In 1993, Haberer et al developed a speed-controlled raster scanning system in which the beam current was varied, but the scanning speed was kept constant. In raster scanning, the beam is moved continuously without turning off the beam. This process is repeated for each sequential spot. Then, the beam is turned on, and the desired beam intensity for that point is delivered. The magnets are adjusted to each point when the beam is off. For spot scanning, the scanning area is covered by a mesh of distinct points that are irradiated separately. Two strategies exist for proton beam scanning, namely, spot scanning and raster scanning. Dose uniformity is then achieved through mathematical optimization of the individual dose delivered by each pencil beam. Through that combination of scanning and energy variation, the placement of the Bragg peak within a tumor can be controlled in 3-dimensional (3D) space. Bragg peaks are stacked depthwise by altering proton energy. This property is exploited in beam scanning to spread the proton beam laterally, such that the narrow pencil beam is no longer broadened through scattering but, rather, is scanned across the tumor by a scanning magnet. The principle of beam scanning is that protons are subject to Lorentz forces, that is, protons are deflected in the presence of a magnetic field. Furthermore, it solves the problem of excess radiation dosage that occurs with passive scattering because of the constant spreading out of Bragg peaks. Second, the technique is more efficient than passive scattering because fewer protons need to be delivered to achieve a prescribed total dose. First, it can be fully automated by computer, such that only Bragg peaks that terminate within the tumor volume are delivered, thereby eliminating the need to use collimators and compensators to achieve dose conformality. The beam-scanning technique has a number of advantages over traditional passive scattering. The concept of beam-scanning, controlled proton therapy was first introduced by Kanai et al.
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