A new FDA approved hydrogel is injected between the prostate and rectum to reduce rectal injury and improve quality of life in prostate cancer patients undergoing radiation therapy or proton beam treatments.

See:

https://www.massgeneral.org/imaging/news/radiology-rounds/february-2018/use-of-spacers-limits-non-target-radiation-exposure-in-radiotherapy-to-treat-prostate-cancer/