C.M. Able

Initial investigation using statistical process control for quality control of accelerator beam steering

BackgroundThis study seeks to increase clinical operational efficiency and accelerator beam consistency by retrospectively investigating the application of statistical process control (SPC) to linear accelerator beam steering parameters to determine the utility of such a methodology in detecting changes prior to equipment failure (interlocks actuated).MethodsSteering coil currents (SCC) for the transverse and radial planes are set such that a reproducibly useful photon or electron beam is available. SCC are sampled and stored in the control console computer each day during the morning warm-up. The transverse and radial - positioning and angle SCC for photon beam energies were evaluated using average and range (Xbar-R) process control charts (PCC). The weekly average and range values (subgroup n = 5) for each steering coil were used to develop the PCC. SCC from September 2009 (annual calibration) until two weeks following a beam steering failure in June 2010 were evaluated. PCC limits were calculated using the first twenty subgroups. Appropriate action limits were developed using conventional SPC guidelines.ResultsPCC high-alarm action limit was set at 6 standard deviations from the mean. A value exceeding this limit would require beam scanning and evaluation by the physicist and engineer. Two low alarms were used to indicate negative trends. Alarms received following establishment of limits (week 20) are indicative of a non-random cause for deviation (Xbar chart) and/or an uncontrolled process (R chart). Transverse angle SCC for 6 MV and 15 MV indicated a high-alarm 90 and 108 days prior to equipment failure respectively. A downward trend in this parameter continued, with high-alarm, until failure. Transverse position and radial angle SCC for 6 and 15 MV indicated low-alarms starting as early as 124 and 116 days prior to failure, respectively.ConclusionRadiotherapy clinical efficiency and accelerator beam consistency may be improved by instituting SPC methods to monitor the beam steering process and detect abnormal changes prior to equipment failure.PACS numbers: 87.55n, 87.55qr, 87.56bd

Quality Control of High-Dose-Rate Brachytherapy: Treatment Delivery Analysis Using Statistical Process Control

PURPOSE Statistical process control (SPC) is a quality control method used to ensure that a process is well controlled and operates with little variation. This study determined whether SPC was a viable technique for evaluating the proper operation of a high-dose-rate (HDR) brachytherapy treatment delivery system. METHODS AND MATERIALS A surrogate prostate patient was developed using Vyse ordnance gelatin. A total of 10 metal oxide semiconductor field-effect transistors (MOSFETs) were placed from prostate base to apex. Computed tomography guidance was used to accurately position the first detector in each train at the base. The plan consisted of 12 needles with 129 dwell positions delivering a prescribed peripheral dose of 200 cGy. Sixteen accurate treatment trials were delivered as planned. Subsequently, a number of treatments were delivered with errors introduced, including wrong patient, wrong source calibration, wrong connection sequence, single needle displaced inferiorly 5 mm, and entire implant displaced 2 mm and 4 mm inferiorly. Two process behavior charts (PBC), an individual and a moving range chart, were developed for each dosimeter location. RESULTS There were 4 false positives resulting from 160 measurements from 16 accurately delivered treatments. For the inaccurately delivered treatments, the PBC indicated that measurements made at the periphery and apex (regions of high-dose gradient) were much more sensitive to treatment delivery errors. All errors introduced were correctly identified by either the individual or the moving range PBC in the apex region. Measurements at the urethra and base were less sensitive to errors. CONCLUSIONS SPC is a viable method for assessing the quality of HDR treatment delivery. Further development is necessary to determine the most effective dose sampling, to ensure reproducible evaluation of treatment delivery accuracy.

Quality assurance: Fundamental reproducibility tests for 3D treatment-planning systems

The use of image‐based 3D treatment planning has significantly increased the complexity of commercially available treatment‐planning systems (TPSs). Medical physicists have traditionally focused their efforts on understanding the calculation algorithm; this is no longer possible. A quality assurance (QA) program for our 3D treatment‐planning system (ADAC Pinnacle3) is presented. The program is consistent with the American Association of Physicists in Medicine Task Group 53 guidelines and balances the cost‐versus‐benefit equation confronted by the clinical physicist in a community cancer center environment. Fundamental reproducibility tests are presented as required for a community cancer center environment using conventional and 3D treatment planning. A series of nondosimetric tests, including digitizer accuracy, image acquisition and display, and hardcopy output, is presented. Dosimetric tests include verification of monitor units (MUs), standard isodoses, and clinical cases. The tests are outlined for the Pinnacle3 TPS but can be generalized to any TPS currently in use. The program tested accuracy and constancy through several hardware and software upgrades to our TPS. This paper gives valuable guidance and insight to other physicists attempting to approach TPS QA at fundamental and practical levels. PACS numbers: 87.53.Tf, 87.53.Xd

Addressing connectivity issues: The Integrating the Healthcare Enterprise-Radiation Oncology (IHE-RO) initiative

In todays world, treating a patient successfully with radiation requires the integration of complex data from a variety of systems. In a typical radiation oncology clinic, data move from the treatment management system to treatment planning system to treatment delivery system. When there is a lack of interconnectivity between the systems, the potential for medical error is increased. Integrating the Healthcare Enterprise-Radiation Oncology (IHE-RO) is dedicated to the identification of connectivity problems encountered in the modern day radiation oncology clinic and the development of solutions to these problems. These solutions are then integrated and made available to the radiation oncology community. This article introduces the IHE-RO initiative, outlines the relevance of IHE-RO for the radiation oncology community, and provides a resource so that therapists, physicists, dosimetrists, administrators, and physicians alike can best understand which vendor equipment can effectively communicate between platforms because it has been deemed IHE-RO compliant through a series of connectivity tests.

SU‐C‐137‐04: Effective Control Limits for Predictive Maintenance (PdM) of Accelerator Beam Uniformity

PURPOSE Predictive maintenance programs employ non-invasive methods to monitor the performance of systems to determine when preemptive intervention is required to maintain high quality performance. The focus of this study is to evaluate the effectiveness of a modified calculation method for the control limits of process control charts (PCC) that will reduce false positive alarms but detect clinically relevant changes in beam uniformity (flatness and symmetry). METHODS Steering coil currents (SCC) for the transverse and radial planes are adjusted such that a reproducibly useful photon or electron beam is available. Performing controlled experiments varying a single SCC, we were able to determine the magnitude of change required to produce a 1% change in beam uniformity. The average and range (Xbar/R) control chart limit calculation was reformulated incorporating a scaling factor (Cm). After monitoring the SCC of 3 accelerators for several months, without detecting any PCC alarms, we intentionally changed the transverse angle and position SCC independently until the beam symmetry differed by ∼1.1% from baseline as confirmed by computerized beam profile scan in water using an ion chamber. These experimental SCC values were plotted in the PCC to determine if they would exceed the modified control limits. RESULTS The experimental SCC value exceeded the PCC limit for each of the steering coils. Additionally, the capability of the new control limit calculation method was confirmed when an anomaly was detected in the operation of an accelerator in clinical use. Instability in the operation of the 18 MV photon beam was predicted by SPC analysis prior to it being confirmed by service and daily check devices. If the analysis had been performed using the original methodology, a number of false positives would have been reported. CONCLUSION Reformulated X-bar/R chart control limits of SCC can provide an effective predictive maintenance tool for accelerator beam uniformity. This project is supported by a grant from Varian Medical Systems.

SU‐E‐T‐205: MLC Predictive Maintenance Using Statistical Process Control Analysis

PURPOSE MLC failure increases accelerator downtime and negatively affects the clinic treatment delivery schedule. This study investigates the use of Statistical Process Control (SPC), a modern quality control methodology, to retrospectively evaluate MLC performance data thereby predicting the impending failure of individual MLC leaves. METHODS SPC, a methodology which detects exceptional variability in a process, was used to analyze MLC leaf velocity data. A MLC velocity test is performed weekly on all leaves during morning QA. The leaves sweep 15 cm across the radiation field with the gantry pointing down. The leaf speed is analyzed from the generated dynalog file using quality assurance software. MLC leaf speeds in which a known motor failure occurred (8) and those in which no motor replacement was performed (11) were retrospectively evaluated for a 71 week period. SPC individual and moving range (I/MR) charts were used in the analysis. The I/MR chart limits were calculated using the first twenty weeks of data and set at 3 standard deviations from the mean. RESULTS The MLCs in which a motor failure occurred followed two general trends: (a) no data indicating a change in leaf speed prior to failure (5 of 8) and (b) a series of data points exceeding the limit prior to motor failure (3 of 8). I/MR charts for a high percentage (8 of 11) of the non-replaced MLC motors indicated that only a single point exceeded the limit. These single point excesses were deemed false positives. CONCLUSIONS SPC analysis using MLC performance data may be helpful in detecting a significant percentage of impending failures of MLC motors. The ability to detect MLC failure may depend on the method of failure (i.e. gradual or catastrophic). Further study is needed to determine if increasing the sampling frequency could increase reliability. Project was support by a grant from Varian Medical Systems, Inc.

SU‐E‐T‐207: Flatness and Symmetry Threshold Detection Using Statistical Process Control

PURPOSE AAPM TG-142 guidelines state that beam uniformity (flatness and symmetry) should maintain a constancy of 1 % relative to baseline. The focus of this study is to determine if statistical process control (SPC) methodology using process control charts (PCC) of steering coil currents (SCC) can detect changes in beam uniformity prior to exceeding the 1% constancy criteria. METHODS SCCs for the transverse and radial planes are adjusted such that a reproducibly useful photon or electron beam is available. Transverse and radial - positioning and angle SCC are routinely documented in the Morning Check file during daily warm-up. The 6 MV beam values for our linac were analyzed using average and range (Xbar/R) PCC. Using this data as a baseline, an experiment was performed in which each SCC was changed from its mean value (steps of 0.01 or 0.02 Ampere) while holding the other SCC constant. The effect on beam uniformity was measured using a beam scanning system. These experimental SCC values were plotted in the PCC to determine if they would exceed the predetermined limits. RESULTS The change in SCC required to exceed the 1% constancy criteria was detected by the PCC for 3 out of the 4 steering coils. The reliability of the result in the one coil not detected (transverse position coil) is questionable because the SCC slowly drifted during the experiment (0.05 A) regardless of the servo control setting. CONCLUSIONS X-bar/R charts of SCC can detect exceptional variation prior to exceeding the beam uniformity criteria set forth in AAPM TG-142. The high level of PCC sensitivity to change may result in an alarm when in fact minimal change in beam uniformity has occurred. Further study is needed to determine if a combination of individual SCC alarms would reduce the false positive rate for beam uniformity intervention. This project was supoorted by a grant from Varian Medical Systems, Inc.

Operational consistency of medical linear accelerators manufactured and commissioned in series

Highlights • Statistical Process Control metrics can assess linac operational performance.• Linacs produced in series have statistically different operating parameters.• Although having different operating parameters, linacs have similar performance.

Impact of bladder volume on radiation dose to the rectum in the definitive treatment of prostate cancer.

BACKGROUND AND OBJECTIVE Our group created and routinely reviewed a dedicated prostate intensity-modulated radiation therapy (IMRT) delivery program. Previously, a retrospective review of our experience demonstrated that a larger bladder volume reduced radiation dose to the rectum. We conducted an observational study to confirm this relationship. METHODS Men receiving definitive radiation for prostate cancer were eligible for the study. Eligible patients received 2 computed axial tomography (CT) scans on the day of their planning CT scan: 1 with a full bladder and 1 with an empty bladder. On each CT data set, the prostate, rectum, bladder, penile bulb, and femoral heads were contoured. 2 IMRT plans were completed on each dataset: 1 by a medical dosimetrist and 1 by a medical physicist. The study plans targeted the prostate to 79.2 Gray (Gy) while respecting predefined dose tolerances to the other contoured structures. Rectal doses were compared on empty and full bladder CT data sets. RESULTS From June 29, 2010 to December 14, 2011, 17 full bladder data sets and 15 empty bladder data sets were available for analysis. Median change in bladder volume was 63 ml. Full vs empty bladder set-up was associated with a statistically significant reduction in the mean rectal dose of 25.41 Gy vs 27.6 Gy (𝑃 = .031). LIMITATIONS Small sample size and small variations in bladder volumes. CONCLUSIONS A greater bladder volume resulted in a reduced mean dose to the rectum irrespective of planning method.

SU-C-BRD-03: Analysis of Accelerator Generated Text Logs for Preemptive Maintenance

PURPOSE To develop a model to analyze medical accelerator generated parameter and performance data that will provide an early warning of performance degradation and impending component failure. METHODS A robust 6 MV VMAT quality assurance treatment delivery was used to test the constancy of accelerator performance. The generated text log files were decoded and analyzed using statistical process control (SPC) methodology. The text file data is a single snapshot of energy specific and overall systems parameters. A total of 36 system parameters were monitored which include RF generation, electron gun control, energy control, beam uniformity control, DC voltage generation, and cooling systems. The parameters were analyzed using Individual and Moving Range (I/MR) charts. The chart limits were calculated using a hybrid technique that included the use of the standard 3σ limits and the parameter/system specification. Synthetic errors/changes were introduced to determine the initial effectiveness of I/MR charts in detecting relevant changes in operating parameters. The magnitude of the synthetic errors/changes was based on: the value of 1 standard deviation from the mean operating parameter of 483 TB systems, a small fraction (≤ 5%) of the operating range, or a fraction of the minor fault deviation. RESULTS There were 34 parameters in which synthetic errors were introduced. There were 2 parameters (radial position steering coil, and positive 24V DC) in which the errors did not exceed the limit of the I/MR chart. The I chart limit was exceeded for all of the remaining parameters (94.2%). The MR chart limit was exceeded in 29 of the 32 parameters (85.3%) in which the I chart limit was exceeded. CONCLUSION Statistical process control I/MR evaluation of text log file parameters may be effective in providing an early warning of performance degradation or component failure for digital medical accelerator systems. Research is Supported by Varian Medical Systems, Inc.