Daily linac output characteristics time-series analysis

• Κύριος συγγραφέας: Ευγενίου, Αντώνης
• Άλλοι συγγραφείς: Ευστάθιος, Ευσταθόπουλος
• Μορφή: Thesis
• Γλώσσα: English
• Έκδοση: 2019
• Θέματα: Linac output; Radiotherapy; 612.014 480 287; Γραμμικοί επιταχυντές; Ακτινοθεραπεία

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.