Assessing the accuracy of electronic portal imaging device (EPID)-based dosimetry: II. Evaluation of a dosimetric uncertainty budget and development of a new film-in-EPID absorbed dose calibration methodology

PURPOSE

The aim of this study is to reduce the uncertainty associated with determining dose-to-water using an amorphous silicon electronic portal imaging detector (EPID) under reference conditions by developing a direct calibration formalism based on radiochromic film measurements made within the EPID panel and detailed Monte Carlo simulations. To our knowledge, this is the first EPID-based dosimetry study reporting an uncertainty budget .

METHODS

Pixel sensitivity and relative off-axis response was mapped by simultaneously irradiating film contained within the imager panel and acquiring an EPID image set. The detector panel was disassembled for the purpose of modeling the EPID in detail using the EGSnrc DOSXYZnrc usercode, which was in turn used to calculate dose-to-film in EPID to dose-to-water in water conversion factors .

RESULTS

A direct comparison of the two correction methodologies investigated in this work, the previously established empirical method and the proposed simultaneous measurement approach involving in-EPID film dosimetry, produced an agreement with an RMS deviation of 1.4 % overall. A combined standard relative uncertainty of 3.3 % (k = 1) was estimated for the determination of absorbed dose to water at the position of the EPID using the proposed calibration methodology .

CONCLUSIONS

This work describes a direct method of calibrating EPID response in terms of absorbed dose to water requiring fewer measurements than other empirical approaches, and without 2D spatial interpolation of correction factors. This article is protected by copyright. All rights reserved.

Planar EPID-Based Dosimetry for SRS and SRT Patient-Specific QA

The study’s purpose was to develop and validate Electronic Portal Imaging Device (EPID)-based dosimetry for Stereotactic Radiosurgery (SRS) and Stereotactic Radiation Therapy (SRT) patient-specific Quality Assurance (QA). The co-operation between extended Source-to-Imager Distance (SID) to reduce the saturation effect and simplify the EPID-based dosimetry model was used to perform patient-specific QA in SRS and SRT plans. The four parameters were included for converting the image to dose at depth 10 cm; dose-response linearity with MU, beam profile correction, collimator scatter and water kernel. The model accuracy was validated with 10 SRS/SRT plans. The traditional diode arrays with MapCHECK were also used to perform patient-specific QA for assuring model accuracy. The 150 cm-SID was found a possibility to reduce the saturation effect. The result of model accuracy was found good agreement between our EPID-based dosimetry and TPS calculation with GPR more than 98% for gamma criteria of 3%/3 mm, more than 95% for gamma criteria of 2%/2 mm, and the results related to the measurement with MapCHECK. This study demonstrated the method to perform SRT and SRT patient-specific QA using EPID-based dosimetry in the FFF beam by co-operating between the extended SID that can reduce the saturation effect and estimate the planar dose distribution with the in-house model.

Commissioning and quality assurance for VMAT delivery systems: An efficient time-resolved system using real-time EPID imaging

PURPOSE

An ideal commissioning and quality assurance (QA) program for Volumetric Modulated Arc Therapy (VMAT) delivery systems should assess the performance of each individual dynamic component as a function of gantry angle. Procedures within such a program should also be time-efficient, independent of the delivery system and be sensitive to all types of errors. The purpose of this work is to develop a system for automated time-resolved commissioning and QA of VMAT control systems which meets these criteria.

METHODS

The procedures developed within this work rely solely on images obtained, using an electronic portal imaging device (EPID) without the presence of a phantom. During the delivery of specially designed VMAT test plans, EPID frames were acquired at 9.5 Hz, using a frame grabber. The set of test plans was developed to individually assess the performance of the dose delivery and multileaf collimator (MLC) control systems under varying levels of delivery complexities. An in-house software tool was developed to automatically extract features from the EPID images and evaluate the following characteristics as a function of gantry angle: dose delivery accuracy, dose rate constancy, beam profile constancy, gantry speed constancy, dynamic MLC positioning accuracy, MLC speed and acceleration constancy, and synchronization between gantry angle, MLC positioning and dose rate. Machine log files were also acquired during each delivery and subsequently compared to information extracted from EPID image frames.

RESULTS

The largest difference between measured and planned dose at any gantry angle was 0.8% which correlated with rapid changes in dose rate and gantry speed. For all other test plans, the dose delivered was within 0.25% of the planned dose for all gantry angles. Profile constancy was not found to vary with gantry angle for tests where gantry speed and dose rate were constant, however, for tests with varying dose rate and gantry speed, segments with lower dose rate and higher gantry speed exhibited less profile stability. MLC positional accuracy was not observed to be dependent on the degree of interdigitation. MLC speed was measured for each individual leaf and slower leaf speeds were shown to be compensated for by lower dose rates. The test procedures were found to be sensitive to 1 mm systematic MLC errors, 1 mm random MLC errors, 0.4 mm MLC gap errors and synchronization errors between the MLC, dose rate and gantry angle controls systems of 1°. In general, parameters measured by both EPID and log files agreed with the plan, however, a greater average departure from the plan was evidenced by the EPID measurements.

CONCLUSION

QA test plans and analysis methods have been developed to assess the performance of each dynamic component of VMAT deliveries individually and as a function of gantry angle. This methodology relies solely on time-resolved EPID imaging without the presence of a phantom and has been shown to be sensitive to a range of delivery errors. The procedures developed in this work are both comprehensive and time-efficient and can be used for streamlined commissioning and QA of VMAT delivery systems.