In the United States, any software used to treat patients must be reviewed and approved by the Food and Drug Administration before it can be used to plan fields and calculate doses and/or control beam parameters. So no, I doubt there is any free software available.

Eclipse v16.1 software

http://www.vinca.rs/~rasa/hopa.htm

All SRNA versions including CT Data and voxels

The newest form of proton therapy, pencil beam scanning, delivers therapy by sweeping a proton beam laterally over the target so that it delivers the required dose while closely conforming to shape of the targeted tumor. Prior to the use of pencil beam scanning, oncologists used a scattering method to direct a wide beam toward the tumor

NIST PSTAR is a program calculating stopping power and range tables for protons in various materials. The link is given as follows;

https://physics.nist.gov/PhysRefData/Star/Text/PSTAR.html

For particle tracking and interaction with mater, The GIANT4 monte carlo code is the best one.

The Geant4 code is the best!