The objective determination of performance standards for radiation therapy equipment requires, ideally, establishing the quantitative relationship between performance deviations and clinical outcome or some acceptable surrogate. In this simulation study the authors analyzed the dosimetric impact of random (leaf by leaf) and systematic (entire leaf bank) errors in the position of the MLC leaves on seven clinical prostate and seven clinical head and neck IMRT plans delivered using a dynamic MLC. In-house software was developed to incorporate normally distributed errors of up to +/- 2 mm in individual leaf position or systematic errors (+/- 1 and +/- 0.5 mm in all leaves of both leaf banks or +1 mm in one bank only) into the 14 plans, thus simulating treatment delivery using a suboptimally performing MLC. The dosimetric consequences of suboptimal MLC performance were quantified using the equivalent uniform doses (EUDs) of the clinical target volumes and important organs at risk (OARs). The deviation of the EUDs of the selected structures as the performance of the MLC deteriorated was used as the objective surrogate of clinical outcome. Random errors of 2 mm resulted in negligible changes for all structures of interest in both sites. In contrast, systematic errors can lead to potentially significant dosimetric changes that may compromise clinical outcome. If a 2% change in EUD of the target and 2 Gy for the OARs were adopted as acceptable levels of deviation in dose due to MLC effects alone, then systematic errors in leaf position will need to be limited to 0.3 mm. This study provides guidance, based on a dosimetric surrogate of clinical outcome, for the development of one component, leaf position accuracy of performance standards for multileaf collimators.
The optimal threshold values that were determined represent a maximized test sensitivity and specificity and are not subject to any user bias. When applied to the datasets that we studied, our results suggest the use of patient specific QA as a safety tool that can effectively prevent large errors (e.g., σ > 3 mm) as opposed to a tool to improve the quality of IMRT delivery.
The performance of a convolution/superposition based treatment planning system depends on the ability of the dose calculation algorithm to accurately account for physical interactions taking place in the tissue, key components of the linac head and on the accuracy of the photon beam model. Generally the user has little or no control over the performance of the dose calculation algorithm but is responsible for the accuracy of the beam model within the constraints imposed by the system. This study explores the dosimetric impact of limitations in photon beam modeling accuracy on complex 3D clinical treatment plans. A total of 70 photon beam models was created in the Pinnacle treatment planning system. Two of the models served as references for 6 MV and 15 MV beams, while the rest were created by perturbing the reference models in order to produce specific deviations in specific regions of the calculated dose profiles (central axis and transverse). The beam models were then used to generate 3D plans on seven CT data sets each for four different treatment sites (breast and conformal prostate, lung and brain). The equivalent uniform doses (EUD) of the targets and the principal organs at risk (OARs) of all plans ( approximately 1000) were calculated and compared to the EUDs delivered by the reference beam models. In general, accurate dosimetry of the target is most greatly compromised by poor modeling of the central axis depth dose and the horns, while the EUDs of the OARs exhibited the greatest sensitivity to beam width accuracy. Based on the results of this analysis we suggest a set of tolerances to be met during commissioning of the beam models in a treatment planning system that are consistent in terms of clinical outcomes as predicted by the EUD.
None of the techniques or criteria tested is sufficiently sensitive, with the population of IMRT fields, to detect a systematic MLC offset at a clinically significant level on an individual field. Patient specific QC cannot, therefore, substitute for routine QC of the MLC itself.
The authors describe a detailed evaluation of the capabilities of imaging and image registration systems available with Varian linear accelerators for image guided radiation therapy (IGRT). Specifically, they present modulation transfer function curves for megavoltage planar, kilovoltage (kV) planar, and cone beam computed tomography imaging systems and compare these with conventional computed tomography. While kV planar imaging displayed the highest spatial resolution, all IGRT imaging techniques were assessed as adequate for their intended purpose. They have also characterized the image registration software available for use in conjunction with these imaging systems through a comprehensive phantom study involving translations in three orthogonal directions. All combinations of imaging systems and image registration software were found to be accurate, although the planar kV imaging system with automatic registration was generally superior, with both accuracy and precision of the order of 1 mm, under the conditions tested. Based on their phantom study, the attainable accuracy for rigid body translations using any of the features available with Varian equipment will more likely be limited by the resolution of the couch readouts than by inherent limitations in the imaging systems and image registration software. Overall, the accuracy and precision of currently available IGRT technology exceed published experience with the accuracy and precision of contouring for planning.
Radiation therapy, along with other branches of medicine, is moving towards a firmer basis in evidence to optimally utilize resources. As new treatment technology and strategies place greater demands on quality assurance resources, the need to objectively evaluate equipment and process performance standards from the perspective of predicted clinical impact becomes more urgent. This study evaluates the appropriateness of recommended quality control tolerance and action levels for linear accelerators based on the calculated dosimetric impact of suboptimal equipment performance. A method is described to quantify the dosimetric changes, as reflected by the changes in the outcome surrogate, equivalent uniform dose (EUD), of machine performance deviations from the optimal, specifically in the range of tolerance and action levels promulgated by the Canadian Association of Provincial Cancer Agencies (CAPCA). Linear accelerator performance deviations were simulated for the treatment of prostate, breast, lung, and brain using 3D conformal techniques, and the impact evaluated in terms of the changes in the EUD of the target volumes and two principal organs at risk (OARs) per site. The eight key performance characteristics examined are: Output constancy, beam flatness, gantry angle, collimator angle, field size indicator, laser alignment (three directions) and, by inference, the optical distance indicator. Currently accepted CAPCA tolerance levels for these eight performance characteristics are shown to maintain average EUD deviations to within 2% for the targets and 2 Gy for the OARs. However, within the 2% or 2 Gy range, the recommended tolerance levels are found to have markedly different effects on the EUDs of the structures of interest.
The Multileaf Collimator (MLC), the most widely used means of intensity modulating beams for IMRT, is subject to random and systematic errors in leaf positions that may compromise the treatment quality. This work is a simulation study of the effect of random and systematic errors in leaf position on delivered dose distributions. The dosimetric effects of random errors of up to 2 mm and systematic errors (±1mm in 2 banks, ±0.5mm in 2 banks and 2mm in 1 bank of leaves) were analysed for a typical head and neck IMRT plan and a typical prostate IMRT plan. Dose Volume Histograms and Equivalent Uniform Doses (EUD) of the target volumes, bladder and rectum for the prostate plan and brainstem, larynx, parotids and spinal cord for the head and neck plan were calculated with and without MLC positioning errors and compared. The results show that if we adopt a 2% change in EUD of the target and 2 Gy for the OARs as acceptable levels of uncertainty in dose due to MLC effects only, then random errors of up to 2mm may be tolerated but systematic errors in leaf position will need to be limited to 0.5mm. Our study provides guidance, based on a surrogate of clinical outcome, for the development of quality control standards for multileaf collimators.
Patient specific IMRT QC remains standard practice in most clinics. We have evaluated the feasibility of detecting systematic errors in MLC leaf position with IMRT QC based on diode and aSi area detectors. 12 head and neck (H&N) and 14 prostate IMRT fields were delivered using MLC files containing systematic errors (±1mm in 2 banks, ±0.5mm in 2 banks and 1mm in 1 bank of leaves). Planar dose maps were measured using both Mapcheck™ (Sun Nuclear Corp.) and the aS1000 EPID (Varian Medical Systems) and compared with maps produced with unperturbed leaves. Results were analyzed using several common criteria including absolute dose difference (AD), relative dose difference (RD), distance to agreement (DTA) and the gamma index (γ). Using Mapcheck™ and the change in percentage of passing points as a measure of sensitivity, the relative sensitivity of the criteria tested in descending order is 3% AD,3mm DTA; γ with 3% AD, 3mm DTA; 5% AD, 3mm DTA and 3% RD, 3mm DTA. This sequence applied to both H&N and prostate fields although the H&N fields, being more highly modulated, exhibited greater sensitivity to systematic MLC leaf offsets. The EPID study, which was software limited to the γ criterion, showed higher sensitivity with [2% AD/2mm DTA] γ criteria. Due to the distribution of passing rates in a population of IMRT fields we conclude that patient specific QC alone is not sufficient to identify potentially clinically significant systematic MLC offsets and must be supplemented with regular QC of the MLC.
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