Assessment and Evaluation of Photon Beam Dosimetric Accuracy Requirement of Tuning Medical Linear Accelerator

Abstract

In radiotherapy, accuracy and precision are key, and thus, a small deviation in dose delivered by a treatment machine can be detrimental to patients. For this reason, in any radiotherapy department, linacs that have been tuned to match each other dosimetrically require critical assessment and evaluation since patients may be moved between linacs without any adjustments to treatment plans. The main aim of this study was to assess and evaluate the photon beam dosimetric accuracy between two beam-matched Elekta HD linacs (Linac 2 and Linac 3) at St. Olav's Hospital in Norway. The dosimetric photon beam data measured during the commissioning of the test Linac 2 were relatively compared to that of Linac 3, which is the reference Linac. Also, as part of this study, the new quality control data measured for the test Linac 2 has been compared to the annual quality control data of the reference Linac 3 as well as three other linacs (Linac 4, Linac 5 and Linac 6) installed at the department which have also been tuned to match the reference, in order to assess and evaluate the degree of deviations among these beam-matched linacs. Comparison was also made in absolute dose terms. The photon beam data (percentage depth doses, beam profiles and output factor) were acquired using an IBA water phantom during the commissioning of the new test Linac 2. Quality control tests were also performed to measure the flatness and symmetry of the beam profiles and the beam quality index (D20/D10). Absolute dose calibration was also performed. These dosimetric beam data measured for the new test linac were compared to that of the department’s reference linac to assess and evaluate the degree of accuracy of the tuning of the test linac and also the consistency among all the linac units. The Percentage depth doses, beam profiles and output factor of the photon beam data evaluated and compared between the test Linac 2 and the reference Linac 3 were well matched for all field sizes and energies and were within the acceptable limit of ±1%. The mean deviations of the photon output factor among three linacs (Linac 2, Linac 3 and Linac 6) compared for open square field sizes ranging from 5 × 5 cm2 to 40 × 40 cm2 were 0.06, 0.18 and 0.11 for 6 MV, 6 MV FFF and 15 MV respectively, with an overall mean deviation of 0.12. The annual quality assurance data of the beam profile parameters (flatness and symmetry) for all the linacs compared were also with the ±1% tolerance limit indicating that the linacs have been well tuned and also their outputs are stable over time (in relation to the new test Linac 2). For the beam quality index (D10/D20), the mean standard deviations among all the linacs were 0.24, 0.26 and 0.21 for 6 MV, 6 MV FFF and 15 MV respectively, with an overall mean standard deviation of 0.24. The absolute dose calibration deviations between the test linac and the reference linac were also within the clinical tolerance limit of ±0.5%. Overall, there was an excellent agreement among the linac units for the photon beam data analyzed and compared. This comparative analysis is plausible and can be adopted by other radiotherapy centres embarking on beam-matching acceptance.