C.J. Hampton

Comparison of accelerated hypofractionation and stereotactic body radiotherapy for Stage 1 and node negative Stage 2 non-small cell lung cancer (NSCLC)

PURPOSE Stereotactic body radiation therapy (SBRT) and accelerated hypofractionated radiation therapy (AHRT) have favorable local control (LC) relative to conventional fractionation in the treatment of stage I non-small cell lung cancer (NSCLC). We report the results of our single institution experience with the treatment of early stage NSCLC with SBRT or AHRT in cases where SBRT was felt to be suboptimal. METHODS One hundred and sixty patients with Stage 1 and node negative Stage 2 NSCLC were treated with SBRT or AHRT from 2003 to 2011. Median follow-up was 29.4 and 19 months (mo), respectively. The median dose was 54Gy in 3 fractions (fx) (SBRT) and 70.2Gy in 26 fx (AHRT). Acute and late toxicities (tox) were graded (G) per CTCAE v4. Time to local (LF), regional (RF) and distant (DF) failure were estimated using the Kaplan-Meier method. The impact of patient and tumor related factors on LF were estimated by multivariate Cox proportional hazard model. RESULTS Three-year LC rates were 87.7% (SBRT) and 71.7% (AHRT). The 3-year freedom from DF was 73.3% and 68.1%. Median OS was 38.4 (95% CI 29.7-51.6) and 35 (95% CI 22-48.3) mo. No G3 or 4 tox were observed. At 1 year, 30% and 50% of complications resolved, while (5-6%) had persistent chest wall pain. Multivariate analysis demonstrated that increasing dose per fraction and tumor size (>5.5 vs. 4cm) in the AHRT and SBRT group were found to be associated with a reduced (HR 0.33 95% CI 0.13-0.84, p=0.021) and increased (HR: 6.372 95% CI 1.23-32.92, p=0.027) hazard for local failure respectively. CONCLUSIONS Our results compare favorably with other reports of treatment for early stage NSCLC. AHRT patients had comparable LC despite increased size and central disease. Toxicity was limited and overall survival, regional and distant recurrences were similar between groups.

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

Radiation safety issues with positron-emission/computed tomography simulation for stereotactic body radiation therapy.

Stereotactic body radiation therapy (SBRT) simulations using a Stereotactic Body Frame (SBF: Elekta, Stockholm, Sweden) were expanded to include 18F‐deoxyglucosone positron‐emission tomography (FDG PET) for treatment planning. Because of the length of time that staff members are in close proximity to the patient, concerns arose over the radiation safety issues associated with these simulations. The present study examines the radiation exposures of the staff performing SBRT simulations, and provides some guidance on limiting staff exposure during these simulations. Fifteen patients were simulated with PET/CT using the SBF. Patients were immobilized in the SBF before the FDG was administered. The patients were removed from the frame, injected with FDG, and allowed to uptake for approximately 45 minutes. After uptake, the patients were repositioned in the SBF. During the repositioning, exposure rates were recorded at the patients surface, at the SBF surface, and at 15 cm, 30 cm, and 1 m from the SBF. Administered dose and the approximate time spent on patient repositioning were also recorded. The estimated dose to staff was compared with the dose to staff performing conventional diagnostic PET studies. The average length of time spent in close proximity (<50 cm) to the patient after injection was 11.7 minutes, or more than twice the length of time reported for diagnostic PET staff. That time yielded an estimated average dose to the staff of 26.5mSv per simulation. The annual occupational exposure limit is 50 mSv. Based on dose per simulation, staff would have to perform nearly 1900 SBRT simulations annually to exceed the occupational limit. Therefore, at the current rate of 50–100 simulations annually, the addition of PET studies to SBRT simulations is safe for our staff. However, ALARA (“as low as reasonably achievable”) principles still require some radiation safety considerations during SBRT simulations. The PET/CT‐based SBRT simulations are safe and important for treatment planning that optimizes biologic dose distribution with highly accurate and reproducible target definition. PACS numbers: 87.57.uk, 87.59.bd

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.

SU‐E‐T‐54: Initial Investigation Applying Statistical Process Control to Accelerator Beam Quality

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.

SU‐E‐T‐53: Statistical Process Control Methodologies for Predictive Maintenance of Linear Accelerator Beam Quality

Purpose: This study prospectively applies statistical process control (SPC) methods as a predictive maintenance tool in linear acceleratorphoton beam quality. Methods: Steering coil currents (SCC) are sampled and stored daily during morning warm‐up. The transverse and radial ‐ positioning and angle SCC for photon beams were evaluated using average and range (Xbar‐R) process control charts (PCC). The weekly average and range values (subgroup n=5) for each SCC was used to develop the PCC. Control limits were calculated using the first sixteen subgroups. Run charts of the daily SCC values were maintained and updated weekly. PCC high alarm action limit was set at six standard deviations. Six subgroups in a row all increasing or decreasing were a low alarm indicator. A sustained high alarm of 3 subgroups in conjunction with a low alarm was used as our action threshold (i.e. independent verification of beam flatness and symmetry constancy). Results: Data collection commenced September 2010, following beam steering adjustments during annual calibration. Only alarms received following establishment of limits (week sixteen) were considered valid indicators for deviation or an uncontrolled process. A high alarm was first detected on Jan 25 (15 MV — transverse position). Simulated intervention was triggered by 6 MV — radial angle SCC on February 18th. Beam scans taken and compared to those from the annual calibration indicated a change in symmetry in the transverse plane (0.6% ‐ 6 MV and 1.0% ‐ 15 MV) while the flatness remained effectively unchanged (<0.5%). There were no changes noted in the radial plane for either energy. Conclusions: SPC techniques are able to effectively detect variations in beam steering currents. SPC monitoring of beam steering has the capability to ensure AAPM TG‐142 constancy guidelines of +/− 1% are maintained. Further investigation is required to develop consistent intervention guidelines.

SU‐GG‐I‐22: Improving Skin‐Surface Delineation in Truncated Cone‐Beam CT Using Laser Profilometry

PURPOSE: Laser profilometry is a technique where three‐dimensional topographic surface profiles of an object are reconstructed using data acquired with a laser range‐finding system. Axial rotation of a laser range‐finding system around an object can provide enough surface data to allow for this reconstruction. In cone‐beam computed tomography(CBCT), a patients imaging dose can be reduced by collimating the incident kilovoltage (kV) field; however this comes at the cost of reducing an imaging systems ability to accurately resolve a patients skin surface. The study described herein is of the implementation of a laser‐profilometry‐based CBCT system which can enhance skin surface delineation while requiring narrower kV imaging beams. MATERIALS AND METHODS: The head‐and‐neck region of an anthropomorphic phantom was imaged using the CBCT feature of the Elekta XVI kV imaging system. During this imaging session, a three‐dimensional skin‐surface was reconstructed using an in‐house laser profilometry system attached to the collimator of the Elekta Axesses MV treatment beam. This profilometry system is capable of measuring the distance to the surface of an object with an uncertainty of +/− 1.5mm. Skin‐surface reconstructions are then compared with CBCTreconstructions with emphasis placed on delineation of skin boundaries. RESULTS: Coregistration of the reconstructed laser‐defined skin surface with XVI‐generated CBCTreconstruction show strong spatial correlation. CONCLUSION: The detail provided by laser data allows for the narrowing of collimators in kV imaging beams without losing the ability to resolve boundary edges in the imaging subject. Research supported by NIH T32‐CA113267.

SU-GG-I-13: CBCT Total Variation Based Image Reconstruction from Limited Projections

PURPOSE: Cone beam computed tomography(CBCT) is commonly used for image‐guided external beam radiation therapy. There is interest and motivation to use a limited number of acquired projections to reconstruct the CBCT in order to reduce patient imagingdose; however, under these conditions filtered back projection (FBP) techniques typically used in CBCTreconstruction produce low quality CBCTimages. We propose to examine the application of a novel iterative reconstruction technique using total variation minimization for this problem. METHODS AND MATERIALS: A previously proposed adaptive steepest descent — projection onto convex set algorithm was modified with an ordered subset convex algorithm replacing the simultaneous algebraic reconstruction technique in the POCS step (ASD‐POCS‐OSC). ASD‐POCS‐OSC was used to reconstruct 60 projections of an anthropomorphic phantom uniformly spaced over 360 degrees to an image space of 512 × 512 × 200. Projections were acquired with an Elekta XVICBCTimagingsystem using the medium field of view with an offset detector panel. Reconstruction results were compared to imagesreconstructed with FBP for several image metrics. RESULTS: After one iteration the ASD‐POCS‐OSC images show improved edge definition, higher contrast‐to‐noise ratios, and decreased artifacts compared to the FBP reconstructed images. ASD‐POCS‐OSC reconstruction requires an increased computational burden and reconstruction time compared to FBP. CONCLUSION: Iterative reconstruction algorithms, such as ASD‐POCS‐OSC, yield reconstructed images with improved image quality compared to FBP algorithms. Iterative algorithms optimized for speed and performance may allow for the use of limited projection CBCTs to be used for patient setup on current clinical imaging devices. Supported in part by NCI T‐32 CA113267.

SU‐FF‐T‐376: SBRT Simulations Using PET/CT: Radiation Safety Concerns

Purpose:SBRT simulations using a Stereotactic Body Frame (SBF — Elekta) were expanded to include a PET study. Because of the time that staff are in close proximity to the patient, some concerns arose over the radiation safety issues associated with these simulations. This study examines the radiation exposures of the staff performing SBRT simulations and provides some guidance on limiting staff exposure during these simulations. Methods and Materials: Fifteen patients were simulated with PET/CT using the SBF. Patients were immobilized in the SBF prior to the administration of FDG (18F‐fluoro‐deoxy‐glucose). The patients were removed from the frame, injected with FDG, and allowed to uptake for 45 minutes. After uptake, the patients were repositioned in the SBF. During the repositioning, exposure rates were recorded at the patients surface, at the SBF surface, and at 15cm, 30cm, and 1m from the SBF, along with administered dose and the approximate time spent on patient repositioning. Comparisons of the estimated dose to the staff were performed with staff performing conventional diagnostic PET studies. Results: The average time spent in close proximity to the patient after injection was 11.4 minutes, or over twice that reported for diagnostic PET staff. This time yielded an estimated average dose to the staff of 26.5 μSv per simulation. Conclusions: Occupational exposure limits are 50 mSv per year. Based on the dose per simulation, the staff would have to perform nearly 1900 SBRT simulations per year to exceed the occupational limit. Therefore, at the current rate of 50–100 simulations per year, the addition of PET studies to the SBRT simulations is safe for our staff. However, ALARA principles still require some radiation safety considerations during the SBRT simulations. PET/CT‐based SBRT simulations are safe and important for treatment planning that optimizes biologic dose‐distribution with highly accurate and reproducible target definition.