Doctors in the United States have begun treatment on Tuesday which they hope will save the life of a four-year-old boy from Leicestershire.
Alex Barnes has been making a good recovery after risky brain surgery to remove a tumour in Florida.
He has had the first of 33 proton therapy sessions at the University of Florida Proton Therapy Institute.
Well-wishers in the East Midlands paid most of the costs. But doctors say there are no guarantees of a cure.
Alex's six-hour operation, to remove a tumour the size of a pea, was performed at the Wolfson Children's Hospital in Jacksonville.
Targeted treatment
Dr Sumeer Keole from the University of Florida said: "The biggest risk is, unfortunately, that the tumour could come back and that is just something that we're going to monitor very, very closely and just pray that it doesn't."
Doctors at the Proton Therapy Institute believe the radiation technique involved targets tumours more precisely than conventional radiotherapy, sparing healthy tissue.
Alex's proton treatment is expected to cost more than £70,000.
Proton therapy is a non-invasive procedure which starts with special equipment separating hydrogen from water.
Part of hydrogen atoms, protons, are then sent at high speed to form a proton beam which is targeted precisely at the tumour.
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