Mg. Karlsson et al., INTENSITY MODULATION WITH ELECTRONS - CALCULATIONS, MEASUREMENTS AND CLINICAL-APPLICATIONS, Physics in medicine and biology, 43(5), 1998, pp. 1159-1169
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.