Áp dụng EPID trong việc đảm bảo chất lượng máy gia tốc xạ trị tuyến tính

Abstract

Objective: Application of electronic portal imaging device - EPID in a number of quality assurance and quality control (QA&QC) examinations for clinical linac; additionally, a specified computer program for processing and analysing EPID and Gafchromic film images is also developed and evaluated. Subject and method: A majority of Linac critical characteristics such as light and radiation field sizes, mechanical and radiation iso-center coincidence, beam profile and beam output are examined in QA&QC process using conventional radio-chromic films as well as other types of dosimeters such as ionisation chambers or diodes. EPID is an on-board imaging system used for patient positioning verification that is constructed with a 2D array of diodes. It has the ability to detect X-ray in MV energy range similar to other dosimeters used in QA&QC; thus, it can also be used for checking some significant parameters of the Linac. In this project, the EPID system attached to Elekta Linacs is used to examine the quality assurance tests mentioned above by taking images of radiation fields, then processing and analysing those images to acquire useful information. The obtained results are evaluated and compared with appropriate measurements from radio-chromic film and ionisation chambers in water phantom in order to investigate the compatibility of EPID in some QA&QC examinations. Result: Equivalent measurements were obtained by EPID in comparison with StartTrack, radio-chromic film as well as ionisation chamber in water phantom. The largest difference between those methods was observed at below 1% for beam output and 0,5% for radiation field-size and beam profile tests. Conclusion: EPID has the capability to replace film dosimetry and StartTrack in a number of QA&QC quick checks for clinical linac such as daily QA. Moreover, EPID is more efficient in terms of time and cost per use compared to other conventional dosimetry methods. However, it is the responsibility of the onsite medical physicists to decide whether the EPID system is appropriate for each particular situation.