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.
Dr. Daniel den Hoed Cancer Center, Department of Clinical Physics, Rotterdam, The Netherlands.
M L
ML
Dirkx
M
M
Kroonwijk
J C
JC
de Boer
B J
BJ
Heijmen
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