medical Association.

The five-year survival rate for testicular cancer in the U.S. is more than 95 per cent, including cases when the disease has spread to lymph nodes, or 72 per cent when it goes beyond, according to the American Cancer Society.

When testicular cancer cells spread to warmer regions of the body, Getzenberg's team believes the increased temperature weakens the protein structure with the nucleus of cancer cells, making it more vulnerable to chemotherapy and radiation.

Sperm from men with undescended testes, a birth defect in which the genitals don't descend into the scrotum, also tend to show problems in the nuclear structure or matrix of their sperm cells, the researchers noted.

They plan to test the idea in animals with prostate cancer by injecting nanoparticles that target tumour cells heated by a magnetic field.

Speculative research

Dr. Laurence Klotz, head of urology at Toronto's Sunnybrook Health Sciences Centre, said it's not clear if the warming idea would work for testicular or other cancers.

"The issue is always whether when you apply the concept, it actually benefits the patient," Klotz said. "And so I think this is a case where in some ways it's technology looking for an application. You've got the nanoparticles and what can you make them do that will help patients?"

"There is no direct or even indirect evidence even remotely supporting this hypothesis," said Dr. Craig Nichols, Armstrong's specialist at Oregon Health & Science University's Cancer Institute.

With files from the Associated Press and Canadian Press