Operational consistency of medical linear accelerators manufactured and commissioned in series.

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.

A model for preemptive maintenance of medical linear accelerators-predictive maintenance.

BACKGROUND: 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. METHODS: The proposed predictive maintenance (PdM) model is as follows: 1) deliver a daily quality assurance (QA) treatment; 2) automatically transfer and interrogate the resulting log files; 3) once baselines are established, subject daily operating and performance values to statistical process control (SPC) analysis; 4) determine if any alarms have been triggered; and 5) alert facility and system service engineers. A robust volumetric modulated arc QA treatment is delivered to establish mean operating values and perform continuous sampling and monitoring using SPC methodology. Chart limits are calculated using a hybrid technique that includes the use of the standard SPC 3σ limits and an empirical factor based on the parameter/system specification. RESULTS: There are 7 accelerators currently under active surveillance. Currently 45 parameters plus each MLC leaf (120) are analyzed using Individual and Moving Range (I/MR) charts. The initial warning and alarm rule is as follows: warning (2 out of 3 consecutive values ≥ 2σ hybrid) and alarm (2 out of 3 consecutive values or 3 out of 5 consecutive values ≥ 3σ hybrid). A customized graphical user interface provides a means to review the SPC charts for each parameter and a visual color code to alert the reviewer of parameter status. Forty-five synthetic errors/changes were introduced to test the effectiveness of our initial chart limits. Forty-three of the forty-five errors (95.6 %) were detected in either the I or MR chart for each of the subsystems monitored. CONCLUSION: Our PdM model shows promise in providing a means for reducing unscheduled downtime. Long term monitoring will be required to establish the effectiveness of the model.

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.

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.

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

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.

Quality assurance: fundamental reproducibility tests for 3-D 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.