INTENSITY MODULATION WITH ELECTRONS - CALCULATIONS, MEASUREMENTS AND CLINICAL-APPLICATIONS

Intensity modulation of electron beams is one step towards truly confo rmal therapy. This can be realized with the MM50 racetrack microtron t hat utilizes a scanning beam technique. By adjusting the scan pattern it is possible to obtain arbitrary fluence distributions. Since the mo nitor chambers in the treatment head are segmented in both x-and y-dir ections it is possible to verify the fluence distribution to the patie nt at any time during the treatment. Intensity modulated electron beam s have been measured with film and a plane parallel chamber and compar ed with calculations. The calculations were based on a pencil beam met hod. An intensity distribution at the multileaf collimator (MLC) level was calculated by superposition of measured pencil beams over scan pa tterns. By convolving this distribution with a Gaussian pencil beam, w hich has propagated from the MLC to the isocentre, a fluence distribut ion at isocentre level was obtained. The agreement between calculation s and measurements was within 2% in dose or 1 mm in distance in the pe numbra zones. A standard set of intensity modulated electron beams has been developed. These beams have been implemented in a treatment plan ning system and are used for manual optimization. A clinical example ( prostate) of such an application is presented and compared with a stan dard irradiation technique.