1. Field of the Invention
The present invention relates to a multi-leaf collimator for, when a radiation beam is irradiated to a target inside a radiation object, forming an irradiation area of the radiation beam in match with a target shape. The present invention also relates to a medical system including an accelerator.
2. Description of the Related Art
In one typical example for irradiating a radiation beam (e.g., a charged particle beam) to a target inside a radiation object, the radiation beam is irradiated to cancer cells (referred to as “diseased part” hereinafter) in a patient body. In such a case, the radiation object corresponds to the patient, and the target corresponds to the diseased part.
When irradiating a radiation beam to the diseased part in a patient body, if an irradiation area of the radiation beam does not match with a target shape, the radiation beam is also irradiated to a normal part around the diseased part. Irradiation of the radiation beam, which has been emitted for remedy of the diseased part, to the normal part around the diseased part may adversely affect the normal part. It is therefore important to define the irradiation area precisely in match with the diseased part, and to minimize irradiation of the radiation beam to the normal part.
A multi-leaf collimator is known as one solution for forming an irradiation area in match with a shape of the diseased part to avoid a radiation beam from being irradiated to the normal part that should be protected against the irradiation.
Such a multi-leaf collimator comprises two leaf plate driving body each including a number of movable shield plates (leaf plates), which are capable of shielding the radiation beam and disposed in the multi-layered form. The leaf plates are arranged so as to sandwich a propagation path of a radiation beam emitted from a radiation source toward the diseased part, and ends of the leaf plates of the two leaf plate driving body are positioned to face each other so that an irradiation field of the radiation beam is formed between the opposing ends. In each leaf plate driver, positions of the leaf plates are individually adjusted by the driving force of driving means, such as an electric motor, to form a space gap, which is similar to the irradiation area, between the leaf plates of one leaf plate driver and the leaf plates of the other leaf plate driver, thereby allowing passage of only the radiation beam that propagates toward the desired irradiation area. Then, the radiation beam having passed the space gap forms the irradiation field, shaped as desired, at the position of the diseased part. With the construction described above, of the radiation beam having reached the multi-leaf collimator, a component directing to other areas than the irradiation area is shielded by the leaf plates, and therefore the irradiation to an unnecessary part (normal part other than the diseased one) can be prevented.
To improve the accuracy in forming contours of the irradiation field when the multi-leaf collimator is used to define the desired shape of the irradiation field, it is required not only to employ a number of leaf plates having a smaller thickness, but also to position each leaf plate with high accuracy. U.S. Pat. No. 4,794,629, for example, is known as the related art in consideration of that point. In a multi-leaf collimator according to that related art, a leaf plate driver on one side and a leaf plate driver on the other side each comprise a number of leaf plates having gears provided at upper edges thereof, a single unit of driving means provided in common to all the leaf plates for adjusting positions of the leaf plates, and a gearing mechanism brought into mesh with the gears of the leaf plates for transmitting the driving force from the driving means. The gearing mechanism is slid in the thickness direction of the leaf plate to be meshed with the gear of each leaf plate successively so that the driving force from the driving means is transmitted to the leaf plates one by one. Each leaf plate can be thereby positioned at a desired position.
The above-cited related art, however, has the problem as follows.
The related art is constructed, as described above, such that the gearing mechanism is brought into mesh with the gear of each leaf plate successively for driving each leaf plate to the desired position in sequence. When forming the irradiation area in practice, therefore, a number of leaf plates must be positioned successively one by one, and a time taken to complete the formation of the irradiation area is prolonged. This results in difficulties in shortening a remedy time, and hence in reducing physical and mental burdens imposed on patients.