| Περίληψη: | The purpose of this study is to characterize and to understand the long term behavior of
various beam characteristics of megavoltage radiotherapy linear accelerators.
QUICKCHECK()*+,-)T42031 device data was used to monitor retrospectively the daily
variation of various beam output characteristics such as Central Axis Dose (CAD) ,Quality of
the Flatness ,Quality of the Symmetry and Beam Quality Factor (BQF).
Machine beam output was masured during daily warm-up as part of the internal departmental
protocol set by the Medical Physics Unit. Annual output trends of 8 beams from two linear
accelerators over a period of one year (01/01/2016 – 31/12/2016) are reported, discussed and
analysed.
The first was Varian 2100c linear accelerator which has two photon beam energies
(nominal values 6MV and 15MV) and multiple electron beam energies
(6MEV,9MEV,12MEV,16MEV and 20MEV).The second was Varian 600c linear accelerator
which has one photon beam energy (6MV). The beam output was measured using the
QUICKCHECK()*+,-)T42031 device . QUICKCHECK()*+,-)T42031 is a cable-free, truly
wireless system, with all required essentials built right in for easy operation and convenience.
Readings from nine chambers were used for calculating the Central Axis Dose (CAD),Quality
of the flatness and Quality of the Symmetry and the other four chambers were used for
calculating of Beam Quality Factor (BQF). Quality assurance protocol of radiotherapy
machines recommends a daily constancy check only for CAD (Cental Axis Dose) beam output.
The output variation is bounded between ±3%. The results over a period of one calendar year
showed that the values of CAD (Central Axis Dose), Quality of the flatness ,Quality of the
Symmetry and Beam Quality Factor (BQF) are within tolerance limits [±3%] for all photon
(9MV and 15MV) and electron (6MeV,9MeV,12MeV,16MeV and 20 MeV) beams. All four
essential dosimetric characteristics of photon and electron beams that this study analysed for a
period of a whole calendar year were found to be very well within the accepted tolerance levels
set by national and international protocols.
The output variation in Cental Axis Dose (CAD) considered as the major dosimetric
quantity of a Linac is bounded mostly between ±1.6% (6MV photon beam clinac 2100c) and
in worst case (electron beam 20MeV) between ±3.3%.
The aforementioned results seem to be in agreement with similar published studies in the
international literature. More in particular our work showed a smaller variation within the
period studied (average variation value), because CAD variation has found to be bounded
mostly between ±3.3% with respect to the published ones which ranged between ±4.0% for 9
beams from 3 linear Accelerators.
The literature appears to have a sortage of published works relating other beam
characterisrics apart from CAD. This is one part that this study hopes to contribute with its
findings.
In addition, it was found that there is no significant variation in Central Axis Dose (CAD)
,in Quality of theFlatness, in Quality of the Symmetry and in Beam Quality Factor (BQF)
before or after the date of absolute dosimetry. None of the above characteristics appeared to
be affected from the absolute dosimetry or any other beam tuning adjustment (e.g. symmetry
adjustment) that took place within the calendar year that the study refers to.
Finally this verifies the optimized state of all the beams of the radiotherapy department in
which the study took place.
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