It is an object of the invention to provide for a system, method and computer program product enabling monitoring of effects of intrafraction motion during the radio therapy treatment. According to a first aspect of the invention this object is achieved by an ultrasound guided radiotherapy system, comprising a radiotherapy system configured for providing a radiation treatment by means of a radiation beam to a treatment target. The ultrasound guided radiotherapy system further comprises an ultrasound imaging system configured for acquiring a 3D online image of the treatment target and/or an organ at risk during the radiation treatment. It further comprises detection means configured for detection, in the 3D online image, parts representing the treatment target and/or organ at risk and selection means configured for selecting a view plane through the treatment target and/or an organ at risk represented by the detected parts in the 3D online image and a display configured to display a 2D image of part of the treatment target and/or organ at risk along the view plane, wherein the display is further configured to display a treatment margin and/or an outline of an actual radiation beam in relation to the treatment target and/or organ at risk.L'objet de l'invention vise à fournir un système, un procédé et un produit-programme informatique permettant de surveiller des effets de mouvement d'intrafraction pendant le traitement par radiothérapie. Selon un premier aspect de l'invention, cet objectif est réalisé au moyen d'un système de radiothérapie guidé par ultrasons, comprenant un système de radiothérapie configuré pour fournir un traitement par rayonnement au moyen d'un faisceau de rayonnement vers une cible de traitement. Le système de radiothérapie guidé par ultrasons comprend en outre un système d'imagerie ultrasonore configuré pour acquérir une image en ligne en 3D de la cible de traitement et/ou un organe à risque au cours du traitement par rayonnement. Il comprend en
