SEOUL, May 1 -- Park Seo-joon revealed on Tuesday a deeply personal letter from the parents of a young boy whose life was forever changed by his quiet act of generosity.

The boy, diagnosed with a brain tumour in January last year, had faced a terrifying medical journey that his family could hardly afford - until help came, quietly, from the 36-year-old South Korean actor.

"We were devastated by the diagnosis, a condition we thought had nothing to do with us," the boy's parent wrote in a letter shared by Park on his Instagram Story.

"Terms like surgery, chemotherapy, radiation and platelet transfusions, combined with the enormous medical costs, left us overwhelmed both emotionally and financially."

What the boy needed was a hematopoiet...