A lightweight, modular radiotherapy treatment chair has a saddle-like seat for unobstructively supporting a patient in a stable and reproducible "straddled stance", allowing ample access to the patient, particularly improved access to the lower abdominal and pelvic regions, during treatment with therapeutic radiation. The chair attaches to a conventional isocentrically mounted radiotherapy treatment table or simulator to enable precise translational and rotational positioning of the patient. An optional frame may be used to secure selected immobilization devices, such as bite-blocks, head rests, and thermoplastic positioning aids, to further support and immobilize the patient during treatment. A method for treating malignant tumors present throughout various anatomical regions of a patient's body includes unobstructively supporting the patient in a straddled stance so that the tumor, or a plurality of tumors, can be treated with predetermined dosages of therapeutic radiation.