Mobile CT lines up for adaptive proton planning

The AIRO mobile CT system is being commissioned for use with the centre's Mevion S250 proton therapy system. Image courtesy of Orlando Health UF Health Cancer Center

CT is an integral part of adaptive image-guided proton therapy (IGPT). It is used to monitor changes in a cancer patient’s anatomy caused by weight loss and/or tumour shrinkage, as well as for treatment plan adaption. CT simulation scans are usually performed in imaging suites outside proton therapy treatment rooms. This set-up, however, can cause workflow inefficiencies and inconvenience for both staff and patients alike.

Compact mobile CT systems may change all this, by enabling scanning to be performed within proton treatment rooms. The Center for Proton Therapy at Orlando Health UF Health Cancer Center has successfully installed a mobile CT system for patient localization and set-up in its compact proton therapy vault. The research team has now described the commissioning process and the dosimetric implications of adaptive planning with the mobile system (J. Appl. Clin. Med. Phys. doi: 10.1002/acm2.12319).

The scanner (the AIRO Mobile CT System) being commissioned for use with the centre’s Mevion S250 proton therapy system is a 16-slice helical scanner that acquires images with 120 kV, 10-250 mA and a field-of-view (FOV) up to 51.2 cm. Designed for intraoperative surgery, the large FOV enables the scanner to capture the entire patient surface including immobilization devices and the treatment couch.

The commissioning process

A critical part of the commissioning process is setting up the stopping power curve for an in-room CT scan so that dose calculations on the scanner are dosimetrically matched to the treatment planning system.

“This groundwork is important so that if adaptive planning is performed on the in-room CT, one can be certain that the changes in dose are due to changes in anatomy and not to differences in the CT scanner or scanning protocols,” medical physicist Twyla Willoughby told Physics World. “This is very important in being able to make clinical decisions regarding adapting a treatment plan. When comparing two different CT scanners for dose comparison, any changes in CT values and in the calculated stopping powers can lead to changes in the dose along the proton path or to a change in the range of proton therapy.”

To do this, Willoughby and colleagues scanned an electron density CT phantom on a simulation CT scanner and the mobile CT, and compared the mean CT numbers to determine differences. They imaged a phantom containing 16 rods and 13 tissue substitute materials with varying plug patterns, table heights, and mA with fixed 120 kV. Images of plugs representing brain, lung 300, lung 450, cortical bone, adipose, breast, liver, solid water, and true water were analysed. They then determined the stopping power ratios (SPRs) by entering averaged CT numbers into a stoichiometric SPR calculation algorithm.

The last step of the commissioning process involved confirming dosimetric equivalence for dose calculated on CT scans from the two scanners. The researchers developed heterogeneous, single-field, non-robust plans on thorax, pelvis and head phantoms, to test the dose accuracy for proton beams traversing large areas of heterogenous media. They also generated five different clinically reasonable treatment plans on five different phantoms to test the accuracy of the adaptive system in common clinical scenarios.

Key findings

Lead author Jasmine Oliver and colleagues reported that proton dose calculations on CT image sets acquired by the mobile CT scanner could be used to calculate dose with relatively high accuracy, similar to the simulation scanner.

They cautioned that dosimetric equivalency testing, using visual display of isodose lines and water-equivalent thickness (WET) values between the planning and in‐room CT scanners, should be performed before any in-room CT system is deployed for adaptive planning purposes.

Test results showed that CT numbers differed between the scanners. Low-density plugs had a higher CT number in the mobile CT compared with the simulation scanner, while high-density plugs had a lower number. Dose on the mobile CT extended deeper by about 5 mm compared with the original treatment plan.

To create equivalent dose distributions, it was necessary to adjust the SPR curve’s low-density data points of the mobile CT, to obtain better proton beam range agreement based on isodose lines. When the authors compared the stochiometric-based SPR curve and the “dose-adjusted” SPR curve, they observed slight improvement on gamma analysis between the treatment plan and the mobile CT plan for single-field plans at the 1%, 1 mm level. Clinical plans at 3%, 3 mm demonstrated equivalent dose.

“Our results demonstrated that performing the stoichiometric analysis for a given phantom and CT scan may not provide dose equivalence between two different CT scans… it was important to verify the dosimetric equivalence of the two CT image sets with their corresponding stopping curves,” wrote the authors. “To achieve this, it was necessary to directly map CT values and adjust them to yield better dosimetric comparisons at the end-of-range.”

The mobile CT system in the proton treatment vault is currently used to perform “re-simulations” for patients who may have anatomical changes due to radiation therapy. “It is used on all of our breast patients to monitor target swelling, on lung patients to monitor fluid in the lungs and tumour changes, and on head-and-neck patients to monitor tumour shrinkage,” Willoughby explained. “These things dramatically affect the proton range and modulation, and can cause significant changes in the treatment plan if they go unmonitored.”

The cancer centre does not offer pencil beam scanning (PBS) proton therapy. However, the authors believe that, based on their experience, the image quality of the mobile CT scanner is good enough for dose-recalculation on PBS as well as double-scatter systems.

Cynthia E Keen is a freelance journalist specializing in medicine and healthcare-related innovations

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Putting image-guided radiotherapy to the test

25 Sep 2018 Sponsored by Modus QA

Early adopters are exploiting a novel motion phantom to explore the possibilities of real-time magnetic-resonance guided radiotherapy for improved cancer treatment and neurosurgery

Upcoming techniques based on magnetic-resonance guided radiotherapy (MRgRT) could enable clinicians to compensate for patient movements to a much higher degree, thanks to the clarity offered by MR imaging. Real-time tracking methods that can pinpoint changes in the position of a tumour translate to improvements in dose conformality by keeping radiation on target and sparing healthy tissue.

As vendors, early adopters and clinicians bring new ideas to fruition, a key part of their success depends on having the right development tools, which includes motion (or 4D) phantoms. Accurate models give researchers the chance to safely explore solutions for overcoming hurdles that can be faced in the clinic as a result of tumour motion. Scenarios include when a patient breathes, causing organs and tumours to move, or when there’s peristaltic motion through the digestive system.

We’ve designed our system to be compatible and expandable, and even – to a certain degree – customizable

Enzo Barberi, director of MR product development at Modus QA
Tumour movement has always challenged cancer treatment and manufacturers have worked hard to mitigate the issue as much as possible.

“Image guidance for radiation therapy has been around for well over a decade and most linacs have some form of cone-beam CT or EPID imaging that allows to them to roughly see where the target is,” says Enzo Barberi, director of MR product development at Modus QA – a developer and manufacturer of quality assurance tools for advanced radiotherapy and medical imaging. “But those imaging techniques provide little information about soft tissue.”

In contrast, MR imaging can reveal soft tissue in exquisite detail, which – when linked to a radiotherapy system – shines a welcome light on where the cancer is at any moment in time.

Barberi, who’s been working in this field for almost three decades, confirms that it’s a very exciting time in terms of the technology and the clinical development of next-generation techniques exploiting MR linacs. “In both systems that are available today, you can image while you are applying radiation,” he points out.

Real-time imaging hits the target

On-board MR imaging offers numerous possibilities for advancing radiotherapy treatment. For example, if gas happens to pass through the intestinal tract of a patient during radiation treatment, real-time MR imaging can detect whether the tumour has moved. And, if the target is now positioned outside the safety margins, the beam can be turned off until the gas has passed through and the tumour moves back into position.

“It’s a dramatic example of how the combination of these two techniques in parallel and in real-time can make a big difference in terms of accuracy in hitting the target when it’s moving,” Barberi comments. Real-time imaging using MR could also see the end of so-called gating, where patients are required to hold their breath to keep their chest stationary – a development that could speed up treatment as well as reducing discomfort.

Bringing these new techniques into the clinic requires reliable tools for quality assurance (QA). MRI-compatible models make it possible to test the ability of novel imaging sequences to track a wide range of movements – such as those resulting from respiration. Verification is important too.

“Using phantoms like Modus’ programmable QUASAR MRI 4D motion product in combination with dosimetry inserts allows early adopters to calculate and measure the dose that is administered to a moving target and ensure that they are actually hitting this moving target and not the surrounding healthy tissue,” says Barberi.

These early adopters are important beta-testers for Barberi and his team, as they are at the frontier of MRgRT. Users require a phantom design that’s flexible, practical and easy to deploy, allowing them to gather as much data as possible for a range of possible patient scenarios.

“Modus focuses very heavily on workflow as we understand that time on the system is valuable,” Barberi comments. “If we can make our QA tools and QA procedures fast and efficient then sites are not only more likely to use them, but they will also appreciate the fact that we’re not taking up a lot of their magnet and linac time simply for setup or integration or when they have to switch over from one mode of measurement to another.”

Early adopters drive development

Features of the QUASAR motion phantom include a spherical target that can mimic numerous trajectories of a tumour in the body, including those seen during breathing. “We can add not only linear motion in and out of the phantom, but we can also add twist and offset that sphere so that it follows a complex 3D path as time plays out,” Barberi explains.

His team acknowledges that different investigators will have different demands, such as when it comes to dosimetry. “Users may wish to use ion chambers or film dosimetry or 3D gel dosimetry,” Barberi notes. “So we’ve designed our system to be compatible and expandable, and even – to a certain degree – customizable.”

Barberi’s group is already working on a second wave of inserts for the MR-safe motion phantom, thanks to the early-adopter programme. The new inserts will focus not just on soft tissue sites, but also modelling deep organ areas and more complex types of motion.

There is no shortage of challenges coming down the pipeline, but Barberi has a great team and is confident in Modus’ approach – having seen its flagship products develop successfully along a similar path. “Working with many different clinicians, physicists and OEMs over the years, we have families of different inserts that we can draw upon,” he says.

Barberi describes MRgRT as a “game changer”, and companies such as Modus are part of a big global effort to support upcoming advances that serve to accelerate the adoption of MR-linac systems for clinical treatment. Initiatives include STARLIT (System Technologies for Adaptive Real-time MR image-guided Therapies), a consortium developing techniques for next-generation motion compensation that includes two large equipment vendors – Elekta and Philips – along with small- and medium-sized companies and academic centres. “We are also equally proud to be a partner with ViewRay, supporting the requirements of an equally respected vendor, and their early adopting customers,” he says.

For more information about the QUASAR phantom, visit https://modusqa.com/mri/motion

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First UK radiation treatment using MR-guided linac

25 Sep 2018 Tami Freeman

The Royal Marsden and the Institute of Cancer Research (ICR) in London have performed the first treatment in the UK using an MR-linac – the Elekta Unity system.

The Elekta Unity, which received its CE mark in June 2018 and is being clinically implemented in European cancer centres, combines high-field (1.5 T) MR imaging, precision radiation therapy and intelligent software to deliver MR-guided radiotherapy.

“It’s hugely exciting to be able to trial this technology here at the ICR and The Royal Marsden,” says Uwe Oelfke, head of the Joint Department of Physics. “Together we’ve made world-leading advances in radiotherapy through our research and we expect Elekta Unity to allow us to make another step change in improving cancer treatment. This trial is for prostate cancer, but we anticipate Elekta Unity will help us improve radiotherapy for a wide range of cancers, including hard-to-treat forms such as lung and pancreatic cancer.”

The patient received treatment as part of the PRISM clinical trial, which will assess the feasibility of delivering radical radiotherapy for prostate cancer using the MR-linac. The patient had a localized prostate cancer and started hormone treatment in May 2018. His PSA (prostate-specific antigen) level indicated that he was ready to start radiotherapy and he was offered treatment on the Elekta Unity.

“Tumour shape and position relative to healthy tissue evolve over the course of treatment and can change during an individual treatment session,” explains Alison Tree, who is leading the PRISM trial. “The ability to detect those changes and adapt therapy in real time allows us to improve the precision of radiation therapy, more effectively treating the tumour while preserving healthy tissue.”

Tree notes that the Elekta Unity will also enable radiation treatment of patients who would not be candidates using more traditional radiation delivery systems.

“For decades, the radiation oncology community has dreamed of the day when we could see what we treat in real time just as our surgical colleagues do, and we are excited that this day has arrived,” says Oelfke. “Radiotherapy is important to the treatment of around 40% of the people who are cured of cancer. But if we want to fully unlock the potential of radiotherapy by making it even more precise, we need to be able to see a patient’s tumour while we deliver the radiation treatment. The MR-linac will make this possible.”

The Royal Marsden and the ICR are founding members of Elekta’s MR-linac Consortium, a collaborative industrial–academic partnership that Elekta founded with seven centres and technology partner, Philips, in 2012.

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