Abstract: A system and a method are provided for intensity modulated radiation treatment. A field on a patient (544) for irradiation is first defined. A beam of radiation (530) is then provided. This beam (530) passing through an opening defined by a beam-shielding device (540, 542). In the preferred embodiment, the beam shielding device is a single jaw (540) and a single collimator section (542). Single jaw (540) and single collimator section (542) are moved in one direction at different velocities to vary the opening for radiation beam (530).

Abstract: An ionization chamber for monitoring a radiation beam includes a housing having a primary beam passageway and a number of secondary beam cells that are adjacent to the primary beam passageway. The primary beam passageway passes entirely through the housing. In the preferred embodiment, a large-area beam measuring electrode and an array of small-area beam measuring electrodes are each coaxial with the primary beam passageway. Also, in the preferred embodiment, each secondary beam cell includes a beam measuring electrode. Because the portions of the radiation beam that enter the secondary beam cells do not pass through the ionization chamber, these portions do not contaminate the treatment beam, allowing the beam measuring electrodes within the cells to be dimensioned and configured so as to maximize signal strength.