Patent ID: 8263954

Claim:
A bolus that is disposed in a particle beam therapy system and modulates energy distribution of a particle beam in accordance with a to-be-irradiated portion, wherein the shape of the bolus is set in such a way that, when a first reference point and a second reference point, which is at the downstream side of the first reference point, are given on a beam axis of a particle beam that enters the bolus at the upstream side thereof, and when an irradiation orbit of a particle beam that penetrates the bolus and reaches the to-be-irradiated portion is defined by a first slant from the beam axis with respect to a first axis that starts from the first reference point, that is perpendicular to the beam axis, and that includes the first reference point and by a second slant from the beam axis with respect to a second axis that is perpendicular to the beam axis and the first axis and includes the second reference point, a path length within the bolus of a particle beam in each of the irradiation orbits defined for combinations within a predetermined range of combinations of the first slant and the second slant, compensates a path length from a body surface situated at the upstream side of the to-be-irradiated portion to the to-be-irradiated portion.