T Simon

On the sensitivity of patient-specific IMRT QA to MLC positioning errors

Accurate multileaf collimator (MLC) leaf positioning plays an essential role in the effective implementation of intensity modulated radiation therapy (IMRT). This work evaluates the sensitivity of current patient‐specific IMRT quality assurance (QA) procedures to minor MLC leaf positioning errors. Random errors of up to 2 mm and systematic errors of ±1mm and ±2mm in MLC leaf positions were introduced into 8 clinical IMRT patient plans (totaling 53 fields). Planar dose distributions calculated with modified plans were compared to dose distributions measured with both radiochromic films and a diode matrix. The agreement between calculation and measurement was evaluated using both absolute distance‐to‐agreement (DTA) analysis and γ index with 2%/2mm and 3%/3mm criteria. It was found that both the radiochromic film and the diode matrix could only detect systematic errors on the order of 2 mm or above. The diode array had larger sensitivity than film due to its excellent detector response (such as small variation, linear response, etc.). No difference was found between DTA analysis and γ index in terms of the sensitivity to MLC positioning errors. Higher sensitivity was observed with 2%/2mm than with 3%/3mm in general. When using the diode array and 2%/2mm criterion, the IMRT QA procedure showed strongest sensitivity to MLC position errors and, at the same time, achieved clinically acceptable passing rates. More accurate dose calculation and measurement would further enhance the sensitivity of patient‐specific IMRT QA to MLC positioning errors. However, considering the significant dosimetric effect such MLC errors could cause, patient‐specific IMRT QA should be combined with a periodic MLC QA program in order to guarantee the accuracy of IMRT delivery. PACS numbers: 87.50.Gi, 87.52.Df, 87.52.Px, 87.53.Dq, 87.53.Tf, 87.53.Kn, 87.56.Fc

Wide field array calibration dependence on the stability of measured dose distributions

PURPOSE The aim of this work was to simulate the effect of dose distribution changes on detector array calibrations and to explore compensatory methods that are used during calibration measurements. METHODS The array calibration technique that was investigated is known as wide field (WF) calibration. Using this method, a linear array [y-axis (65 detectors) of the IC PROFILER (Sun Nuclear Corporation, Melbourne, FL)] is calibrated with three measurements (alpha, theta, and lamda); each measurement uses the same radiation field, which is larger than the array. For measurement configuration theta, the array is rotated by 180 degrees from its position in a; for lamda, the array is shifted by one detector from its position in theta. The relative detector sensitivities are then determined through ratios of detector readings at the same field locations (using theta and lamda). This method results in error propagation that is proportional to the number of detectors in the array. During the procedure, the calibration protocol operates under three postulates, which state that (a) the beam shape does not change between measurements; (b) the relative sensitivities of the detectors do not change; and (c) the scatter to the array does not change as the array is moved. The WF calibrations sensitivity to a postulate (a) violation was quantified by applying a sine shaped perturbation (of up to 0.1%) to a, theta, or lamda, and then determining the change relative to a baseline calibration. Postulate (a) violations were minimized by using a continuous beam and mechanized array movement during theta and lamda. A continuously on beam demonstrated more stable beam symmetry as compared to cycling the beam on and off between measurements. Additional side-scatter was also used to satisfy postulate (c). RESULTS Simulated symmetry perturbations of 0.1% to theta or lamda resulted in calibration errors of up to 2%; alpha was relatively immune to perturbation (<0.1% error). Wide field calibration error on a linear accelerator with similar symmetry variations was +/- 1.6%. Using a continuous beam during theta and lamda with additional side-scatter reduced the calibration error from +/- 1.6% to +/- 0.48%. CONCLUSIONS This work increased the reproducibility of WF calibrations by limiting the effect of measurement perturbations primarily due to linear accelerator symmetry variations. The same technique would work for any array using WF calibration.

Tomographic physical phantom of the newborn child with real-time dosimetry I. Methods and techniques for construction

A tomographic phantom representing a newborn female patient was constructed using tissue-equivalent materials previously developed at the University of Florida. This phantom was constructed using contoured images from an actual patient data set, a whole-body computed tomography of a newborn cadaver previously described by Nipper et al. [Phys. Med. Biol. 47, 3143-1364 (2002)]. Four types of material are incorporated in the phantom: soft tissue, bone tissue, lung tissue, and air. The phantom was constructed on a slice-by-slice basis with a z-axis resolution of 5 mm, channels for dosimeters (thermoluminescent dosimeter (TLD), metal-oxide-semiconductor field-effect transistor, or gated fiber-optic-coupled dosimeter (GFOC)) were machined into slices prior to assembly, and the slices were then fixed together to form the complete phantom. The phantom will be used in conjunction with an incorporated dosimetry system to calculate individual organ and effective doses delivered to newborn patients during various diagnostic procedures, including, but not limited to, projection radiography and computed tomography. Included in this paper are images detailing the construction process, and images of the completed phantom.

Characterization of a multi-axis ion chamber array

PURPOSE The aim of this work was to characterize a multi-axis ion chamber array (IC PROFILER; Sun Nuclear Corporation, Melbourne, FL, USA) that has the potential to simplify the acquisition of LINAC beam data. METHODS The IC PROFILER (or panel) measurement response was characterized with respect to radiation beam properties, including dose, dose per pulse, pulse rate frequency (PRF), and energy. Panel properties were also studied, including detector-calibration stability, power-on time, backscatter dependence, and the panels agreement with water tank measurements [profiles, fractional depth dose (FDD), and output factors]. RESULTS The panels relative deviation was typically within (+/-) 1% of an independent (or nominal) response for all properties that were tested. Notable results were (a) a detectable relative field shape change of approximately 1% with linear accelerator PRF changes; (b) a large range in backscatter thickness had a minimal effect on the measured dose distribution (typically less than 1%); (c) the error spread in profile comparison between the panel and scanning water tank (Blue Phantom, CC13; IBA Schwarzenbruck, DE) was approximately (+/-) 0.75%. CONCLUSIONS The ability of the panel to accurately reproduce water tank profiles, FDDs, and output factors is an indication of its abilities as a dosimetry system. The benefits of using the panel versus a scanning water tank are less setup time and less error susceptibility. The same measurements (including device setup and breakdown) for both systems took 180 min with the water tank versus 30 min with the panel. The time-savings increase as the measurement load is increased.

Multileaf collimator characteristics and reliability requirements for IMRT Elekta system.

Understanding the characteristics of a multileaf collimator (MLC) system, modeling MLC in a treatment planning system, and maintaining the mechanical accuracy of the linear accelerator gantry head system are important factors in the safe implementation of an intensity-modulated radiotherapy program. We review the characteristics of an Elekta MLC system, discuss the necessary MLC modeling parameters for a treatment planning system, and provide a novel method to establish an MLC leaf position quality assurance program. To perform quality assurance on 40 pairs of individual MLC leaves is a time-consuming and difficult task. In this report, an effective routine MLC quality assurance method based on the field edge of a backup jaw as referenced in conjunction with a diode array as a radiation detector system is discussed. The sensitivity of this test for determining the relative leaf positions was observed to be better than 0.1 mm. The Elekta MLC leaf position accuracy measured with this system has been better than 0.3 mm.

An MLC calibration method using a detector array

PURPOSE The authors have developed a quantitative calibration method for a multileaf collimator (MLC) which measures individual leaf positions relative to the MLC backup jaw on an Elekta Synergy linear accelerator. METHODS The method utilizes a commercially available two-axis detector array (Profiler 2; Sun Nuclear Corporation, Melbourne, FL). To calibrate the MLC bank, its backup jaw is positioned at the central axis and the opposing jaw is retracted to create a half-beam configuration. The position of the backup jaws field edge is then measured with the array to obtain what is termed the radiation defined reference line. The positions of the individual leaf ends relative to this reference line are then inferred by the detector response in the leaf end penumbra. Iteratively adjusting and remeasuring the leaf end positions to within specifications completes the calibration. Using the backup jaw as a reference for the leaf end positions is based on three assumptions: (1) The leading edge of an MLC leaf bank is parallel to its backup jaws leading edge, (2) the backup jaw position is reproducible, and (3) the measured radiation field edge created by each leaf end is representative of that leafs position. Data from an electronic portal imaging device (EPID) were used in a similar analysis to check the results obtained with the array. RESULTS The relative leaf end positions measured with the array differed from those measured with the EPID by an average of 0.11+/-0.09 mm per leaf. The maximum leaf positional change measured with the Profiler 2 over a 3 month period was 0.51 mm. A leaf positional accuracy of +/-0.4 mm is easily attainable through the iterative calibration process. The method requires an average of 40 min to measure both leaf banks. CONCLUSIONS This work demonstrates that the Profiler 2 is an effective tool for efficient and quantitative MLC quality assurance and calibration.

SU‐GG‐T‐168: Measurement Errors Associated with Linear Accelerator Commissioning Data

Purpose:Linear accelerator(LINAC) commissioning employs 3‐D water phantoms (WP) to accurately assess LINAC dosimetric characteristics. The accuracy of the treatment planning system (TPS) modeling is based on the collected data. This investigation aims to assess the errors associated with the collection of commissioning data. Method and Materials: Possible sources of error within 3‐D WP measurements that were assessed are water tank mechanics, data acquisition, and chamber aspects. WP scanning software, mechanical measurements, and infra‐red motion tracking cameras were used to evaluate the mechanical performance of the WP. Continuous motion and point by point measurements were compared for a variety of standard detectors. Minor field size variations were scaled to the nominal field size for TPS import purposes using two methods, 1) geometric scaling of the whole profile or 2) geometric scaling of 80% of the field width. The detector orientation effects were evaluated for profiles and percentage depth dose. Results: WP positional accuracy and reproducibility was within 0.4mm and mechanical hysteresis within 0.46mm. The collection method impacted the results obtained for small detectors (<0.01cc) with the point measurement technique resulting in reduced noise level. However, the time scale increased considerably with the point by point technique. For field width scaling the 80% scaling method showed improved results compared to scaling the whole profile. The detector orientation showed variations in the build up region for small detectors and small variations were observed for penumbral widths of profile measurements. Conclusion: Error sources in data collection have been identified and quantified in this work. Detailed error analysis of experimental set up is presented. This work supported in part with Federal funds from the National Cancer Institute, Contract No HHSN261200522014C, and by Sun Nuclear Corporation.

SU‐FF‐T‐266: Characterizing a Multi‐Axis Ion Chamber Array

Purpose: To characterize a commercially available multi‐axis ion chamber array for use as a scanning water tank alternative. Method and Materials: The ion chamber array used in this study was the IC Profiler (Sun Nuclear Corporation: Melbourne, FL). We characterized four items of the array: reproducibility, dose linearity, backscatter response, and water tank agreement. Short and long term reproducibilitys were established on a 60Co teletherapy unit (Eldorado 6; Atomic Energy of Canada Limited: Mississauga, Canada). The remaining tests were conducted with a Synergy (Elekta: Crawley, UK) linear accelerator(LINAC) operated at a nominal photon energy of 6MV. Results: Over a short time period the array displayed a maximum standard deviation of 0.55% and a mean standard deviation of 0.15%; over a long time period the array displayed a maximum standard deviation of 1.80% and a mean standard deviation of 0.76%. The array was sensitive to startup characteristics of the LINAC when operating in pulsed mode; this affected the dose linearity relative to a Farmer chamber operating under the same geometry. This effect was not observed when the array was operated in continuous mode. Both the arrays central axis detector and a Farmer chamber displayed a similar increase in measured signal with increasing backscatter. However, with increasing backscatter (up to 16.6 cm) the arrays in‐beam‐profile shape changed by less than 0.7% relative to a setup with no additional backscatter. The agreement between the array and a scanning water tank differed by less than 1% in the beam. Conclusion: The IC Profiler is a viable option for water tank ‘like’ measurements. The device provides a stable platform with good dose linearity, minimal backscatter response, and uniform profile measurements. Conflict of Interest: This work was supported in part by SBIR Contract No. HHSN261200522014C, the University of Florida, and Sun Nuclear Corporation

Appropriate Technology and Inappropriate Politics

Over the past two decades “appropriate technology” has been used as code for new ways of thinking about the social implications of technological choice. Appropriate technology is seen variously: as a means of ushering in a New Age, as an alternative to high technology, as a social movement, and, by some, as Utopian delusion. The debate over appropriate technology is raising some of the most difficult questions facing a philosophy of technology, including: The relationship between technology and development, between ideology and industrialization, and more fundamentally, between man and machine (Rybcynski, 1980, p. v.).

WE‐D‐BRB‐07: Wide Field Array Calibration Dependence on Beam Shape Stability

Purpose: To simulate the effects of beam symmetry instabilities on detector arraycalibrations and explore compensation methods during actual calibration with a linear accelerator(LINAC).Methods: The array calibration method that was investigated is known as wide field (WF) calibration. With this method, a linear array [y‐axis (65 detectors) of the IC PROFILER™; Sun Nuclear Corporation, Melbourne, FL USA] requires three calibrationmeasurements (α, θ, and λ) using a radiation field that is larger than the array. Measurement θ is a 180° rotation from α, and λ is a physical shift by one detector from θ. The relative detector sensitivities are then determined through ratios of detector readings at the same field locations. This method results in error propagation that is proportional to the array size. The WF calibration postulates that the dose distribution is constant for each calibrationmeasurement. Postulate violations were quantified by applying a sine shaped perturbation (of up to 0.1%) to a calibrationmeasurement and determining the error relative to a baseline calibration. Actual postulate violations were limited by using a continuously on beam and mechanized array movement during θ and λ. Symmetry is more stable for a continuously on beam. Results: Simulated perturbations to θ or λ resulted in calibration errors of up to 2%, while α was relatively immune (<0.1% error). Wide field calibration error on a LINAC with similar symmetry instabilities was (±) 1.6%. Using a continuous beam during θ and λ reduced the calibration error to (±) 0.68%. Conclusion: This work increased the reproducibility of WF calibrations by limiting the effect of measurement perturbations due to LINAC symmetry instabilities. The same technique would work for any array using WF calibration.Conflict of Interest: Research sponsored by Sun Nuclear Corporation.