I believe the Island only cures the diseases people have got before they came to the Island. That's why Locke's legs and Rose's cancer were fixed while other survivors or THE OTHERS injured in the crash were not, e.g. the US marshal ,Colleen, and people died from unknown virus. The Island just doesn't cure new diseases or injuries people got after them came to the Island. So, perhaps Ben got his tumor after he landed on the Island.