Abstract: An apparatus and method for treating a tumor mass location in a breast of a patient. The breast is stabilized in space, preferably by creating a reduced pressure environment around the breast. Particle beams are applied to a tumor mass location positioned at the center of a substantially spherical (or cylindrical) portion of a chamber filled with a fluid having an electron density substantially similar to the breast. The patient treatment platform is translated to aim the particle beam at the tumor mass location while traversing and beaming perpendicular to the portion of the chamber.