@lady86 and @panorman3, here is information from Mayo Clinic.
- Photodynamic therapy https://www.mayoclinic.org/tests-procedures/photodynamic-therapy/about/pac-20385027

EXCERPT
"Photodynamic therapy is a two-stage treatment that combines light energy with a medicine called a photosensitizer. The photosensitizer kills cancerous and precancerous cells when activated by light, usually from a laser.

The photosensitizer is nontoxic until it is activated by light. After light activation, however, the photosensitizer becomes toxic to the targeted tissue.

Several photosensitizer drugs are available today to treat many conditions, including acne, psoriasis, age-related macular degeneration and several types of cancer.

In addition to treating these conditions, photodynamic therapy also helps treat bacterial, fungal and viral infections. Studies have shown that this light-based therapy can trigger the body's immune response, giving the body another means to help destroy cancerous and precancerous cells."