In Japan today, cancer is the highest cause of death, and more than 300,000 domestic people die of cancer every year. Under the circumstances, a particle radiation therapy using a carbon beam and a proton beam with excellent features of high therapeutic effects and few side effects is drawing attention. In the therapy, a particle beam emitted from an accelerator can be directed to cancer cells to destroy the cancer cells while reducing influence on normal cells.
In the therapeutic method, a currently used particle beam irradiation method is a method called a broad beam method. In the broad beam method, a diameter of the particle beam is expanded to a size greater than the affected area based on a method called a wobbler method or a double scatter method. A multi-leaf collimator formed by a large number of metallic plates (leaves) limits an irradiation area to direct the beam in accordance with a shape of the affected area. A beam range expansion apparatus called a ridge filter expands the beam in a beam travelling direction (beam axis direction). A polyethylene beam range shaping apparatus called a compensator adjusts a beam termination position according to a shape (outline) of the affected area at a deep position to direct the beam.
However, the broad beam method is not capable of precise three-dimensional adjustment of the beam in accordance with the shape of the affected area, and there is a limit to reducing the influence on the normal cells around the affected area. The geometric collimator and the compensator are created for each affected area (and for each irradiation direction relative to the affected area), and there is a problem that radioactive wastes are generated after therapeutic irradiation.
Consequently, scanning irradiation for dividing the affected area inside of a body into three-dimensional lattices before irradiation is being developed as a further advanced form of irradiation in the particle beam treatment. In the scanning irradiation, the beam can be accurately adjusted to the affected area in the beam axis direction without using the geometric collimator or the compensator, and exposure to the normal cells can be reduced compared to conventional two-dimensional irradiation.
For example, each point is irradiated as follows in three-dimensional irradiation called spot scanning irradiation.
When a predetermined dose is directed to a point (operation of determining the irradiation dose for each irradiation point is called treatment planning), a scanning control apparatus receives a completion signal from a dose monitor and outputs a spot switch command. A beam emission control apparatus terminates beam emission based on the spot switch command. At the same time, an electromagnet power supply that provides an exciting current to a scanning electromagnet that scans the particle beam starts setting a current value corresponding to coordinates of a next irradiation point. When receiving a completion signal of the current value setting of the electromagnetic power supply, the scanning irradiation apparatus outputs a beam start command to the beam emission control apparatus, and irradiation for the next point is started. This is sequentially repeated to irradiate a treatment region with respect to one irradiation slice. When the irradiation is finished, the beam emission is temporarily terminated. Energy of the beam emitted from the accelerator is changed, or a range adjustment apparatus called a range shifter is controlled to change a beam termination position (slice) in the beam travelling direction. In this way, the scanning irradiation and the slice switch are sequentially performed for irradiation of the entire treatment region.
A weak point of the spot scanning irradiation is that the beam emission cannot be actually immediately terminated even if the beam emission control apparatus outputs the beam termination command. Therefore, a leakage dose is directed to the affected area when an exciting current of the electromagnet is changed, i.e. when the irradiation position is moved. This is particularly a problem when the irradiation dose (set dose) for each point is small, because a ratio of the leakage dose (leakage dose/set dose) is large. To prevent the problem, beam intensity needs to be reduced to make the ratio of the leakage dose relatively small. However, the reduction in the beam intensity leads to an increase in the time for treatment, and a physical burden of the patient increases.
A method called a raster scanning method is newly developed to solve the problem that the beam intensity cannot be increased in the spot scanning method (see Non-Patent Document 1 or the like). In the method, the beam is not terminated when the irradiation point is moved, unlike in the spot scanning method. Therefore, the beam is irradiated when the particle beam moves between a termination irradiation position (a point for directing a dose that is set when the particle beam is terminated, not when the particle beam is moving, will be called a termination irradiation point) and a termination irradiation point. The treatment planning including an amount of irradiation during the irradiation, i.e. irradiation dose at each termination point, is optimized.
In the scanning irradiation, an irradiation port includes a position monitor to ensure that the beam is directed to a correct position. If there is a difference between a predetermined irradiation position and a position measured by the position monitor due to a current setting abnormality of the scanning electromagnet power supply or due to displacement in the beam position at a beam transport section from an upstream accelerator to a scanning apparatus, a position monitoring mechanism (called “position monitoring controller” here) in the scanning control apparatus outputs an interlock signal, and the therapeutic irradiation is temporarily suspended.
An example of the position monitor for scanning irradiation is disclosed in, for example, Patent Document 1. In the position monitor, for example, an electrode section of accumulated electrodes of an ionization chamber is divided into a large number of electrically non-contact strips in one axial direction, and a measurement circuit is connected to each strip. The measurement circuit includes an integration portion that stores, for each irradiation spot, electricity equivalent to charge accumulated on the strip. Voltages output from the integration portions are extracted as digital signals by A/D converters (hereinafter, described as “ADC circuits”). Computation, such as calculation of a center of gravity, is applied to the digital signals to calculate spot positions.