Browsing by Subject "EPID"

Background: The verification of VMAT delivery accuracy is widely performed with measurement-based QA methods and gamma index test evaluation. Having the gantry speed as an element of modulation requires that VMAT QA methods resolve the gantry angle accuracy during delivery. EPIDs have increasingly been used for VMAT QA and its minimal size limitation make it advantageous for the measurement of large fields. In this work, we implemented a gantry-resolved EPID-based QA method for patient-specific QA and evaluated its performance for large VMAT fields based on gamma index analysis and process-based tolerance and action limits.

Materials and Methods: Our method created gantry-resolved pseudo-3D dose distributions from XIM files acquired in TrueBeam using dosimetry mode acquisition. The method was used for the evaluation of 35 large VMAT fields measured with two different EPID models and two energies, accounting a total of 140 fields divided into 4 different processes. Predicted portal dose distributions were calculated based on MU information contained in the image headers. An independent calibration procedure that only requires MATLAB for the full implementation of the method was developed. Gamma index analyses were performed with a two-step calculation algorithm that increases accuracy in steep dose gradient regions. Acquisition artifacts causing MU information variability and banding patterns were addressed. Gamma pass rates for pseudo-3D and composite 2Ddose distributions were used to calculate process-based tolerance and action limits following the TG-218 methodology.

Results: The methods to increase gamma index calculation accuracy and reduce artifacts greatly improved the performance and reduced the variation of our results. An independent calibration procedure was successfully implemented. All calculated tolerance limits were stricter than the action limits, and no gamma pass rate from the pseudo-3D distributions with global normalization fell outside of the recommended universal action limit of 90%.

Conclusion: We have demonstrated that our software is suitable for use in patient-specific QA of large VMAT fields. Our results met the recommendation of TG-218. The differences in performance among the processes illustrated they are affected by different sources of variation, indicating that improvements are possible to obtain stricter process-specific tolerance and action limits.

Keywords: VMAT QA, EPID, gantry-resolved, gamma analysis, TG-218, tolerance limits

Purpose: Pre-treatment IMRT and VMAT QA techniques are often commissioned without knowledge of their sensitivity to clinically relevant delivery errors. The purpose of this work is to develop a method to quantify the sensitivity and specificity of pre-treatment IMRT and VMAT QA techniques to treatment delivery errors.

Materials and Methods: To evaluate a QA technique, a population of treatment plans and a population of clinically relevant delivery errors are defined. For each delivery error, a threshold magnitude is determined that induces a substantial change in clinically relevant dosimetric indices. Errors at the threshold magnitude are introduced into the plans and QA is performed with and without intentionally introduced errors. The QA technique is treated as a binary classifier to predict error plans using Receiver Operator Characteristic (ROC) analysis. We applied this technique to evaluate portal imager and 2D ion chamber array based QA for VMAT treatment of brain lesions. Delivery errors included discrepancies in MLC positioning (single leaf and leaf bank); lag of MLC trajectory; and discrepancy in dose rate per control point or gantry angle. The threshold magnitude was determined by achieving a 5% change in target conformity index.

Results: The area under the curve (AUC) for the ROC analysis was 0.592 and 0.509 for the ion chamber array and portal imager, respectively, using a gamma index of 3%, 3mm. The AUC increased to 0.632 and 0.777 when 2%, 2mm was used for the ion chamber array and portal imager, respectively. Comparison based on 3% dose agreement resulted in an AUC of 0.557 and 0.693, respectively.

Conclusion: For both portal imager and ion chamber array based QA, stricter tolerance than 3%, 3mm is needed to detect clinically relevant delivery errors. This method can be used to quantitatively compare the sensitivity of various QA techniques to clinically relevant dosimetric errors.

Purpose: Changes in patient volume, due to tumor shrinkage, dehydration, dysphagia and atrophy, could present issues in the accuracy of dosimetry throughout the course of treatment. The aim of this work is to study the dosimetric impacts of the volumetric changes during IMRT and to investigate the feasibilities of electronic portal imaging device (EPID) in predicting the impacts. Materials and Methods: An anthropomorphic head and neck phantom was used to represent two scenarios: symmetric and asymmetric volume loss. The phantom was simulated and planned according to the head and neck protocols used in our clinic. Dose volume histograms (DVH) were generated for each set up scenario and were used to calculate the integral dose expected at the coincident volume of the phantom. During treatment delivery, the EPID captured exit fluence of each beam at each level of bolus thickness. These images were quantitatively analyzed using gamma analysis with criteria of 3% and 3mm dose difference and distance-to-agreement respectively. Results: Comparing maximum to minimum volume in the symmetric situation with DVH generated in Eclipse show substantial fluctuations in dose. When comparing five layers of bolus material to zero layers of bolus material, the changes were most significant. The asymmetric volume change predicted dose fluctuations that were less significant than the symmetric phantom. As for gamma analysis, a quantitative evaluation of the integrated dose fluence, captured by the EPID, showed extreme variability in the images with five layers of bolus when compared to images with no bolus. Less significant variation was shown in layers of closer thicknesses, as expected. Conclusions: The phantom study indicates that volume loss could contribute to clinically considerable changes in the dose delivered to target and organs at risk. The proposed technique using EPID could provide valuable information about the variation of dose due to volumetric changes and might be potentially useful.