There are actually medical tattoo technicians (used in radiology to place targeting tattoos) it helps the oncology team line up the equipment perfectly every time.

And also reconstruction tattoo artists that are considered part of the allied healthcare industry, they work with reconstructive plastics and cosmetic surgeons. They can tattoo over hypopigmented scar tissue, they can create photorealistic nipples on people who had radical mastectomies, they can help blend the surrounding skin to seamlessly match a prosthesis through what is essentially permanent contouring.

It requires specialist training it's not something any old tattoo artist could do. Scarred, burned and surgically damaged skin does not respond the same to ink and the skill level required is immense. But the quality of life and pshyc-social benefit to a patients health is worth it for those that get these tattoos.