Colleen and Beth- As usual science has to make things as complicated as possible with acronyms for everything. There is a difference between SABR and SBRT and that is where the cancer is in the lung, the size of the tumor, and other factors.
Essentially SABR aims concentrated beams at small tumors at the edge of organs such as lungs, prostrate, and livers.
https://news.cancerresearchuk.org/2017/08/21/sabr-radiotherapy-thats-smart-fast-and-to-the-point/
From what I can understand SBRT is a type of external radiation therapy that uses special equipment to position the patient and precisely give a single large dose of radiation to a tumor. Radiation distorts the DNA of the tumor cells. It's used to get rid of tumors where people can't or shouldn't have surgery.

Proton and Photon are the main atoms used in SBRT. I might have opened up a conversation regarding an area that is so beyond my understanding....this is so futuristic. Each one also has a different size atom and it depends on the size of the tumor which one is used.