In May 2016, Victor Barreiro learned his melanoma, originally treated in his hometown of Mexico City in 2014, had returned and spread throughout his body. There were growths in his lungs, liver, spleen and brain – a total of 15 separate spots where cancer had spread, or metastasized.

Today, after metastatic melanoma treatment at MD Anderson, most of those growths are gone. The few that remain are just a fraction of their original...