217 Development of a scanning system for intensity modulated radiation therapy with photon pencil beams

Abstract

The increasing cancer incidence requires more accurate and cost-effective diagnostic as well as therapeutic methods. Intensity modulated radiation therapy is now rapidly being introduced at many radiation therapy departments with a clear trend towards radiobiological treatment planning using 4D-diagnostic imaging and -treatment delivery. Therefore a magnetic system for scanning of narrow photon pencil beams has been developed for fast 4D-IMRT. The magnetic scanning system has been modelled in full 3D taken into account all aspects such as fringing field, magnetic leakage and cooling. The magnetic system consists of two perpendicular scanning magnets, a bending magnet and a purging magnet placed after a thin bremstrahlung target and in the same plane as the last scan magnet. The scanning magnets operate in orthogonal planes and the first scanning magnet (bending plane) is placed so that the image of its scanning centre lies near the effective centre of the second scan magnet (cross plane). In this way a very compact deflection system is obtained, with the position of the effective radiation source coinciding for the two orthogonal planes. The focus of the intrinsic electron beam is at the top of the target placed near the end of the last scanning magnet. The composition and thickness of this target determines the energy of the transmitted electrons and the full width half maximum (FWHM) of the photon beam. Here the target is made of 3 mm beryllium creating a therapeutic beam of 10-15 mm (FWHM) for an incident electron beam of 70-50 MeV. The transmitted electron beam has only lost around 0.5-1.0 MeV in the target and must be removed from the therapeutic beam. Therefore downstream of the target a strong and fast purging magnet integrated with an electron collector has been developed. The purging magnet operates in the same plane as the last scanning magnet and deflects the transmitted high energy electrons to the same area on the electron collector independent of the direction of the scanned photon beam. The overall design of the treatment head is compact and the narrow photon beam will allow a very accurate dose delivery particularly when combined with a thin efficient leaf collimator designed for pencil beam IMRT.