A radiation treatment system (100) for accurately delivering radiation to a targeted site within a cancer patient (108) that includes a modular patient support system and a patient positioner (114). The modular patient support system includes a modularly expandable patient pod (200) and at least one immobilization device, such as, for example, a rigid moldable foam cradle (350). The patient pod (200) includes a generally hemi-cylindrical support shell (212) that extends longitudinally between proximal edge (214) and distal edge (216), and transversely between two lateral edges (222, 224). In one embodiment, the lateral edges (222, 224) are tapered to minimize edge effects that result when radiation beams traverse the lateral edges (222, 224).