Accuracy and associated measurement uncertainties in clinical dosimetry data for static small fields

• Main Author: Msimang, Zakithi Lungile Mpumelelo
• Format: Others
• Language: en
• Published: 2021
• Online Access: https://hdl.handle.net/10539/31402

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy, 2020 === Developments in radiotherapy techniques and technologies have contributed to an increase in the use of small fields. Small fields are used in stereotactic treatments and large uniform or non-uniform fields that are composed of small fields such as for intensity modulated radiation therapy (IMRT). Implementation has predated guidance documents for clinical dosimetry. The first international Code of Practice (CoP) for small field dosimetry was only published in 2017 by the International Atomic Energy Agency (IAEA), in collaboration with the American Association of Physicists in Medicine (AAPM). There is a lack of data quantifying the accuracies linked with the use of small fields. Estimating and publishing uncertainties for measurement capabilities is standard practice for primary and secondary standard laboratories that operate within a rigorous total quality management system. This is not necessarily the case with clinical dosimetry measurements performed at hospitals, where there is alack of published uncertainties for each of the steps used in the determination of the field output actors (FOF) for small fields. In this study, the accuracy of detector positioning in small field clinical dosimetry measurements were evaluated in 6 MV and cobalt teletherapy beams with different collimation systems. In addition, the impact of two different methods of calculating the equivalent square, constancy and reproducibility of field output factors (FOFs) for different detectors, and machine stability over time, was evaluated. The influence of a reference detector was investigated. The uncertainties of all measurements were determined. For the linear accelerator data, the integrated multileaf collimator (MLC) and jaw were used as well as demountable stereotactic circular cones. The data from the study highlighted the importance of verifying Central Axis (CAX) for independent measurement set ups and not relying on a visual set up using the field projection, the manufacturer’s specifications of a detector or an automated water tank positioning system. A variation in CAX of 0.8 mm for in plane and 1.6 mm for cross plane was found to yield a 32 % variation in the FOF for Sclin of 0.6 cm. The solid-state detectors used in the study performed better than the air ionisation detectors and are thus recommended as detectors of choice. The study proved that the need for and frequency of the MLC calibration greatly affects the FOF, and lack of MLC maintenance will result in a gradual, unpredictable change in Sclin. A 3-monthly calibration period of the machine used in this study yielded results that were within the measurement uncertainties for the determination of Sclin, and it was concluded that this frequency was sufficient to achieve the required outcomes. Data were compared to measurements provided by other hospitals in South Africa, standard data sets (BJR25) and other international hospitals that participated in the IAEA coordinated research project [E24021: “Testing of Code of Practice on Small Field Dosimetry”].Comparison of measured data to that published in the British Journal of Radiology (BJR) Supplement No. 25 of 1996 showed that BJR 25 data for 6MV and cobalt teletherapy should not be adopted for Sclin ≤ than 6 cm. Local data should be determined experimentally. For reference measurements, the standard uncertainty contributed by the traceability process of daisy-chaining, contributed the most significant uncertainty. For relative dosimetry measurements, the standard uncertainty associated with the determination of the FOF contributed the most significant uncertainty. These were identified as the two high risk areas in the dosimetry chain for small static field dosimetry. As such, dosimetry audits for small fields should focus on the FOF and reference dose determination in field sizes ≤ 2 cm === CK2021