ABSTRACT We hypothesize that a room-temperature, 360°, isocentric proton therapy gantry fits inside a Linac vault, a.k.a the Mini-Gantry. This would enable the conversion of two neighboring Linac vaults (one for the Mini-Gantry and one for the accelerator) into an ultra-compact proton therapy center and increase the accessibility of proton therapy, especially in urban centers with existing linear accelerator vaults where space is a constraint. Existing proton therapy gantries are larger than 24’ in diameter. The proton gantries at Massachusetts General Hospital occupy 3 stories. The IBA ProteusOne gantry is 25’ in diameter. None of these fit in a typical 12’ high basement of a high-rise building and require custom-designed spaces. The aim is to optimize the proton maximum energy and treatment field size in the Mini-Gantry while maintaining 12’ diameter and produce designs. This effort determines the feasibility of the Mini-Gantry concept which is a stepping stone to publications, grants, patents, prototypes, and partnering with a proton therapy vendor. Contains proprietary information that B. Clasie requests not be released to persons outside the government, except for purposes of review and evaluation._