"There is a one-in-four to one-in-five chance that he would not survive a first term," New Scientist magazine quoted John Alam of a Cambridge, Massachusetts, as saying.
His estimate is higher than that of McCain's physician at the Mayo Clinic, who had once said that the Senator had a less than 10 per cent chance of deadly relapse.
Alam's estimate is based on studies of the survival rates of others with melanoma, where the patients were grouped by the size and location of their tumour, their gender, and age.
His estimate, though nothing more than an educated guess, attains significance as it falls between the previous estimates of 10 and 60 per cent.
Though Alam is a supporter of Democratic presidential hopeful Barack Obama and has donated money to his campaign, says he insists that he performed the analysis independently.
"Because the analysis is an objective, evidence-based analysis, I believe my having contributed has no bearing on the results of the analysis," he says.
A report on his analysis has been published in the journal The Lancet.