The American Association of Physicists in Medicine (AAPM) is a nonprofit professional society whose primary purposes are to advance the science, education, and professional practice of medical physics. The AAPM has more than 8000 members and is the principal organization of medical physicists in the United States.The AAPM will periodically define new practice guidelines for medical physics practice to help advance the science of medical physics and to improve the quality of service to patients throughout the United States. Existing medical physics practice guidelines will be reviewed for the purpose of revision or renewal, as appropriate, on their fifth anniversary or sooner.Each medical physics practice guideline represents a policy statement by the AAPM, has undergone a thorough consensus process in which it has been subjected to extensive review, and requires the approval of the Professional Council. The medical physics practice guidelines recognize that the safe and effective use of diagnostic and therapeutic radiology requires specific training, skills, and techniques, as described in each document. Reproduction or modification of the published practice guidelines and technical standards by those entities not providing these services is not authorized.The following terms are used in the AAPM practice guidelines:Must and Must Not: Used to indicate that adherence to the recommendation is considered necessary to conform to this practice guideline. While must is the term to be used in the guidelines, if an entity that adopts the guideline has shall as the preferred term, the AAPM considers that must and shall have the same meaning.Should and Should Not: Used to indicate a prudent practice to which exceptions may occasionally be made in appropriate circumstances.

Over the past 3 years, our department has evolved from using multiple Excel spreadsheets to track the planning workflow to our new Safety Checklist with a customized Good Catch application. Anomalies, simple mistakes, major issues, or concerns that are "caught" are entered into the system. The system allows stratification of the event, where it first occurred, what checks it passed through, and who entered it, which allows us to examine areas of need and safety. Since "go live", we have reviewed over 1700 events, which have resulted in numerous recommendations, staff reminders, flow changes, and process and procedure changes for improved patient safety. Secondary analysis of our new flow and good catch procedures showed that items noted in weekly physics chart checks were not systematically being documented in the new Good Catch software, which would require double reporting and wasted physics effort. To improve physics efficiency and to capture these events, we developed a new physics weekly chart check application that works with our Good Catch software. We reviewed some of the treatment details examined during our physics weekly checks and how they are presented in an electronic patient information management system, our R&V system. In this system, much of this data is often stored under separate tabs or windows, which require numerous clicks for complete chart review. The goal was to create a system where a large portion of the patient's weekly record could be easily viewable on one page for efficiency. Our new software generates a list of patients to be checked for each Linac. Once a patient is selected the following data are available in one snapshot view for the week: calendar date(s), fraction number(s), appointment status, images taken, image approval status, daily dose delivered, and Linac number. Additionally, prescription data including start date, total dose, technique, prescription approval, and comments are shown. Electronic notes are also shown, which include a table for weekly SSD measurements. The software helps track which patients have been checked and which still need checking by showing the last treatment date, the last chart check date, and the status. Comments can be entered for each patient when a physics note is needed. The software tracks all of this data in a SQL database. When all weekly checks are completed, the software can be used to generate a weekly physics report for distribution throughout the department. Events from this report can then be selected to be incorporated into our Good Catch program by the reviewing physicist. The addition of this software has improved the efficiency of weekly physics chart checks by presenting a large portion of the data in one view and enabled us to "catch" a portion of the Good Catches for analysis that were being missed prior.

The purpose of this study was to determine if medical linear accelerators (linac) produced by the same manufacturer exhibit operational consistency within their subsystems and components. Two linacs that were commissioned together and installed at the same facility were monitored. Each machine delivered a daily robust quality assurance (QA) irradiation. Linacs and their components operate consistently, but have different operational parameter levels even when produced by the same manufacturer and commissioned in series. These findings have implications on the feasibility of true clinical beam matching.

Purpose: To develop an integrated statistical process control (SPC) framework using digital performance and component data accumulated within the accelerator system that can detect dysfunction prior to unscheduled downtime. Methods: Seven digital accelerators were monitored for twelve to 18 months. The accelerators were operated in a ‘run to failure mode’ with the individual institutions determining when service would be initiated. Institutions were required to submit detailed service reports. Trajectory and text log files resulting from a robust daily VMAT QA delivery were decoded and evaluated using Individual and Moving Range (I/MR) control charts. The SPC evaluation was presented in a customized dashboard interface that allows the user to review 525 monitored parameters (480 MLC parameters). Chart limits were calculated using a hybrid technique that includes the standard SPC 3σ limits and an empirical factor based on the parameter/system specification. The individual (I) grand mean values and control limit ranges of the I/MR charts of all accelerators were compared using statistical (ranked analysis of variance (ANOVA)) and graphical analyses to determine consistency of operating parameters. Results: When an alarm or warning was directly connected to field service, process control charts predicted dysfunction consistently on beam generation related parameters (BGP)– RF Driver Voltage, Gun Grid Voltage, and Forward Power (W); beam uniformity parameters – angle and position steering coil currents; and Gantry position accuracy parameter: cross correlation max-value. Control charts for individual MLC – cross correlation max-value/position detected 50% to 60% of MLCs serviced prior to dysfunction or failure. In general, non-random changes were detected 5 to 80 days prior to a service intervention. The ANOVA comparison of BGP determined that each accelerator parameter operated at a distinct value. Conclusion: The SPC framework shows promise. Long term monitoring coordinated with service will be required to definitively determine the effectiveness of the model. Varian Medical System, Inc. provided funding in support of the research presented.

Unscheduled accelerator downtime can negatively impact the quality of life of patients during their struggle against cancer. Currently digital data accumulated in the accelerator system is not being exploited in a systematic manner to assist in more efficient deployment of service engineering resources. The purpose of this study is to develop an effective process for detecting unexpected deviations in accelerator system operating parameters and/or performance that predicts component failure or system dysfunction and allows maintenance to be performed prior to the actuation of interlocks.

Purpose: To determine the effectiveness of SPC analysis for a model predictive maintenance process that uses accelerator generated parameter and performance data contained in trajectory log files. Methods: Each trajectory file is decoded and a total of 131 axes positions are recorded (collimator jaw position, gantry angle, each MLC, etc.). This raw data is processed and either axis positions are extracted at critical points during the delivery or positional change over time is used to determine axis velocity. The focus of our analysis is the accuracy, reproducibility and fidelity of each axis. A reference positional trace of the gantry and each MLC is used as a motion baseline for cross correlation (CC) analysis. A total of 494 parameters (482 MLC related) 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 parameter/system specifications. 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: TG-142 and published analysis of VMAT delivery accuracy. Results: All errors introduced were detected. Synthetic positional errors of 2mm for collimator jaw and MLC carriage exceeded the chart limits. Gantry speed and each MLC speed are analyzed at two different points in the delivery. Simulated Gantry speed error (0.2 deg/sec) and MLC speed error (0.1 cm/sec) exceeded the speed chart limits. Gantry position error of 0.2 deg was detected by the CC maximum value charts. The MLC position error of 0.1 cm was detected by the CC maximum value location charts for every MLC. Conclusion: SPC I/MR evaluation of trajectory log file parameters may be effective in providing an early warning of performance degradation or component failure for medical accelerator systems.

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.

Purpose: Dynamic conformal arc (DCA) techniques offer many advantages for thoracic stereotactic body radiotherapy (SBRT) compared to VMAT, including the elimination of interplay effect between machine and target motion and increased treatment efficiency. Despite these advantages, DCA techniques are underutilized. One potential explanation for the limited DCA use is the perceived restricted user control on dose rate and conformality. We present a method for DCA planning that divides a full arc into sub‐arcs to provide better dose optimization. Methods: Thoracic SBRT patients were simulated free‐breathing and a 4DCT was acquired and binned into ten phase‐based scans to determine internal target volumes. Dynamic conformal sub‐arc (DCSA) and VMAT plans were created on the free‐breathing scan such that at least 95% of the planning target volume received the prescribed dose. For DCSA plans, IMRT‐like dose modulation was achieved by varying field weights and angle ranges for each arc to optimize dose rate and conformality. Plans for five patients were created with DCSA and VMAT. Dose metrics including ratios of prescription dose and 50% isodose to PTV volume, volume of dose outside the PTV receiving >105% of the prescription, maximum dose at 2cm from the PTV and volume of lung receiving 20Gy were measured and compared to assess DCSA and VMAT plan quality. Results: DCSA delivery offers reduced treatment time of up to a factor of 2. Although the conformity index was closer to the ideal value of 1 for all VMAT plans, conformity is also clinically acceptable with DCSA plans. Lung V20% is also comparable and in one case is lower for the DCSA technique. Conclusion: DCSA delivery techniques are an excellent alternative to VMAT for thoracic stereotactic radiotherapy.

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.

Purpose: To introduce and compare dynamic conformal sub‐arc (DCSA) planning to static 3D planning for SBRT lung treatments. Methods: Discussions about standard dynamic conformal arcs revealed little control to adjust the dose cloud. This was especially relevant in areas such as a tumor in between the chest wall and lung. DCSA was devised to overcome this lack of adjustability. DCSA reduces an arc field into multiple conformal sub‐arcs, which can then be adjusted using field weights to alter dose contribution and shift the dose cloud. In this initial study, we investigated two DCSAs, where each sub‐arc covered 45°: 8‐field 360° DCSA and 6‐field 270° DCSA. The 360° plans used the fit and shield tool in Eclipse for any field that entered through the spine. The 270° partial arc plans were positioned to have no entry dose into the spine or contralateral lung. These DCSA plans were compared to static 3D SBRT plans for isodose and RTOG dosimetric measurements. Results: Results show minimal differences with the largest differences being a slight increase in volume of the 50% isodose coverage. 360° DCSA plans showed similar MU versus 3D, while 270° DCSA plans showed reduced MU. All organs‐of‐interest doses were significantly below RTOG criterion. The 3D plans incorporated 3 couch kicks, while the DCSA plans had none. Treatment time for the 3D plans was approximately 20–30 minutes because of the couch kicks and repeat imaging, while the DCSA plans can be treated in under 4 minutes at 600 MU/min and beam automation on a Varian Truebeam linac. Isodose comparisons are presented, as well. Conclusion: Full arc DCSA and partial arc DCSA offer comparable treatment plans to 3D planning but can be delivered much more efficiently. This reduces patient treatment time, alleviates the need for any second CBCT localization, removes couch kicks, and may reduce overall MU.

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.

To determine the variance between the planned and actual relative positions of sequentially treated isocenter pairs following IGRT localization In our clinic, patients receiving stereotactic body radiation therapy (SBRT) to 2 or more isocenters in a single therapy session are planned with a known relative shift between each isocenter. In practice, each isocenter is localized independently and treated sequentially. In this study, we propose to investigate if replacing the second IGRT localization with the planned relative shift can be accomplished with minimal variance between the planned and actual relative positions of the treated isocenters, thereby reducing the total treatment time by an average of 6 min. To examine this variance, the couch parameters and treatment plans of 8 patients who received SBRT were analyzed. The difference between the final couch positions recorded in the R&V system for the examined 10 isocenter pairs (2 patients had 3 isocenters) were extracted and compared to the relative isocenter shift from the treatment plan. All of the patients were immobilized using vacuum cushions except one who utilized a wingboard with knee and foot immobilization combination. Seven patients received therapy to the lung, one to the liver. Five of the isocenter pairs were located ipsilaterally in the lung while four others were located bilaterally (the final pair was in the liver). A total of 51 fractions were analyzed. We defined delta as the delivered isocenter pair position distance minus the planned isocenter shift from the treatment plan in three dimensions. Table shows the results of our data analysis. While the average delta in each dimension was approximately 0.3 cm, the maximum in each dimension was greater than 1 cm. Localization using planned relative isocenter shifts instead of independent IGRT localization during sequential treatment of an isocenter pair would result in reduced geometric precision when compared with CBCT isocenter localization and should be cautiously considered. An ITV-PTV expansion of 5 may be sufficient to mitigate the dosimetric effects of reduced geometric precision on average; however, in some instances the variance would create a dosimetric misalignment with the PTV. The impact on dose volume histograms of the observed variance is the subject of a future investigation.Poster Viewing Abstract 3621; TableDelta values (N = 51 fractions) [cm]A/PLateralS/IVectorAverage0.290.290.300.51Standard deviation0.250.250.230.42Max1.191.071.081.93 Open table in a new tab

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.

BACKGROUND:This 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).METHODS:Steering 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.RESULTS:PCC 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.CONCLUSION:Radiotherapy 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.

Purpose: Investigations over the past few years have demonstrated some of the benefits and shortfalls of tomosynthesis in radiation therapy for patient positioning or dose tracking. Tomosynthesis generates excellent image quality in one reconstruction plane; however, there is a loss of edge and frequency information in the additional planes due to the spatial incompleteness of the projection data. In our previous study, we demonstrated improved results from a new discrete frequency interpolation technique (DFIT), which is used to incorporate tomosynthesis limited arc projection data and three additional “filling” projections. Here, we continue investigation into DFIT by studying noisy projections for the data and incorporation of total variation (TV) noise reduction into the reconstruction processing. Methods: For this investigation, we simulated a Shepp‐Logan phantom to generate 93 parallel beam projections with ±5% uniform random noise. 90 of these projections were over a 90‐degree tomosynthetic arc. The remaining three projections were evenly spaced (i.e. 22.5, 45, and 67.5 degrees from the end of the arc) to help “fill in” the incomplete data. DFIT+TV was used to reconstruct images by interpolating frequency values into the gaps of the Fourier spectrum, inverse Fourier transforming from frequency to image space, adding the resultant image to a filtered backprojection (FBP) image, and finally applying a TV noise reduction step. Results: The images by traditional FBP, DFIT, FBP+TV and DFIT+TV are compared visually and using horizontal and vertical line profile plots. Additionally, contrast results are examined. The DFIT+TV images show improved edge delineation in the usual tomosynthesis “blurred” direction and improved reconstructed contrast. Conclusions: The addition of limited “filling” discrete projections and DFIT+TV can be used to improve the reconstruction accuracy and quality of short arc imaging. Future work will study DFIT+TV in the cone beam geometry.Research supported by Nucletron, B.V.

Purpose: Investigations over the past few years have demonstrated some of the benefits and shortfalls of tomosynthesis in radiation therapy for patient positioning or dose tracking. Tomosynthesis generates excellent image quality in one reconstruction plane; however, there is a loss of edge and frequency information in the additional planes due to the spatial incompleteness of the projection data. In this study, we investigate the results of a new discrete frequency interpolation technique (DFIT), which is used to incorporate tomosynthesis limited arc projection data and three additional “filling” projections. Methods: For the initial investigations, we simulated a Shepp‐Logan phantom with parallel beam geometry to generate 93 projections. 90 of these projections were over a tomosynthetic arc at one projection per degree. The remaining three projections were generated to fill in the next 90 degrees evenly (i.e. 22.5, 45, and 67.5 degrees from the end of the arc) to help “fill in” the incomplete data. Simple filtered backprojection (FBP) results show no improvement as FBP is designed for uniform distribution of projections. Per the projection slice theorem, the tomosynthesis data fills in a portion of the frequency plane. The remaining sparsely sampled area of the frequency plane is filled using linear interpolation from the end arc projections and the 3 discrete projections. The DFIT image is the sum of the FBP image and the inverse Fourier transform of the interpolated frequencies Results: The images by traditional FBP and DFIT are compared visually and using line profile plots. Additionally, contrast results are examined. The DFIT images show improved edge delineation in the usual tomosynthesis “blurred” direction and improves reconstructed contrast. Conclusions: The addition of limited “filling” discrete projections and DFIT can be used to improve the reconstruction accuracy and quality of short arc imaging.Research supported by Nucletron, B.V.

Purpose: This study retrospectively applies statistical process control (SPC) methods to determine its utility in detecting changes in linear accelerator beam steering parameters prior to equipment failure (interlocks actuated) thereby increasing patient safety. Methods: Steering 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 on June 28th were evaluated. PCC limits were calculated using the first twenty subgroups. Appropriate action limits were developed using conventional SPC guidelines. Results: PCC high alarm action limit was set at six standard deviations from the mean. Low alarm indicators were: (1) nine points in a row on either side of the mean, and (2) two out of three points in a row greater than two standard deviations from the mean. Alarms received following establishment of limits (week twenty) are indicative of a special cause for deviation (Xbar chart) and/or an uncontrolled process (R chart). Transverse angle SCC for 15X and 6X indicated a high alarm on March 11th and March 29th respectively. A downward trend in this parameter continued, with high alarm, until failure. Transverse position and radial angle SCC for both energies indicated a high or low alarm starting in February or March. Conclusions: Patient safety may be improved by instituting SPC methods to monitor the beam steering process and detect abnormal changes prior to equipment failure.