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

Patient specific evaluation of breathing motion induced interplay effects

PURPOSE

In lung SABR, interplay between target motion and dynamically changing beam parameters can affect the target coverage. To identify the potential need for motion-management techniques, a comprehensive methodology for pre-treatment estimation of interplay effects has been implemented.

METHODS

In conjunction with an alpha-version of VeriSoft and OCTAVIUS 4D (PTW-Freiburg, Germany), a method is presented to calculate a virtual, motion-simulated 3D dose distribution based on measurement data acquired in a stationary phantom and a subsequent correction with time-dependent target-motion patterns. In-house software has been developed to create user-defined motion patterns based on either simplistic or real patient-breathing patterns including the definition of the exact beam starting phase. The approach was validated by programmed couch and phantom motion during beam delivery. Five different breathing traces with extremely altered beam-on phases (0 % and 50 % respiratory phase) and a superior-inferior motion altitude of 25 mm were used to probe the influence of interplay effects for 14 lung SABR plans. Gamma analysis (2 %/2mm) was used for quantification.

RESULTS

Validation measurements resulted in >98 % pass rates. Regarding the interplay effect evaluation, gamma pass rates of <92 % were observed for sinusoidal breathing patterns with <25 number of breaths per delivery time (NBs) and realistic patterns with <18 NBs.

CONCLUSION

The potential influence of interplay effects on the target coverage is highly dependent on the patient's breathing behaviour. The presented moving-platform-free approach can be used for verification of ITV-based treatment plans to identify whether the clinical goals are achievable without explicit use of a respiratory management technique.

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

Simple Summary

The interplay of breathing and beam motion reduces the efficacy of particle irradiation in moving tumours. The effect of motion on protons and carbon ion treatments was investigated dosimetrically and the results were benchmarked against each other by employing an anthropomorphic thorax phantom that was able to simulate tumour, rib, and lung motion. The critical question was whether target coverage and organ-at-risk sparing could be maintained when the application of simple motion mitigation was addressed. Special focus was put on unique synchrotron characteristics, such as pulsed beam delivery and beam intensity variations. It could be demonstrated that the effect of motion was greater for carbon ions than for protons. These findings demonstrated the need for applying motion mitigation techniques depending on the motion amplitude, particle type, and treatment prescription considering complex time correlations.

Abstract

Motion amplitudes, in need of mitigation for moving targets irradiated with pulsed carbon ions and protons, were identified to guide the decision on treatment and motion mitigation strategy. Measurements with PinPoint ionisation chambers positioned in an anthropomorphic breathing phantom were acquired to investigate different tumour motion scenarios, including rib and lung movements. The effect of beam delivery dynamics and spot characteristics was considered. The dose in the tumour centre was deteriorated up to 10% for carbon ions but only up to 5% for protons. Dose deviations in the penumbra increased by a factor of two when comparing carbon ions to protons, ranging from 2 to 30% for an increasing motion amplitude that was strongly dependent on the beam intensity. Layer rescanning was able to diminish the dose distortion caused by tumour motion, but an increase in spot size could reduce it even further to 5% within the target and 10% at the penumbra. An increased need for motion mitigation of carbon ions compared to protons was identified to assure target coverage and sparing of adjacent organs at risk in the penumbra region and outside the target. For the clinical implementation of moving target treatments at a synchrotron-based particle facility complex, time dependencies needed to be considered.

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

Four-dimensional dose calculation (4D-DC) is crucial for predicting the dosimetric outcome in the presence of intra-fractional organ motion. Time-resolved dosimetry can provide significant insights into 4D pencil beam scanning dose accumulation and is therefore irreplaceable for benchmarking 4D-DC. In this study a novel approach of time-resolved dosimetry using five PinPoint ionization chambers (ICs) embedded in an anthropomorphic dynamic phantom was employed and validated against beam delivery details. Beam intensity variations as well as the beam delivery time structure were well reflected with an accuracy comparable to the temporal resolution of the IC measurements. The 4D dosimetry approach was further applied for benchmarking the 4D-DC implemented in the RayStation 6.99 treatment planning system. Agreement between computed values and measurements was investigated for (i) partial doses based on individual breathing phases, and (ii) temporally distributed cumulative doses. For varied beam delivery and patient-related parameters the average unsigned dose difference for (i) was 0.04 ± 0.03 Gy over all considered IC measurement values, while the prescribed physical dose was 2 Gy. By implementing (ii), a strong effect of the dose gradient on measurement accuracy was observed. The gradient originated from scanned beam energy modulation and target motion transversal to the beam. Excluding measurements in the high gradient the relative dose difference between measurements and 4D-DCs for a given treatment plan at the end of delivery was 3.5% on average and 6.6% at maximum over measurement points inside the target. Overall, the agreement between 4D dose measurements in the moving phantom and retrospective 4D-DC was found to be comparable to the static dose differences for all delivery scenarios. The presented 4D-DC has been proven to be suitable for simulating treatment deliveries with various beam- as well as patient-specific parameters and can therefore be employed for dosimetric validation of different motion mitigation techniques.

A novel dynamic robotic moving phantom system for patient-specific quality assurance in real-time tumor-tracking radiotherapy

In this study, we assess a developed novel dynamic moving phantom system that can reproduce patient three-dimensional (3D) tumor motion and patient anatomy, and perform patient-specific quality assurance (QA) of respiratory-gated radiotherapy using SyncTraX. Three patients with lung cancer were enrolled in a study. 3D printing technology was adopted to obtain individualized lung phantoms using CT images. A water-equivalent phantom (WEP) with the 3D-printed plate lung phantom was set at the tip of the robotic arm. The log file that recorded the 3D positions of the lung tumor was used as the input to the dynamic robotic moving phantom. The WEP was driven to track 3D respiratory motion. Respiratory-gated radiotherapy was performed for driving the WEP. The tracking accuracy was calculated as the differences between the actual and measured positions. For the absolute dose and dose distribution, the differences between the planned and measured doses were calculated. The differences between the planned and measured absolute doses were <1.0% at the isocenter and <4.0% for the lung region. The gamma pass ratios of γ_3 mm/3% and γ_2 mm/2% under the conditions of gating and no-gating were 99.9 ± 0.1% and 90.1 ± 8.5%, and 97.5 ± 0.9% and 68.6 ± 17.8%, respectively, for all the patients. Furthermore, for all the patients, the mean ± SD of the root mean square values of the positional error were 0.11 ± 0.04 mm, 0.33 ± 0.04 mm, and 0.20 ± 0.04 mm in the LR, AP, and SI directions, respectively. Finally, we showed that patient-specific QA of respiratory-gated radiotherapy using SyncTraX can be performed under realistic conditions using the moving phantom.

Development of a novel methodology for QA of respiratory-gated and VMAT beam delivery using Octavius 4D phantom

The objective of this study was to develop and evaluate a series of quality assurance (QA) techniques based on Octavius 4D phantom for testing of respiratory-gated treatment delivery, integrity of dose rate vs gantry speed in volumetric-modulated arc therapy (VMAT) commissioning, and multileaf collimator (MLC) positioning accuracy of a linear accelerator. An Octavius 4D phantom capable of rotating with the gantry and recording the detector signal with a sampling rate of 10 Hz was isocentrally set up and an inclinometer was also installed to measure the gantry angle simultaneously. A simple arc test was created and delivered with gating function activated to measure the timing accuracy of the gating window. A tailor-made dose rate vs gantry speed plan was also designed to test the accuracy of measured dose rate, gantry speed, and actual control points. All experiments were conducted while machine log files were collected for comparison. The variations of beam flatness, symmetry, and field size were analyzed as a function of gantry angle to evaluate the influence from the modulation of dose rate and gantry speed. MLC position accuracy was evaluated based on specific garden fence plans. The time of gating window was measured to be less than 10-millisecond deviation from the log data. Gantry backlash was observed and quantified to be 1.72° with an extra stabilization time of 1.16 seconds for a gating arc with gantry speed of 6°/s. In the dose rate vs gantry speed test, the mean deviation between measured gantry angle and log data was less than 0.2° after a time delay of 0.25 second was corrected. The measured dose rate agreed with the log data very well with a mean deviation of 0.05%, and even the transit of modulation was tracked successfully. There was a statistically significant difference on the variation of beam parameters between a VMAT plan and a simple arc plan. The induced MLC position errors were detected with an accuracy of 0.05 mm. The leaf position reproducibility was found to be better than 0.02 mm, whereas the routine MLC position accuracy was better than 0.1 mm. A time-resolved method using Octavius 4D phantom has been developed and proven to be convenient for respiratory gating QA, dose rate vs gantry speed test, and MLC QA. Gating time, dose rate, and gantry speed-induced leave position error could be directly measured with high accuracy after comparison with the machine log data. This study also highlights the capability of the phantom in quantifying the variation of flatness, symmetry, and field size during gantry rotation.

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

PURPOSE

The aim of this study was to present a novel 2041 liquid-filled ionization chamber array for high-resolution verification of radiotherapy treatments.

MATERIALS AND METHODS

The prototype has 2041 ionization chambers of 2.5 × 2.5 mm² area filled with isooctane. The detection elements are arranged in a central square grid of 43 × 43, totally covering an area of 107.5 × 107.5 mm² . The central inline and cross-line are extended to 227 mm and the diagonals to 321 mm to be able to perform profile measurements of large fields. We have studied stability, pixel response uniformity, dose rate dependence, depth and field size dependence and anisotropy. We present results for output factors, tongue-and-groove, garden fence, small field profiles, irregular fields, and verification of dose planes of patient treatments.

RESULTS

Comparison with other detectors used for small field dosimetry (SFD, CC13, microDiamond) has shown good agreement. Output factors measured with the device for square fields ranging from 10 × 10 to 100 × 100 mm² showed relative differences within 1%. The response of the detector shows a strong dependence on the angle of incident radiation that needs to be corrected for. On the other hand, inter-pixel relative response variations in the 0.95-1.08 range have been found and corrected for. The application of the device for the verification of dose planes of several treatments has shown gamma passing rates above 97% for tolerances of 2% and 2 mm. The verification of other clinical fields, like small fields and irregular fields used in the commissioning of the TPS, also showed large passing rates. The verification of garden fence and tongue-and-groove fields was affected by volume-averaging effects.

CONCLUSIONS

The results show that the liquid filled ionization chamber prototype here presented is appropriate for the verification of radiotherapy treatments with high spatial resolution. Recombination effects do not affect very much the verification of relative dose distributions. However, verification of absolute dose distributions may require normalization to a radiation field which is representative of the dose rate of the treatment delivered.