A system comprising: a withdrawal catheter comprising means for occluding a target vessel; a venous pressure catheter; an infusion catheter comprising a compliant balloon; a pressure transducer in communication with the venous pressure catheter; circulation monitoring means arranged to monitor the circulation of a fluid through the infusion catheter; imaging mechanisms arranged to capture an image of the catheters (40, 50, 60) and to provide image data; a data processing system for monitoring retrograde venous perfusion therapy of a tumor in a patient's body as the therapy is occurring, the data processing system comprising: a data output display arranged to provide the results of monitoring, comprising one or more visual monitors arranged to display the image of the catheters (40, 50, 60); and a processor connected by an input/output interface to: the pressure transducer; the circulation monitoring means; the imaging mechanisms; and the data output display, the processor being arranged to: receive image data from the imaging mechanisms; monitor the positioning of the withdrawal catheter for monitoring the positioning of the withdrawal catheter within vasculature of a target vessel in the patient's body near the tumor; monitor the positioning of the infusion catheter for monitoring the position of the infusion catheter within the vasculature of the target vessel extending beyond the withdrawal catheter and near the tumour monitor the position of the venous pressure catheter for monitoring the position of the venous pressure catheter within the vasculature of the target vessel intermediate the infusion catheter and the withdrawal catheter; monitor venous pressure via the pressure transducer; monitor the circulation of a fluid through the infusion catheter via the circulation monitoring means; and provide the results of monitoring to the data output display. a withdrawal catheter comprising means for occluding a target vessel; a venous pressure catheter; an infusion catheter comprising a compliant balloon; a pressure transducer in communication with the venous pressure catheter; circulation monitoring means arranged to monitor the circulation of a fluid through the infusion catheter; imaging mechanisms arranged to capture an image of the catheters (40, 50, 60) and to provide image data; a data processing system for monitoring retrograde venous perfusion therapy of a tumor in a patient's body as the therapy is occurring, the data processing system comprising: a data output display arranged to provide the results of monitoring, comprising one or more visual monitors arranged to display the image of the catheters (40, 50, 60); and a processor connected by an input/output interface to: the pressure transducer; the circulation monitoring means; the imaging mechanisms; and the data output display, a data output display arranged to provide the results of monitoring, comprising one or more visual monitors arranged to display the image of the catheters (40, 50, 60); and a processor connected by an input/output interface to: the pressure transducer; the circulation monitoring means; the imaging mechanisms; and the data output display, the pressure transducer; the circulation monitoring means; the imaging mechanisms; and the data output display, the processor being arranged to: receive image data from the imaging mechanisms; monitor the positioning of the withdrawal catheter for monitoring the positioning of the withdrawal catheter within vasculature of a target vessel in the patient's body near the tumor; monitor the positioning of the infusion catheter for monitoring the position of the infusion catheter within the vasculature of the target vessel extending beyond the withdrawal catheter and near the tumour monitor the position of the venous pressure catheter for monitoring the position of the venous pressure catheter within the vasculature of the target vessel intermediate the infusion catheter and the withdrawal catheter; monitor venous pressure via the pressure transducer; monitor the circulation of a fluid through the infusion catheter via the circulation monitoring means; and provide the results of monitoring to the data output display. monitor the positioning of the withdrawal catheter for monitoring the positioning of the withdrawal catheter within vasculature of a target vessel in the patient's body near the tumor; monitor the positioning of the infusion catheter for monitoring the position of the infusion catheter within the vasculature of the target vessel extending beyond the withdrawal catheter and near the tumour monitor the position of the venous pressure catheter for monitoring the position of the venous pressure catheter within the vasculature of the target vessel intermediate the infusion catheter and the withdrawal catheter; monitor venous pressure via the pressure transducer; monitor the circulation of a fluid through the infusion catheter via the circulation monitoring means; and provide the results of monitoring to the data output display.