To answer this, German start-up iRT is creating what it says is a completely new approach to radiotherapy QA. The company’s Integral Quality Monitor (IQM), developed in collaboration with the Princess Margaret Cancer Centre in Toronto, is a real-time beam verification system that monitors the accuracy of radiation delivery throughout each patient treatment without any user interaction. 

IQM continuously reviews every segment of every beam during every treatment fraction,” explained iRT’s managing director Jürgen Oellig. “It is the first intra-fractional verification system available on the market.” If beam delivery deviates from the prescribed plan at any point, the treatment can be paused or halted. “This device replaces error management with error prevention – errors cannot find their way to the patient any more,” he added.

As well as improving patient safety, the IQM also increases efficiency, said Oellig. He described an example radiotherapy centre treating 700 patients per year, 300 of which are complex intensity-modulated radiotherapy (IMRT) or volumetric-modulated arc therapy (VMAT) cases. Each linac requires daily QA, plus routine annual and monthly checks. The IMRT and VMAT treatments also require pre-treatment QA, leading to an average of 650 verification sessions per year.

So if each patient receives 20 fractions, and IQM is used to measure every fraction, this adds 14000 QA sessions,” Oellig told medicalphysicsweb. “It’s a completely different level – for the first time it is possible to make every patient treatment into a verification session.

Keep it simple

Unlike existing QA systems, which comprise arrays of hundreds of detectors, the IQM uses a single large-area ion chamber. The IQM detector comprises three 1.5 mm-thick aluminium plates: two polarizing electrodes sandwiching a collector electrode. Spatial sensitivity is achieved by linearly varying the separation between electrodes across the chamber.

The IQM is positioned directly underneath the linac collimator, enabling the treatment beam to pass through the ion chamber. Oellig noted that the device does not impact the beam quality. It does attenuate the beam slightly – by 6.5% and 5% for 6 and 18 MV photons, respectively – but this attenuation is homogenous over the entire field and can thus be accounted for during treatment planning.

Clinical workflow

The IQM system is fully automated at all stages, only requiring export of a DicomRT Plan data from the treatment planning system. The IQM’s calculation algorithm uses these imported data to calculate an expected signal for each individual beam segment, based on the prescribed monitor units (MU), field size, field shape and location. The IQM chamber sensitivity map used in this calculation comprises a matrix of 801 x 801 data points, equivalent to a grid of 641601 data points. The system then expands each calculated signal into a “corridor”, to allow tolerance for expected daily linac fluctuations, and displays this inner corridor in green. In addition, a user-defined outer corridor, which separates acceptable from potentially harmful deviations, is displayed in red…
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