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C.-M. Nilsson / Medical Physics in the Baltic States 7 (2009) 60 - 63 of beam data into a radiotherapy treatment planning system, development of operational procedures and training of all users in the operation of the accelerator. Daily QC Each day a routine output check with a phantom (see Fig. 1) is performed by nurses at the accelerator. Fig. 1. A polystyrene phantom with an RK-chamber inside. If a deviation appears, a physicist is summoned to do more accurate measurements. The data from the daily check is put into a homemade Excel sheet to be plotted. The phantom consists of a block of solid water, where an ion-chamber [RK chamber] is placed inside. The phantom is then placed on the treatment table and SSD, field size, alignment etc are set, and then the phantom is irradiated three times with a predetermined number of monitor units (MU). The value is then corrected for pressure and temperature and compared to a reference value. The reference value is obtained by a physicist when the RK chamber is calibrated, and when the output from the accelerator is well determined and at zero deviation. Daily QC is performed for all photon energies as well as for one electron energy. Laser and light field check is also done daily to see if the lasers begin to drift and to see if a standard field (10 cm x 10 cm) still is the same. Weekly QC A weekly measurement with a phantom from IBA, called StarTrack (see Fig. 2), is performed on each accelerator. Fig. 2. Phantom from IBA, with ion chambers. Used for weekly checks of accelerators. 61 This check includes measurements of profiles, hard/dynamic wedge, energy and output for both photons and electrons. The calibration here is very important, and it should be done together with a more accurate measurement of the absolute dose. Monthly/Annual Service This is a more extensive service where the medical physicist together with an engineer works on the machine for half a day. First the engineer does a mechanical integrity check/service of the machine, checking everything from imaging plate, and MLC, to lubricants, breaks, etc. A visual inspection is done of water hoses, couplings and manifolds for leakage and wear. All fans are checked for proper function. Water- and gas-pressure and water temperature are checked and water is refilled if need be. The table is also inspected: Table movements and brakes, emergency lowering of table, and there are more specific tests for Varian and Electa tables. The coordinate system and iso-rotation is checked. The way the MLCs are checked depends on its construction. In general the movement of the MLC is checked with different field sizes, and also with asymmetric fields. There are also some parameters to have in mind as a physicist when thinking of the MLC QC. The leaves must go to the required position, which has three aspects: The independent motion of each leaf, the leakage through and between leaves, the weight of the collimator may cause the alignment of the leaves to be different at different gantry angles and that the relationship between the optical field and the radiation field may differ. On an Electa machine where the position of the MLC leaves are positioned via reflectors, the intensity and position of the reflectors are checked. On Varian, the MLC is initialized, fans to MLC driver card is checked and visual inspection of MLCs is done. Gantry and collimator angle readout is checked with a water level device. Isocenter check is important, especially when IGRT is involved. A related parameter is the mechanical and optical distance indicator. Alignment of radiation field with field light is done with Gafchromic film. Lasers are checked and aligned if need. Lasers are also compared to the position of the light field and together with a phantom. Dose rate for the different energies, default dose rate value and PLS server are checked. The imaging system is checked: Anti Collision switches, full detector calibration and imaging with a PTW phantom and analyses in software is performed, (applies for both EPID and cone-beam CT). After the mechanical check the physicist and engineer together goes through each energy and looks at the profiles, i.e. flatness and symmetry. The energy for the electrons is also checked. When that is done and – if needed - corrected, the physicist measures absolute dose for the photons in an in house built water phantom (see Fig. 3) and if the need arises corrects the output if variations are larger than 1%. This is always done
C.-M. Nilsson / Medical Physics in the Baltic States 7 (2009) 60 - 63 together with another medical physicist to minimize the risks for mistakes. Fig. 3. In house built water phantom for measurements of absolute dose. Room for an ion-chamber is also seen. In the larger annual services that lasts a week, a large water tank (see Fig. 4) is used to take up dose profiles and depth doses. These are compared with the results of previous measurements of absorbed dose in the water tank and also the profiles taken up previously. Depth dose and profiles for open and wedge fields, absolute dose, TMR 20 10, and R50 are measured. These services also include preventive exchanges on the accelerators for example if one sees a MLC leaf beginning to falter, one exchanges it beforehand, or lubricates etc, all to keep the accelerator working properly. Fig. 4. Water tank from IBA, where an electrometer, diode, computer etc. is connected. Patient Specific QC IMRT QC QC of IMRT is performed before each patient starts his/her treatment. The plan is exported and calculated on a phantom, IBA MatriXX (see Fig. 5). 62 Fig. 5. IMRT phantom from IBA, with MULTICube polystyrene. The phantom is then irradiated in two ways. One series of irradiations where the phantom is irradiated in gantry and collimator 0 ◦ which is to check the fluence from the accelerator. The second irradiation is performed as if the phantom was the patient, and then each field is summed up to give a view of the complete dose to the patient/phantom. Then the measured and calculated dose distribution on the phantom is compared using γ evaluation and a visual comparison of the profiles. We use the criteria 3% and 3mm in our γ evaluation. In vivo QC For an in vivo system it is important to check initially that the system is stable and constant over time. This can be made that one irradiates the diodes 10-15 times and the spread should be within 0.5%. The measurements should be remade at different days for 1- 2 weeks to see that the diodes are constant. 3. Results We have an uptime of 97-98%. Using the in house made Excel sheet the physicist can follow the output from the daily QA check as can be seen in Fig. 6 for 4 MV photons and in Fig. 7 for 10 MV photons. Fig. 6. Per cent deviation from reference output in the daily morning QA for 4 MV photons.
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2009 PROCEEDINGS OF THE 7 th INTERN
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Executive editor: D.Adlienė CONFER
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11.15-11.30 Daiva Leščiute, Valda
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MEDICAL PHYSICS IN THE BALTIC STATE
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