Independent QA: Catching, Understanding and Correcting Errors Before Radiotherapy Begins

In this article, originally published in Physics World, two Sun Nuclear customers discuss catching errors introduced in a Treatment Planning System upgrade through use of Sun Nuclear's independent QA solutions.

Independence is everything. For Sun Nuclear Corporation, a US-based manufacturer of QA solutions for radiotherapy and diagnostic imaging practices, independence is also non-negotiable, informing a product development roadmap that’s focused on relentless improvement in pursuit of patient safety, treatment outcomes and workflow efficiencies in the radiation oncology clinic.

Fundamentally, independent QA products and services enable the essential – and granular – auditing of the always evolving radiotherapy delivery system. That could mean the commissioning of a new treatment machine or clinical workflow; daily, weekly or monthly machine QA checks; as well as all aspects of patient-specific QA. At the clinical sharp-end, meanwhile, independent QA gives the radiation oncology team confidence that treatment is being delivered to the tumour site as intended while minimizing collateral damage to healthy tissues and organs at risk (OARs).

“There will always be residual risk from unforeseen failure modes in today’s complex treatment systems,” explains Jeff Kapatoes, senior director for regulatory and research at Sun Nuclear. That risk, he argues, is best addressed through independent QA to avoid any conflict of interest associated with the equipment maker’s machine self-checks. “As such, our goal is to give medical physicists the QA tools they need to do their job better as the independent auditors of radiation treatment and patient safety.”

Independent measurement

If that’s the back-story from the QA vendor perspective, what of the clinical customer when it comes to identifying – and mitigating – system errors in an expanding universe of complex treatment variables? Chris Bowen is one such end-user, a solo medical physicist working at Mosaic Life Care, a busy community clinic with two linacs providing radiation therapy services to a metro area of around 100,000 people in St Joseph, Missouri, US.

“Our radiotherapy QA effort at Mosaic is sourced almost exclusively from Sun Nuclear through its SunCHECK Quality Management Platform,” explains Bowen. In this way, SunCHECK – a single interface and database offering a unified, yet independent, view of patient and machine QA – is key to streamlining Mosaic’s radiotherapy workflows while maximizing patient safety (and throughput). Furthermore, Bowen’s work schedule now incorporates a heavy stereotactic treatment load, with up to 90 patients a year assigned for stereotactic body radiotherapy (SBRT) and another 40 or so patients for cranial stereotactic radiosurgery (SRS) procedures.

As the clinic’s SRS/SBRT programme gathered pace, Bowen needed a QA solution that would allow him to perform his patient-specific QA accurately and efficiently. The answer he settled on is Sun Nuclear’s SRS MapCHECK, a high-density diode array for patient-specific QA and end-to-end testing that’s billed as an “efficient digital alternative to film for small-field dosimetry”. Designed to insert into the StereoPHAN phantom, the SRS MapCHECK comprises 1013 silicon diodes, each with an active area of only 0.23 mm², in a 77×77 mm effective measurement area – an arrangement that enables absolute dose measurement of field sizes as small as 5 mm with the tight spatial resolution (2.47 mm centre-to-centre) needed for SRS/SBRT.

For the medical physicist, the main benefits of the SRS MapCHECK include film equivalence, the ability to measure in the true composite geometry (including vertex couch rotations), proven small-field absolute dose accuracy, as well as ease of use and enhanced workflow efficiency. Think patient QA in minutes rather than hours as per film QA (which in turn requires tight process control to get consistently accurate absolute dose measurements). “For stereotactic treatments,” notes Bowen, “SRS MapCHECK delivery is what we hang our hat on as far as confirming whether a plan has been successfully QA’d. The device enables you to verify your targeting and dose distribution accuracy.”

The heart of the matter

With hundreds of SRS MapCHECK QA sessions completed, Bowen was accustomed to pass rates of 97% or higher, even with a 2%/1 mm gamma criteria. Towards the end of last year, however, following an upgrade to the treatment planning system (TPS) for Mosaic’s twinned linacs, those results slipped dramatically. Post-upgrade, the very first SBRT lung QAs presented with passing rates only in the mid-80% range, with the first SRS cranial case in the low-90% range – both far lower than normal. His first inclination was to test the SRS MapCHECK to see if something had changed in the array, so he ran several previous QAs and achieved nearly identical results to his initial runs (97% or higher).

Knowing it wasn’t the array, Bowen reached out to the linac manufacturer’s physics team, who traced the root cause to an error introduced in the latest software update (such that the TPS did not employ the user-selected 1 mm dose grid for SRS plan calculations, instead reverting to the planning system’s default 2.5 mm grid). What’s more, with no indication to the end-user, the TPS functionality for air-cavity correction had also defaulted to “off” in the same TPS update. “It wasn’t just our cranial SRS cases where we were seeing dosimetric QA anomalies,” explains Bowen. “Our lung SBRT cases were also not hitting the usual passing rates versus our gamma criteria.”

It’s worth adding that although Bowen uses the linac vendor-supplied portal dosimetry (EPID) system as a secondary QA method, the machine’s EPID self-checks failed to pick up the TPS errors. In any case, after implementing manual workarounds to ensure the correct optimization, calculation preferences and grid sizes, Bowen’s SRS MapCHECK results reverted to their previous excellent passing rate.

This isn’t the first time that the SRS MapCHECK has picked up errors that may otherwise have gone unnoticed in the clinic. During the commissioning and twinning of Mosaic’s two linacs, the array also identified a 0.4 mm difference in the vendor-set physical leaf-gap settings of the respective multileaf collimators. “The SRS MapCHECK has become my QA gold standard,” concludes Bowen. “For starters, it’s an excellent patient surrogate, but it also offers a level of detail and precision that significantly enhance the fine-tuning of our treatment machines.”

Independent calculations

Like Mosaic Life Care, the team at the Nancy N and J C Lewis Cancer & Research Pavilion at St Joseph’s/Candler (Georgia, US) has implemented Sun Nuclear’s SunCHECK Quality Management Platform for machine and patient-specific QA. A key building block within the latter is DoseCHECK, a Collapsed-Cone Convolution Superposition algorithm for 3D analysis and MU/dose comparisons to identify clinically relevant deviations within the entire dose volume.

Last year, when performing a second check on an SRS plan with DoseCHECK, St Joseph’s/Candler medical physicist Joey Spring noted the 2.5mm dose grid in the DoseCHECK report did not match the 1.0 mm dose grid in the TPS. Spring initially reached out to the Sun Nuclear support team in an effort to shed light on the discrepancy, with subsequent investigations reviewing the plan information sent over from the TPS and the 3D RT-Dose file.

The conclusion: while the TPS reported a 1.0 mm dose grid being used, the actual plan was calculated at 2.5 mm. It turns out that St Joseph’s/Candler physicists had also recently upgraded their TPS and, thanks to independent QA via DoseCHECK, they had stumbled upon the same issue as Chris Bowen at Mosaic Life Care. To confirm the finding, Spring used the SRS MapCHECK to carry out pretreatment dose measurements, with the same reduced passing rates reported by Mosaic Life Care.

This article was originally published in Physics World.