Patent Number: 
Section: claims

1. An irradiation apparatus for radiating a radiation beam transported from a particle accelerator onto a location to be irradiated that is positioned on an irradiation table, said apparatus comprising:a beam interruption part for interrupting said radiation beam;a position control part for controlling a position of said irradiation table such that said radiation beam is radiated onto an entire surface of a target in a plurality of irradiation zones including an overlapping zone formed by a plurality of radiations of said radiation beam; anda multileaf collimator control part for controlling said radiation beam to provide a slope to a dose distribution in said overlapping zone of said irradiation zones, respectively, such that the dose distribution is made flat over the entire surface of said target including said overlapping zone by the plurality of radiations of said radiation beam. 2. The irradiation apparatus of claim 1, wherein said slope is approximated by a straight line. 3. The irradiation apparatus of claim 1, wherein said slope has different gradients and is approximated by two or more straight lines connected with each other. 4. The irradiation apparatus of claim 1, wherein said slope changes in a stepwise manner. 5. The irradiation apparatus of claim 1, wherein said slope is approximated by a curved line. 6. The irradiation apparatus of claim 1, wherein said multileaf collimator control part includes a multileaf collimator provided with a plurality of pairs of opposed leaves, and said multileaf collimator control part decreases a dose irradiated to said overlapping zone in a direction from a boundary between said overlapping zone and a non-overlapping zone toward another irradiation zone by moving at least one of said opposed leaves in each pair. 7. The irradiation apparatus of claim 6, wherein said multileaf collimator is operated by remote control. 8. The irradiation apparatus of claim 6, wherein a direction in which said leaves are driven to move is parallel to the direction in which said dose decreases. 9. The irradiation apparatus of claim 6, wherein a direction in which said leaves are driven to move is perpendicular to the direction in which said dose decreases. 10. The irradiation apparatus of claim 1, wherein said target encloses an area to which said radiation beam is not radiated. 11. The irradiation apparatus of claim 1, further comprising a compensating filter commonly usable with at least two of said irradiation zones, wherein when irradiation is changed from one of said irradiation zones to another a filter driving mechanism drives said compensating filter to move to a position suitable for irradiation. 12. The irradiation apparatus of claim 11, further comprising a filter position verification mechanism for verifying the position of said compensating filter. 13. The irradiation apparatus of claim 1, wherein said irradiation apparatus is incorporated in a radiotherapy system. 14. An irradiation method for radiating a charged particle beam transported from a particle accelerator onto a location to be irradiated that is positioned on an irradiation table, said method comprising:dividing a location to be irradiated into a first irradiation zone and a second irradiation zone, the first irradiation zone and the second irradiation zone partially overlapping creating an overlapping zone;radiating a radiation beam to said first irradiation zone such that a distribution of a dose radiated to said overlapping zone has a slope that decreases from a boundary between said overlapping zone and a non-overlapping zone of said first irradiation zone toward said second irradiation zone; andradiating a radiation beam to said second irradiation zone such that a distribution of a dose radiated to said overlapping zone has a slope that decreases from a boundary between said overlapping zone and a non-overlapping zone of said second irradiation zone toward said first irradiation zone, with a dose distribution in a target being made flat. 15. An irradiation method for radiating a charged particle beam transported from a particle accelerator onto a location to be irradiated that is positioned on an irradiation table, said method comprising:dividing a location to be irradiated into first, second, and third irradiation zones that partially overlap, each of the first, second, and third irradiation zones having non-overlapping zones and overlapping zones adjoining each other;radiating a radiation beam to said first irradiation zone such that a distribution of a dose radiated to two overlapping zones in said first irradiation zone overlapping with at least one of said second irradiation zone and said third irradiation zone has a slope that decreases from boundaries between said overlapping zones and a non-overlapping zone in said first irradiation zone toward at least one of said second and third irradiation zones;radiating the radiation beam to said second irradiation zone in such a manner that a distribution of a dose radiated to two overlapping zones in said second irradiation zone overlapping with at least one of said first and third irradiation zones has a slope that decreases from boundaries between said overlapping zones and a non-overlapping zone in said second irradiation zone toward at least one of said first and third irradiation zones, with a total dose distribution in said overlapping zone of said second irradiation zone that overlaps with said first irradiation zone alone being made flat; andradiating the radiation beam to said third irradiation zone in such a manner that a distribution of a dose radiated to two overlapping zones of said third irradiation zone overlapping with at least one of said first and second irradiation zones has a slope that decreases from boundaries between said overlapping zones and a non-overlapping zone in said third irradiation zone toward at least one of said first and second irradiation zones, with a total dose distribution in said overlapping zones of said third irradiation zone that overlap with one of said first and second irradiation zones being made flat.