Abstract: A lateral decubitus precision patient support system and method for accurately and reproducibly positioning a patient's breast relative to a patient support surface to receive therapy, such as, but not limited to, radiation treatment, while the patient is in a lateral decubitus position (lying on the patient's side) upon a patient support surface is disclosed. The lateral decubitus precision support assembly may be securely attached to a treatment table during use. The lateral decubitus precision support assembly may include a contralateral breast module having at least one contralateral breast receiving area into which a contralateral breast of the patient may reside during breast therapy. The lateral decubitus precision support assembly may also include a treatment breast support that contacts a treatment breast of the patient to separate the treatment breast from the contralateral breast of patient to limit exposure of the contralateral breast to therapeutic procedures administered to the patient.