COMP report: CPQR technical quality control guideline for medical linear accelerators and multileaf collimators

Abstract

The Canadian Organization of Medical Physicists (COMP), in close partnership with the Canadian Partnership for Quality Radiotherapy (CPQR) has developed a series of Technical Quality Control (TQC) guidelines for radiation treatment equipment. These guidelines outline the performance objectives that equipment should meet in order to ensure an acceptable level of radiation treatment quality. The TQC guidelines have been rigorously reviewed and field tested in a variety of Canadian radiation treatment facilities. The development process enables rapid review and update to keep the guidelines current with changes in technology (the most updated version of this guideline can be found on the CPQR website). This particular TQC details recommended quality control testing for medical linear accelerators and multileaf collimators.

Technological quality requirements for stereotactic radiotherapy

“…Recently, comprehensive small-field dosimetry and systemspecific quality assurance guidelines have been published for stereotactic radiotherapy [172,187,188,201,208,209,[220][221][222]. However, the translation from the old to the new methods has only just begun and the clinical implications are not yet fully understood (e.g., when correcting the output factors and hence changing the absolute dose for high-dose small-field SRS for trigeminal neuralgia).…”

“…Once the stereotactic radiotherapy device is fully commissioned, regular checks of the geometric and dosimetric accuracy have to be performed in order to ensure the system's integrity and to find system drifts and inaccuracies early before clinical consequences can arise. There are a wide range of system specific recommendations available which are considered mandatory state-of-the-art practice for each of the systems in description [172,187,188,201,209,221,222]. Parts of these recommendations are specific details on daily quality assurance which minimally requires verification of the consistency of the stereotactic frame and/or the image-guidance system with the treatment isocenter (which is a modern version of the so called Winston-Lutz test [223], normally reduced in number of inspected axes for a daily check).…”

Seasonal variations in measurements of linear accelerator output

“…Daily quality control (QC) tests for linear accelerator output have been recommended by most national guidelines in North America and Europe . Output measurements that exceed tolerance levels require investigation, create delay in patient treatment, and may require adjustments to restore functionality of the equipment to within tolerance.…”

Calibrating TrueBeam jaws by considering collimator walkout to improve the dose uniformity at abutting field junctions

“…Accurate jaw calibration of medical linear accelerators (Linac) with a precision on the order of a millimeter is a requirement in modern quality assurance protocols . Accurate field sizes and a homogeneous junction dose benefit clinical applications such as mono‐isocentric half‐beam block breast cancer or head and neck cancer treatment with field matching .…”

“…Accurate field sizes and a homogeneous junction dose benefit clinical applications such as mono‐isocentric half‐beam block breast cancer or head and neck cancer treatment with field matching . Junction requirements in the Canadian Partnership for Quality Radiotherapy (CPQR) protocol specify variances in terms of the dosimetry – 5% tolerance and 10% action levels for the dose peak/valley across the junction of the abutting fields – but the reality is this constrains the jaw position's accuracy and precision to be less than half a millimeter . To add to the complexity one often wishes to match jaws that have a 90° collimator rotation between them and the collimator walkout becomes a serious consideration …”