To enable 3D imaging prior to irradiation, physicists in the US have commissioned and characterized a compact CT scanner. Now in clinical use at the UF Health Cancer Center - Orlando Health, phantom measurements demonstrated the AIRO Mobile CT system has a high absolute localization accuracy of 0.28 mm and 0.6 degrees (J. Appl. Clin. Med. Phys. doi: 10.1002/acm2.12084).

"The AIRO provides soft tissue-based localization accuracy that kV/kV [imaging] can't provide," said first author Jasmine Oliver. "It also provides 3D imaging that can potentially reveal tumour and surrounding anatomical changes, which kV/kV can't. Additionally, the AIRO's simple workflow allows the physician to monitor tumour and anatomy more easily and more frequently."

While several systems are available for in-room CT, Oliver and colleagues chose the AIRO system, made by Mobius Imaging, for its small footprint of 1.28 m2 and the ease with which it could be moved in and out of the treatment room. Size was an important factor as the centre has a compact Mevion S250 treatment system with limited space in the treatment room. Their study is the first to report AIRO's application to proton therapy; the system is typically used for surgical imaging.

The team investigated image quality, localization accuracy and imaging dose of the AIRO system. A CATPHAN phantom was scanned to assess image quality. Images of the phantom, which were acquired using a pre-set clinical head protocol, were compared with images from two CT simulators and one cone-beam imager used to plan and deliver photon-based radiotherapy at the centre.

Examination of several parameters revealed the AIRO system had inferior image quality. Spatial resolution, for example, assessed using CATPHAN's line pair gauge, was 0.21 line pairs per mm compared to 0.35–0.40 line pairs per mm for the other systems. Separate measurements also revealed higher imaging doses using the AIRO system compared to the other systems. The higher doses should be taken into account if multiple scans are to be performed on each day of treatment, wrote the authors.

Despite inferior image quality, however, measurements of the AIROS system revealed superior localization accuracy. In one of two experiments, the authors assessed AIRO against the kV/kV gold standard used clinically at the centre using a Stereophan phantom. Shifts measured by image registration software upon comparison of AIRO and planning CT images were within 0.2 mm of those reported for kV/kV images.

In a second assessment of localization accuracy used to commission the AIRO system for clinical use, the researchers measured the system's ability to detect translations and rotations of a MIMI phantom. Here, it achieved an absolute localization accuracy of 0.28 mm and 0.6°. Accuracies of 1.4 mm and 0.3 mm have been reported in the literature for other proton image-guidance systems. "Based on our study, the AIRO can be used accurately and reliably for image guidance and adaptive purposes," said Oliver.

The Orlando clinic has been using the AIRO system since October last year in thorax, breast and head-and-neck cases. Its use requires one therapist to move the system into the room, while a second therapist sets up the patient. An extra 5–10 minutes is needed to perform the scan.

"We are still exploring its use on all disease sites and we hope to expand its use on paediatric patients soon," Oliver told medicalphysicsweb. In ongoing research, the authors are also investigating the use of AIRO for adaptive proton therapy and plan to publish their findings within the next 12 months.

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