The invention relates to a method of quality assurance of an apparatus for radiotherapy (10) by a photon beam (20) directed toward an object or a patient (30), comprising the following steps: the object or the patient (30) is galvanically isolated from a reference potential； a pico-ammeter (60) is linked between the object or the patient (30) and the reference potential； the photon beam (20) is directed toward the object or the patient (30)； the electric charge (Q) arising in the object or the patient (30) and/or the electric current (I) flowing between the object or the patient (30) and the reference potential are/is measured by means of the pico-ammeter (60). The invention also pertains to an apparatus suitable for the execution of this method.
