A little boy with a rare cancer like rhabdomyosarcoma is still a little boy.

And like many 3-year-old boys, Mason Macri is fascinated with superheroes. 

That's why his parents tell him that each chemotherapy and proton therapy treatment recharges his superpowers. It works much the same as the spoonful of sugar about which Mary Poppins sings.

"We knew he would have questions about what was happening, and...