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Radiotherapy

Radiotherapy

Optical surface tracker monitors patient position during brain radiotherapy

18 May 2020
Optical surface tracking
The experimental setup, showing a mannequin training head on the couch of a TrueBeam linac. The cameras of the Catalyst HD optical surface tracking are attached to the ceiling. (Courtesy: CC BY 4.0/J. Appl. Clin. Med. Phys. 10.1002/acm2.12866)

A commercial optical surface tracking (OST) system can monitor a patient’s position with submillimetre accuracy during radiotherapy, according to tests conducted at Maastro Clinic in the Netherlands. With this system, non-coplanar single isocentre stereotactic radiosurgery (SRS) treatments of multiple brain metastases are feasible and safe.

Progress in linac-based treatments of brain metastases has led to the use of volumetric-modulated arc therapy (VMAT) techniques, preferably using a single isocentre to make dose delivery faster and more efficient. It is imperative, however, to reduce the GTV–PTV (gross tumour volume–planning target volume) margin to 1 mm to minimize or prevent radionecrosis and the development of debilitating side effects. And to preserve this 1 mm GTV–PTV margin, the patient needs to be monitored in real time to maintain accurate positioning.

OST systems for radiotherapy use advanced technologies to precisely monitor a patient’s external surface. In addition to reducing setup errors, OST can provide continuous intra-fractional motion surveillance during treatment, without the use of ionizing radiation.

The research team evaluated a three-camera Catalyst HD system, which uses LEDs to project three wavelengths of light onto the patient and a CCD camera to detect the reflected light. The system uses the reflected signals to generate a real‐time 3D surface of the patient, which is compared to a reference surface to verify setup. The system can be used for different skin tones, by employing individualized camera settings for gain and saturation.

For the study, lead author Ans Swinnen and colleagues used a TrueBeam STx linac equipped with a high-definition multileaf collimator and a six-degrees-of-freedom couch. To evaluate setup accuracy, they compared the isocentre shifts calculated by the OST system with the ones suggested after image verification with the on-board kilovoltage imaging system, at couch angles of 0° and 270°. Deviations between the isocentre shifts in rotational and translational directions were within 0.2° for both couch positions, and within 0.1 and 0.5 mm at 0° and 270°, respectively.

The researchers also performed film measurements at three depths in a Rando-Alderson phantom using a single isocentre non-coplanar VMAT plan containing four brain lesions. They report that dose deviations between the film-measured and treatment planning system-predicted doses in the centres of the four target lesions were –1.2%, –0.1%, 0.0% and –1.9%.

To verify that the OST system could accurately visualize a patient at various couch angles, the researchers used a mannequin training head in an open face mask. They subsequently tested the OST on seven volunteers, each of whom were monitored three times to represent three consecutive treatment fractions. They collected setup data to evaluate the accuracy and reproducibility of the OST system at couch rotation angles of 0°, 45°, 90°, 315° and 270°.

When the volunteers were tested, the couch rotational drift was impacted by their individual weights, as well as the position of the couch relative to the couch pedestal. The mean translational isocentre shifts for the seven volunteers were less than 0.6 mm. The largest isocentre displacements monitored by the OST system were obtained in the lateral and longitudinal directions of a couch positioned at 45°.

Because of this, the researchers recommend that a regular Winston-Lutz (WL) test (a procedure for verifying the linac isocentre) should be part of the SRS-specific linac quality assurance programme. They are in the process of developing a practical WL test-based phantom that integrates measurement of the isocentre congruence of imaging systems, radiation beam and couch rotation axis with submillimetre accuracy, and measurements of the isocentricity of the OST system.

Additional research will focus on testing similar OST systems for brain tumour patients treated with proton beam therapy on a Mevion S250i accelerator. For this purpose, the radiotherapy centre has installed a four-camera Catalyst HD system to monitor a patient on the robotic couch moving in and out of the in-room cone-beam CT scanner and towards the different treatment positions. A proton-compatible open-face mask has been manufactured and will be tested in the near future.

As the accuracy in patient setup during the various couch movements may be even more important in brain treatments with proton therapy, we expect the potential of this intra-fraction monitoring system to be even higher,” the researchers write.

This research is reported in the Journal of Applied Clinical Medical Physics.

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