According to one embodiment, a radiotherapy planning apparatus includes processing circuitry. The processing circuitry calculates initial irradiation directions of particle beams and an initial dose distribution corresponding to the initial irradiation directions by using a three-dimensional medical image concerning an object. The processing circuitry disperses some or all of the initial irradiation directions in response to a dispersion instruction via an input device. The processing circuitry modifies the initial dose distribution based on the dispersed irradiation directions.
