A comparative study between the imaging system and the optical tracking system in proton therapy at CNAO

Abstract

The synergy between in-room imaging and optical tracking, in co-operation with highly accurate robotic patient handling represents a concept for patient-set-up which has been implemented at CNAO (Centro Nazionale di Adroterapia Oncologica). In-room imaging is based on a double oblique X-ray projection system; optical tracking consists of the detection of the position of spherical markers placed directly on the patient's skin or on the immobilization devices. These markers are used as external fiducials during patient positioning and dose delivery. This study reports the results of a comparative analysis between in-room imaging and optical tracking data for patient positioning within the framework of high-precision particle therapy. Differences between the optical tracking system (OTS) and the imaging system (IS) were on average within the expected localization accuracy. On the first 633 fractions for head and neck (H&N) set-up procedures, the corrections applied by the IS, after patient positioning using the OTS only, were for the mostly sub-millimetric regarding the translations (0.4 ± 1.1 mm) and sub-gradual regarding the rotations (0.0° ± 0.8°). On the first 236 fractions for pelvis localizations the amplitude of the corrections applied by the IS after preliminary optical set-up correction were moderately higher and more dispersed (translations: 1.3 ± 2.9 mm, rotations 0.1 ± 0.9°). Although the indication of the OTS cannot replace information provided by in-room imaging devices and 2D-3D image registration, the reported data show that OTS preliminary correction might greatly support image-based patient set-up refinement and also provide a secondary, independent verification system for patient positioning.

Keywords: IGRT; head and neck; optical tracking system; particle therapy; patient positioning; pelvis localizations.

Fig. 1.

One of the three treatment rooms at CNAO. Each room is composed of three different devices. The Patient Positioning System (PPS) (A) on which is docked the couch, the Patient Verification System (PVS), an imaging system fixed to the ceiling from which kV X-ray tubes (B) and flat panels (C) are deployed, and the Optical Tracking System (OTS) (D) composed of three infrared cameras mounted on the nozzle and detecting the positions of reflecting markers fixed on the patient's contention device.

Fig. 2.

The patient to be simulated or treated is immobilized by the immobilization device (a mask for head and neck regions, on the left of the image – a cast for pelvic regions, on the right of the image) fixed to the couch. The markers (six for head and neck regions, seven for pelvic regions) are fixed on the mask/cast in a position that will be visible to the three infrared cameras in the treatment room, in order to be exploitable by the OTS.

Fig. 3.

Patient workflow performed at the CNAO before the dose delivery. The patient is immobilized outside the treatment room and afterwards is introduced into the treatment room, docked to the PPS and driven to the planned position for treatment. Only then, corrections (firstly from the OTS and secondly by the PVS) are applied and repeated if needed. Once this process is concluded, the patient is in the expected (or nominal) position and the dose can be delivered.