Daily dosimetric quality control of the MM50 Racetrack Microtron using an electronic portal imaging device.
The MM50 Racetrack Microtron, suited for advanced three-dimensional conformal radiotherapy techniques, is a complex machine in various respects. Therefore, for a number of gantry angles, daily quality control of the absolute output and fluence profiles of the scanned beams are mandatory. For the applied photon beams, a fast method for these daily checks, based on dosimetric measurements with the Philips SRI-100 Electronic Portal Imaging Device (EPID), has been developed and tested. Open beams are checked for four different gantry angles; for gantry angle 0, a wedged field is checked as well. Performing and analyzing the measurements takes about 10 min. The applied EPID has favourable characteristics for dosimetric quality control measurements: absolute output measurements reproduce within 0.5% (1 SD) and the reproducibility of relative (2D) beam profile measurements is 0.2% (1 SD). The day-to-day sensitivity stability over a period of one month is 0.6% (1 SD). Measured grey scale values are within 0.2% linear with the applied dose. The 2D fluence profile of the 25 MV photon beam of the MM50 is very stable in time: during a period of 5 months a maximum fluctuation of 2.2% has been observed. Once, a deviation in the cGy/MU-value of 6% was detected. There is no interlock in the MM50-system that would have prevented patient treatment with this strongly deviating output. Based on the results of this study and on clinical requirements regarding acceptability of deviations of beam characteristics, a protocol has been developed including action levels for additional investigations and, if necessary, adjustment of the beam characteristics.
Dirkx
M L
ML
Dr. Daniel den Hoed Cancer Center, Department of Clinical Physics, Rotterdam, The Netherlands.
Kroonwijk
M
M
de Boer
J C
JC
Heijmen
B J
BJ
D002138:Calibration; D005471:Fluoroscopy / Q000295:instrumentation; D006801:Humans; D008961:Models, Structural; D017785:Photons; D011786:Quality Control; D011856:Radiographic Image Enhancement; D011859:Radiography / Q000295:instrumentation; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation* / Q000592:standards / Q000706:statistics & numerical data; D011881:Radiotherapy, Computer-Assisted / Q000295:instrumentation; D011882:Radiotherapy, High-Energy / Q000295:instrumentation / Q000592:standards / Q000706:statistics & numerical data; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted; D014963:X-Ray Intensifying Screens