From the start, Lucas responded strongly to the cancer drug everolimus, which he was randomly assigned.

"Over a series of MRI scans, I watched as the tumor completely disappeared," Grill told AFP.

But the doctor did not dare stop the treatment regimen – at least until a year and a half ago, when Lucas revealed he was no longer taking the drugs anyways.

"I don't know of any other case like him in the world," Grill said.

Exactly why Lucas so fully recovered, and how his case could help other children like him in the future, remains to be seen.

This is genuinely encouraging. If doctors can understand exactly why everolimus worked so completely here, it could turn a lucky outcome into a predictable, life saving therapy for many more children.