Towards clinical application: prompt gamma imaging of passively scattered proton fields with a knife-edge slit camera

Abstract

Prompt γ-ray imaging with a knife-edge shaped slit camera provides the possibility of verifying proton beam range in tumor therapy. Dedicated experiments regarding the characterization of the camera system have been performed previously. Now, we aim at implementing the prototype into clinical application of monitoring patient treatments. Focused on this goal of translation into clinical operation, we systematically addressed remaining challenges and questions. We developed a robust energy calibration routine and corresponding quality assurance protocols. Furthermore, with dedicated experiments, we determined the positioning precision of the system to 1.1 mm (2σ). For the first time, we demonstrated the application of the slit camera, which was intentionally developed for pencil beam scanning, to double scattered proton beams. Systematic experiments with increasing complexity were performed. It was possible to visualize proton range shifts of 2-5 mm with the camera system in phantom experiments in passive scattered fields. Moreover, prompt γ-ray profiles for single iso-energy layers were acquired by synchronizing time resolved measurements to the rotation of the range modulator wheel of the treatment system. Thus, a mapping of the acquired profiles to different anatomical regions along the beam path is feasible and additional information on the source of potential range shifts can be obtained. With the work presented here, we show that an application of the slit camera in clinical treatments is possible and of potential benefit.

Classification of the source of treatment deviation in proton therapy using prompt‐gamma imaging information

“…This study assumes that the treatment cases are monitored with an IBA slit camera. 16,25,29 This camera consists of two rows of 20 individual scintillation detectors and a knife-edge slit of tungsten alloy collimator. It detects energy depositions of gamma rays between 3 and 6 MeV.…”

“…23,24 Previous studies have demonstrated the accuracy and sensitivity of the PGI slit camera in both homogeneous and heterogeneous phantoms. [25][26][27] Furthermore, proof-of-principle clinical applications of the slit camera have been performed in both, double scattering and pencil-beam scanning (PBS) treatments. 23,24 However, pure detection of spot-wise range shift might be difficult to interpret concerning clinical relevance and potential source of deviation.…”

“…Detection of deviations beyond a threshold by means of machine learning analysis of PGI information was introduced by Gueth et al 28 and demonstrated for one spot of a prostate treatment plan, with restriction to patient translation errors. The purpose of the present simulation study was to investigate the feasibility of a PGI-based automated classification of several types of the error sources for head and neck tumor treatments when using the PGI slit camera system from Ion Beam Applications (IBA, Louvain-la-Neuve, Belgium), 16,25,29 shifting focus from spot-wise detection of deviations to global analysis of patterns of deviations from all spots. A decision tree model was heuristically established and developed with known error scenarios in phantom and patient scenarios of different geometrical complexity.…”

Positron emission tomography for quality assurance in proton therapy

“…The other coordinates can be obtained on a spot-by-spot basis for pencil beam scanning via a synchronization with the beam delivery system. Clinical tests have been performed using the knife-edge slit camera at the OncoRay facility for passively double scattered proton beams (Priegnitz et al, 2016). Shifts in the proton range of 2 to 5 mm could be detected using this technique.…”

“…Prompt gamma ray imaging using a knife-edge slit camera has been used in a clinical setting for a double scattering proton therapy irradiation Priegnitz et al (2016); Richter et al (2016). The sensitivity of the imaging device for several beam delivery techniques was investigated by Nenoff et al (2017), showing that applied range shifts were measured with 1 to 3 mm accuracy for IMPT and single field uniform dose (SFUD) beam delivery.…”