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Radiotherapy

Radiotherapy

The myQA SRS detector: clinical validation to clinical application in SRS/SBRT patient QA

26 Jul 2021 Sponsored by IBA Dosimetry

Real-world clinical evaluation confirms that IBA Dosimetry’s myQA SRS provides a unique patient QA capability, verifying stereotactic treatment plans with film-class resolution and the proven workflow efficiency of a digital detector

Patient-focused QA
Patient-focused QA: Chris Stepanek of UHBW NHS Foundation Trust, UK, adjusts the myQA SRS set-up during a comprehensive evaluation of the detector, phantom and supporting software using SBRT plans for a range of beam energies and clinical indications. (Courtesy: UHBW NHS Foundation Trust)

The myQA SRS is being billed as a “game-changer” when it comes to the complex task of verifying stereotactic treatment plans in the radiation oncology clinic – enhancing treatment quality, workflow efficiency and patient safety in the process. Developed by IBA Dosimetry, a German supplier of specialist QA products and services for radiotherapy treatment centres, this next-generation 2D digital detector array is designed to support the medical physics team with patient-specific QA and commissioning of their stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT) systems.

Following commercial release at the end of March, the development team at IBA Dosimetry is now focused squarely on real-world implementation, evaluation and validation of myQA SRS in the clinical setting, synthesizing inputs from a mix of early-adopting customers and beta sites. Near term, the detector’s capabilities are being road-tested by several clinics in Europe and the US, with more and more data emerging daily on the benefits of myQA SRS for patient safety and throughput within the SRS/SBRT workflow. Related abstracts have already been accepted for presentation at the virtual AAPM Annual Meeting later this month and the ESTRO 2021 Annual Congress in Madrid, Spain, at the end of August.

Innovate, evaluate, validate

Among the treatment centres putting myQA SRS through its paces is University Hospitals Bristol and Weston (UHBW) NHS Foundation Trust in the UK. Radiotherapy physicist Chris Stepanek and UHBW’s head of radiotherapy physics, Sally Fletcher, carried out their evaluation of myQA SRS on the Trust’s Elekta Versa HD linacs, using SBRT plans calculated in the RayStation v7.0 treatment planning system (TPS) for a range of beam energies and clinical indications (including spine, lung, liver and prostate).

In the clinic, the UHBW physicists integrated the detector into the cylindrical myQA SRS phantom (compatible with static and rotational delivery) or in water-equivalent Scanplas. Field and plan measurements were subsequently compared to RayStation calculations using gamma analyses, also with commissioned plan verification systems (e.g. radiochromic film and IBA Dosimetry’s CC04 ion chamber).

Operationally, the emphasis on integrated product design is key to the combined detector–phantom assembly, minimizing uncertainties in set-up, calibration and QA checks. “The detector comes with its own dedicated phantom and inserts for ion chambers and radiochromic film – all of which are straightforward to set up,” Stepanek explains. “The myQA software is also very intuitive, enhancing ease of use and efficiency within the SRS/SBRT workflow.”

Ease of use, in turn, translates into a streamlined workflow for patient-specific QA, sidestepping the lengthy and cumbersome process controls needed to get consistently accurate absolute dose data with film dosimetry. “For SRS/SBRT, QA is all about patient safety and workflow efficiency,” adds Fletcher. “As such, the myQA SRS detector will speed up time to treatment delivery for the patient, while providing reassurance that what your TPS has calculated is what your stereotactic treatment system is delivering. That reassurance is doubly important given the steep dose gradients implicit with SRS/SBRT modalities.”

A new-look QA platform

In terms of the underlying technology, the myQA SRS detector is based on a silicon complementary metal-oxide-semiconductor (CMOS) platform, which enables a compact design, fast read-out and high pixel density along the x and y coordinates (with each pixel representing a radiation-sensitive element comprising a photodiode, capacitor and three transistors). Spatial resolution is 0.4 mm, with more than 100,000 pixels across a large active area of 12×14 cm2.

myQA SRS

Those specifications promise significant time savings for the QA of patients with several treatment volumes. Put simply, there is no need for the physicist to choose which targets they want to QA when everything can fit in one irradiation session to verify the complex dose distributions required for mono-isocentric SRS plans with multiple targets.

Notwithstanding the growing clinical adoption of stereotactic treatment systems, the extreme physics of SRS/SBRT – focusing high-dose radiation very precisely on a small lesion and having it fall off as quickly as possible – remains a non-trivial dosimetric and QA challenge for the radiation oncology team. In other words: it’s not easy to confirm targeting accuracy and dose-distribution accuracy when the stereotactic treatment volume can be as small as a few millimetres in diameter – and notably so for traditional QA solutions.

While film provides excellent precision in terms of dose resolution, it is cumbersome to use, time-consuming and temperamental, owing to the uncertainties in handling, calibration and development. Conversely, 2D diode arrays and ion-chamber arrays are able to generate results rapidly, though they lack the necessary spatial resolution and error-detection sensitivity for SRS/SBRT QA.

“With myQA SRS, the accuracy versus efficiency trade-off no longer applies,” claims Sandra Kos, product manager for patient QA solutions at IBA Dosimetry. “It’s that value proposition we set out to verify during our in-house evaluation of the detector in Q4 of last year and with the help of our clinical beta sites through 2021.”

Quantitative validation

The main importance, of course, is in the dosimetric detail and the ability of the myQA SRS detector to verify dose distribution accuracy in minutes rather than hours and with the necessary spatial resolution for SRS/SBRT QA. With this in mind, Stepanek, Fletcher and their UHBW colleagues used ion-chamber measurements to assess myQA SRS dose linearity, dose-rate dependence and field-size dependence, while TPS calculations and radiochromic film enabled assessment of off-axis square fields and step-and-shoot off-axis stripes.

“We validated the performance of myQA SRS through the measurement of clinical plans and subsequent comparison with small-volume ion chambers, radiochromic film and TPS doses,” explains Stepanek. “The project also evaluated the detector versus a variety of errors simulated within a selection of treatment plans, focusing in the main on sensitivity to single MLC position errors as well as gantry and collimator miscalibrations.”

Stepanek, for his part, will present full results of the UHBW study at ESTRO 2021 next month. In summary, though, the detector demonstrated good dose linearity and good dose-rate independence above 200 MU/min. Measurements of field-size dependence agreed well with small-volume ionization chambers, as did measurements of small off-axis fields versus TPS doses. Further investigations included the measurement of 6 MV and 6 MV FFF clinical plans with the detector integrated into the myQA SRS phantom. Those results also demonstrated excellent agreement versus TPS doses, ion-chamber readings and radiochromic film.

Into the clinical workflow

Through the second half of this year, the UHBW medical physics team will continue its clinical evaluation of myQA SRS, while Kos and her colleagues are focused on collating feedback from customers and various beta sites to inform the next phase of myQA SRS product development.

“Right now,” Kos adds, “the detector is validated and released for standard C-arm linacs capable of delivering stereotactic treatments, though we already have further clinics working on operational validation of the detector with different treatment delivery machines.”

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