The present invention is related to a method and apparatus of radiation processing of a product package in a device having a radiation source, a collimator having a variable aperture, and a turntable, said radiation processing resulting in a point in the product package where the dose is minimal (Dmin point) and a point in said product package where the dose is maximal (Dmax point) comprising the steps of: determining a first value of the collimator aperture, by increasing said aperture from a small value, where the Dmax point is located near the centre of the product package, up to a value where the Dmax point moves near to the centre of a small side of said package's rectangular horizontal cross-section; determining a second value of the collimator aperture, by further increasing the collimator aperture up to a point where the Dmin point moves from a point near the corner of the product package to the centre of said package; processing said package with radiation, the collimator aperture being kept at a constant value comprised between said first and said second value, the turntable being rotated at a variable speed.