Development of a dosimetry audit methodology for advanced radiotherapy
Date
2024
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Abstract
Radiotherapy treatment technologies are advancing rapidly and this necessitates the establishment of new dosimetry audit methodologies for quality improvements. The International Atomic Energy Agency / World Health Organization introduced a postal dosimetry audit service for high energy photon beams in reference conditions in 1969 using thermoluminescent dosimetry. Together with other dosimetry audit networks including Imaging and Radiation Oncology Core Houston, more complex methodologies to audit a range of radiotherapy activities have been developed over the years. Most routine audits are carried out for high energy photon reference beam output only, owing to the coverage and resources needed. End-to-end dosimetry audits include the entire radiotherapy process associated with a particular technique. This study focuses on the development of a remote end-to-end audit methodology using ionization chamber and radiophotoluminescent glass dosimeters, for complex radiotherapy treatments and requires participation of the entire radiotherapy team.
The methodology that was developed uses a head and neck phantom (with inserts for an ionization chamber or solid-state dosimeters) that was scanned, planned and treated in accordance with a given patient histology and treatment prescription based on a typical diagnosis. Five clinical teams were requested to use their local scanning protocol, delineate target volumes and other organs at risk associated with the treatment site using the local clinical and margin protocols, and then prepare a treatment plan. After treatment planning, local quality control checks and final plan approval were carried out and the phantom was then treated on a linear accelerator. During treatment, the dose at a specified point (defined in the centre of the phantom in the vicinity of the target and represented by three external markings on the phantom), was measured using a PTW 3- D pinpoint ionization chamber (Type: 31022). A second treatment delivery was then performed with a set of radiophotoluminescent dosimeters embedded at a fixed position in the phantom. In addition, small field output factors were measured for a range of selected equivalent square field sizes and compared with the onsite treatment planning iii commissioning data from the centres. The data were then anonymized and centrally analyzed.
Major differences were found in the delineation of target volumes and as a result, comparative data analysis of the planning was limited, which highlighted the need for clinicians to participate in external audit teams. Differences of up to 7.5% were observed when comparing the treatment planning calculated doses to the doses obtained from the dosimeters placed in the phantom. Differences of up to 17.0% were observed in the small field output factors for equivalent square field sizes of 0.5 cm x 0.5 cm. The study also demonstrated that the end-to-end audit methodology was not sensitive to the errors in the extrapolation of small field output factors in the planning systems.
The results of this study have shown that there is a need for additional steps to be audited separately prior to performing end-to-end dosimetry audits for complex treatment techniques. In addition, differences in clinical approaches, which include clinical interpretation of a prescription, volume definitions, and the evaluation and reporting of dose objectives and constraints, need to be addressed separately. Following this, methodologies that provide pre-defined target and organ at risk volumes are likely to be more efficient for an end-to-end audit programme. On-site patient specific quality assurance alone is not sensitive to errors in the commissioning of the treatment planning systems and some treatment planning modelling parameters should be audited separately
Description
A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the Faculty of Science, School of Physics, University of the Witwatersrand, Johannesburg, 2023
Keywords
Radiotherapy, Dosimetry audit, Dosimetry