System for monitoring a hadron beam during particle therapy treatment of a subject comprising tumour cells labelled with a radiopharmaceutical product having a radio-isotope quasi-coincidentally emitting a positron and a de-excitation gamma ray, wherein the particle therapy treatment is delivered in accordance with a treatment plan comprising parameters that are predefined as a function of time so as to define at least one characteristic of the hadron beam over time during the particle therapy treatment, wherein the hadron beam comprises a plurality of discrete hadron bursts emitted at a predefined frequency by an acceleration device, the system comprising: - a beam imaging module configured to receive data acquired from a liquid-xenon Compton telescope when a burst impacts the subject and to analyse these data so as to determine the point of emission of prompt gammas generated by the interaction of the burst with the tissue of the subject, which is taken as a basis for reconstructing an image of the interaction volume within which the hadrons of the burst interact with the tissue; - a tumour imaging module configured to receive the data acquired by the Compton telescope, in particular a liquid-xenon Compton telescope, when no burst impacts the subject, and to analyse these data so as to extract the position of the tumour cells labelled with the radiopharmaceutical product by simultaneously detecting the de-excitation gamma ray and two annihilation gamma rays produced by the positron and to use the position of the tumour cells to reconstruct an image of the total volume of the tumour to be treated by the hadron beam during the particle therapy treatment; - an evaluation module configured to compare the image of the interaction volume and the image of the total volume of the tumour so as to locate the measured interaction volume with respect to the measured total volume of the tumour and, whenever the measured interaction volume is at least partially contained within the measured total volume of the tumour, to calculate the deviation between the position of the measured interaction volume with respect to the measured total volume of the tumour, on the one hand, and a predefined position of the interaction volume within the total volume of the tumour defined in the treatment plan, on the other hand. - a beam imaging module configured to receive data acquired from a liquid-xenon Compton telescope when a burst impacts the subject and to analyse these data so as to determine the point of emission of prompt gammas generated by the interaction of the burst with the tissue of the subject, which is taken as a basis for reconstructing an image of the interaction volume within which the hadrons of the burst interact with the tissue; - a tumour imaging module configured to receive the data acquired by the Compton telescope, in particular a liquid-xenon Compton telescope, when no burst impacts the subject, and to analyse these data so as to extract the position of the tumour cells labelled with the radiopharmaceutical product by simultaneously detecting the de-excitation gamma ray and two annihilation gamma rays produced by the positron and to use the position of the tumour cells to reconstruct an image of the total volume of the tumour to be treated by the hadron beam during the particle therapy treatment; - an evaluation module configured to compare the image of the interaction volume and the image of the total volume of the tumour so as to locate the measured interaction volume with respect to the measured total volume of the tumour and, whenever the measured interaction volume is at least partially contained within the measured total volume of the tumour, to calculate the deviation between the position of the measured interaction volume with respect to the measured total volume of the tumour, on the one hand, and a predefined position of the interaction volume within the total volume of the tumour defined in the treatment plan, on the other hand.