Dosimetric end-to-end tests in a national audit of 3D conformal radiotherapy

Abstract

Background and purpose: Independent dosimetry audits improve quality and safety of radiation therapy. This work reports on design and findings of a comprehensive 3D conformal radiotherapy (3D-CRT) Level III audit. Materials and methods: The audit was conducted as onsite audit using an anthropomorphic thorax phantom in an end-to-end test by the Australian Clinical Dosimetry Service (ACDS). Absolute dose point measurements were performed with Farmer-type ionization chambers. The audited treatment plans included open and half blocked fields, wedges and lung inhomogeneities. Audit results were determined as Pass Optimal Level (deviations within 3.3%), Pass Action Level (greater than 3.3% but within 5%) and Out of Tolerance (beyond 5%), as well as Reported Not Scored (RNS). The audit has been performed between July 2012 and January 2018 on 94 occasions, covering approximately 90% of all Australian facilities. Results: The audit pass rate was 87% (53% optimal). Fifty recommendations were given, mainly related to planning system commissioning. Dose overestimation behind low density inhomogeneities by the analytical anisotropic algorithm (AAA) was identified across facilities and found to extend to beam setups which resemble a typical breast cancer treatment beam placement. RNS measurements inside lung showed a variation in the opposite direction: AAA under-dosed a target beyond lung and over-dosed the lung upstream and downstream of the target. Results also highlighted shortcomings of some superposition and convolution algorithms in modelling large angle wedges. Conclusions: This audit showed that 3D-CRT dosimetry audits remain relevant and can identify fundamental global and local problems that also affect advanced treatments.

A comparison of IROC and ACDS on‐site audits of reference and non‐reference dosimetry

“…This fundamental part of dosimetry testing remains relevant to identify flaws in individual implementation of beam models and limitations in commercial algorithms. 29 The IROC recommendations similarly continue to show fundamental issues, particularly with small field output factors and wedge factors. 9 The broader scope of IROC on-site visit also includes QA program review, with shared aspects with the QUATRO clinical audit approach.…”

“…Despite the expansion of the audit into advanced modalities, there are continued recommendations generated from the conformal audit component. This fundamental part of dosimetry testing remains relevant to identify flaws in individual implementation of beam models and limitations in commercial algorithms . The IROC recommendations similarly continue to show fundamental issues, particularly with small field output factors and wedge factors .…”

Development of a CT number calibration audit phantom in photon radiation therapy: A pilot study

“…The theoretical CT numbers for the three-parameter fit model corresponded with the measured CT numbers better than those of the two-parameter fit model, for air and lung #7112, because the theoretical CT numbers for air are described as À1000a HU, according to Eq. (5). The difference between the theoretical and measured CT numbers of the "a number of materials" group was increased by adding the free parameter, a, while the difference between the theoretical and measured CT numbers for the "a number of elements" group was similar to that for the "all materials and elements" group.…”

“…To validate dose calculation accuracy in the inhomogeneous medium, a comparison between the calculation and measurements, using lung or bone equivalent phantoms, is usually conducted. [1][2][3][4][5] Final dose calculation results in the inhomogeneous medium vary according to the four factors above; however, practitioners such as radiation therapists or medical physicists can only adjust two of these, patient imaging and the calibration process. Therefore, it is valid to review patient imaging and the calibration processes using a third party; however, such patient imaging and calibration process reviews have not been performed.…”

“…Dose calculation in an inhomogeneous medium is influenced by four factors: patient imaging (itself influenced by CT scanner manufacturer, scanning parameters, and patient size), the calibration process (influenced by tissue‐equivalent phantom manufacturer and selection of tissue‐equivalent materials), the difference between tissue‐equivalent material and standard tissues, and the dose calculation algorithm applied. To validate dose calculation accuracy in the inhomogeneous medium, a comparison between the calculation and measurements, using lung or bone equivalent phantoms, is usually conducted . Final dose calculation results in the inhomogeneous medium vary according to the four factors above; however, practitioners such as radiation therapists or medical physicists can only adjust two of these, patient imaging and the calibration process.…”