Time-resolved dosimetric verification of respiratory-gated radiotherapy exposures using a high-resolution 2D ionisation chamber array

Abstract

The aim of this work was to track and verify the delivery of respiratory-gated irradiations, performed with three versions of TrueBeam linac, using a novel phantom arrangement that combined the OCTAVIUS(®) SRS 1000 array with a moving platform. The platform was programmed to generate sinusoidal motion of the array. This motion was tracked using the real-time position management (RPM) system and four amplitude gating options were employed to interrupt MV beam delivery when the platform was not located within set limits. Time-resolved spatial information extracted from analysis of x-ray fluences measured by the array was compared to the programmed motion of the platform and to the trace recorded by the RPM system during the delivery of the x-ray field. Temporal data recorded by the phantom and the RPM system were validated against trajectory log files, recorded by the linac during the irradiation, as well as oscilloscope waveforms recorded from the linac target signal. Gamma analysis was employed to compare time-integrated 2D x-ray dose fluences with theoretical fluences derived from the probability density function for each of the gating settings applied, where gamma criteria of 2%/2 mm, 1%/1 mm and 0.5%/0.5 mm were used to evaluate the limitations of the RPM system. Excellent agreement was observed in the analysis of spatial information extracted from the SRS 1000 array measurements. Comparisons of the average platform position with the expected position indicated absolute deviations of <0.5 mm for all four gating settings. Differences were observed when comparing time-resolved beam-on data stored in the RPM files and trajectory logs to the true target signal waveforms. Trajectory log files underestimated the cycle time between consecutive beam-on windows by 10.0 ± 0.8 ms. All measured fluences achieved 100% pass-rates using gamma criteria of 2%/2 mm and 50% of the fluences achieved pass-rates >90% when criteria of 0.5%/0.5 mm were used. Results using this novel phantom arrangement indicate that the RPM system is capable of accurately gating x-ray exposure during the delivery of a fixed-field treatment beam.

Time-resolved dosimetry for validation of 4D dose calculation in PBS proton therapy

“…In the studies listed above, even though some detectors could perform time-resolved measurements, no temporal dosimetric analysis was considered. The use of a 2D IC array (Octavius SRS 1000, PTW-Freiburg, Germany) in time-separated mode was partially demonstrated by King et al (2016) in their work on the validation of a gating technique for photon beam deliveries. Time-resolved dosimetry by a prototype 2D diode array (Sun Nuclear Corporation-Melbourne, FL, USA) was applied for proton radiography in a passive -scattering proton beam (Testa et al 2013).…”

Development and clinical characterization of a novel 2041 liquid‐filled ionization chambers array for high‐resolution verification of radiotherapy treatments

“…3,4 Devices developed in the 2000s for the verification of IMRT broad fields 5,6 may lack the desired resolution and/or fill factor for the verification of modern radiotherapy techniques that involve small fields and steep dose gradients. 7 New devices have been made comercially available in recent years, including planar detectors like OCTAVIUS 1000 SRS (liquid isooctane) [8][9][10] and OCTAVIUS 1500 (air), 11,12 developed by PTW (Freiburg, Germany), the biplanar diode detector Delta4 (Scandidos, Sweden), 13 and the ArcCheck (SunNuclear, USA), 14,15 which has diodes arranged on a helical geometry, and new prototypes are being developed. 16 However, most of these still present insufficient sampling rates and low fill factors (except for the OCTAVIUS 1000 SRS), 7,17 which may require performing different measurements with displacements to perform a verification with appropiate sampling rate.…”

The Influence of Motion on the Delivery Accuracy When Comparing Actively Scanned Carbon Ions versus Protons at a Synchrotron-Based Radiotherapy Facility

“…Hence, measurements should be the ground truth for the implementation of motion mitigation techniques for carbon ions, which could also serve for validation of 4DDT at a later stage. For a more detailed understanding of the dosimetric effects during dose delivery to a moving system, time-resolved dosimetry with a high sampling rate using point measurements or employing ionization chamber arrays is the ideal choice [ 12 , 17 , 18 , 19 ].…”