Constancy Checks Research Articles

Patient‐specific QA and delivery verification of scanned ion beam at NIRS‐HIMAC

To evaluate a patient-specific QA program and system for constancy checking of a scanning delivery system developed at the National Institute of Radiological Sciences. For the patient-specific QA, all the planned beams are recalculated on a water phantom with treatment planning software (TPS). The recalculated dose distributions are compared with the measured distributions using a 2D ionization chamber array at several depths, and evaluated using gamma index analysis with criteria of 3% and 3 mm and a pass rate of 90%. For the constancy check, the authors developed the multiwire proportional chamber (MWPC), which can record the delivered 2D fluence images in a slice-by-slice manner. During irradiation for dosimetric QA with the 2D ionization chamber array and an accordion-type water phantom, the 2D fluence images are recorded using the MWPC in the delivery system. These recorded images are then compared to those taken in the treatment session to check the constancy check. This analysis also employs gamma index analysis using the same criteria as in the patient-specific QA. These patient-specific QA and constancy check evaluations were performed using the data of 122 patients. In the patient-specific QA, the measured dose distributions agreed well with those calculated by the TPS, and the QA criteria were satisfied in all measurements. The additional check of the fluence comparison ensured the constancy of the delivered field during each treatment irradiation. The authors established a patient-specific QA program and additional check of delivery constancy in every treatment session. Fluence comparison is a strong tool for constancy checking of the delivery system.

SU-E-T-55: A Study of Film Dosimetry for Routine Beam Profile and PDD Constancy Checks in Proton Beams

Purpose: Radiographic film dosimetry suffers from energy dependence in Proton dosimetry, and thus is not suitable for absolute dosimetry. In this study, we investigate film dosimetry for the constancy check of percentage depth dose (PDD) and beam profile measurements in proton beams. Methods: From PDD measured by film and ion chamber (IC), calibration factors as a function of depth (IC/film) was obtained. These factors imply variable slopes (with energy and depth) of linear characteristic curves that relate film response to dose. They were used to convert day‐to‐day film measurements into dose. Film dosimetry of a 186 MeV proton beam was performed to investigate this hypothesis. In addition, Monte Carlos simulation of a 250 MeV proton beam was performed calculating proton fluence spectrum along the off‐axis direction. By multiplying stopping powers of film emulsion and water, respectively, to the spectrum, doses to film and water were calculated. The ratio of film dose to water dose was evaluated across the off‐axis distance to understand film response. Results: The measured and calibrated PDD approached to that of IC, but near the end of spread‐out‐bragg‐peak (SBOP), a spurious peak is observed due to the mismatch of distal edge between calibration film and measurement film. The SBOP width was measurable within 1mm. The distal edge was reproducible within 1.5mm. Entrance dose was reproducible within 5.5%. The possible sources of such errors include developer uncertainty, film emulsion nonuniformity, and misalignment of film edge to the phantom surface. For off‐axis evaluation, the dose ratio varied within 3%, and thus film is shown to be accurate for dosimetry across off‐axis distance. Conclusions: Radiographic film can be suitable for beam profile measurements and may be suitable for PDD constancy check for proton beams. Routine use will confirm such error which will be presented in this study.

SU-E-T-210: Surviving a Visit by the Radiological Physics Center.

To demonstrate an objective approach to determining if a negative report from the Radiological Physics Center (RPC) of greater than 10% error is valid or has clinical significance. The discrepancy involved the clinical activity (mgRaEq) of Cs-137 sources, some manufactured by 3M and some by Amersham. Measurements were made in the proprietary RPC Well Counter calibrated by the MD Anderson ADCL and our Well Counter (CNMC, Model 44D) calibrated by the same laboratory as well as the University of Wisconsin ADCL. In addition, we possess an Amersham Cs-137 Check Source that had been calibrated by the UW-ADCL in 2002. All clinical sources were checked in both Well Counters on the first visit. One clinical source and the Check Source were measured in a second visit that occurred 51 days later. On the initial RPC visit, 9 of 25 sources had a minimum of an 8% discrepancy between the RPC and the Institution, with a maximum of 11%. Contributing errors included using the incorrect straw position by us, an unexplained 2.3% error in the RPC data identified 73 days post-visit, a 2% variation in Chamber Factors for our Well Counter from the two ADCL's. When we use the 2004 value of Air Kerma Strength for the Check Source to determine a Calibration Factor of the Well Counter, all sources were within 0.5% of their decayed value established in 2002. This work emphasizes the value of having simple Constancy Check systems in a Quality Assurance program as 'Accuracy' has error bars. The disagreement in calibration data between the ADCL Laboratories, which was at the 2% maximum quoted in their Calibration Reports, is a reminder that there is uncertainty in measurements. Constancy Checks allow one to sort out discrepancies and to answer challenges to the validity of your program.

SU-E-T-187: A Dedicated Phantom for Electron Density Calibration and Image QA of Linac Cone Beam CT Options

Purpose: In order to use linac Cone‐Beam‐CT (CBCT) data for dosimetric planning an electron density calibration is compulsory. Studies on available phantoms for CT performance checks/QA have shown that these are not sufficient for an adequate HU calibration. Therefore we designed a new dedicated phantom and carried out a planning study to determine the usability of such calibrated CBCT data in comparison with diagnostic CT data. Methods: Available linacCBCT options acquire in a single gantry rotation a volumetric CT data set. In order to 3D‐calibrate the system with regard to HU a special phantom has been designed to include the whole imager area. Removable inserts allow flexible positioning of the probes in x‐z as well as z‐direction. Furthermore a performance and image QA part according AAPM recommendations No. 1 has been integrated and evaluated. We compared 20 plan setups (10 in the pelvic and 10 in the thoracic region). Results: Within the field of view the HU variation is up to 5%. Towards the imager edges hounsfield units show small deviations relative to the imager center (max. 8%). Relative dose distributions in CBCT‐based plans show minor differences to plans calculated using a diagnostic CTimage dataset (p=0.002) and absolute dosage deviations are within 1% (p=0.001). Image quality comparisons using the performance and image QA part showed that CBCTimages are of lower quality than diagnostic images, i.e. spatial and low contrast resolution are 2–3 levels below. Conclusions: The newly designed phantom allows real 3D‐calibration of linacCBCT. It is made of water equivalent material and consists of probes with tissue equivalent materials. The phantom has additional scatter material to simulate a real patients' situation. CBCT data can be used for treatment planning. Furthermore the image QA part allows to carry out recommended constancy checks in straightforward approach. We received financial support from Varian Medical Systems to manufacture the prototypes.

Quantitative MR-Spektroskopie: Implementierung und Qualitätssicherung an einem klinischen MR-Tomografen

The aim of this study was the implementation of quantitative MR-Spectroscopy with external reference signals on a Philips Gyroscan Intera 1,5 T MR-Scanner for routine clinical use. We established measurements for quality assurance of the used phantoms and methods. In detail we analyzed the dependence of the signal on concentration and volume as well as on the local variation within the coil and the impact of these factors on quantification. Furthermore we established measurements for routine constancy checks. The method as well as the software for concentration calculation was checked by an in vitro measurement. In a blinded measurement the concentrations of NAA, Cho and Kr in the PTB reference phantom 10614 mM were determined correctly within their error limits.

SU-GG-T-274: A Practical MLC Pattern for Comprehensive Routine MLC QA

Purpose:Quality assurance (QA) for Multi‐leaf Collimators(MLC) is becoming more important as IMRT is becoming popular. Its importance will further increase when tumor tracking using an MLC becomes clinically applicable. This study aims to develop a practical MLC routine QA pattern that can be used in clinics to perform various MLC QAs using an Electronic Portal Imaging Device(EPID).Method and Materials: An MLC pattern of 17 cm width, a modified Picket fence, has been developed. The pattern consists of seven regions, including 0, 10, and 20 MU, and two gradient regions (for increasing and decreasing dose). In each region, the MLCs move with various speeds, namely 0, 0.25, 0.3, 0.8, and 2.5 cm/sec, and with varying sliding window width. Offsets of 0.5, 1, 2, and 3 mm were introduced for some of the leaves to simulate defectiveness. The MLC sequence was delivered with a Varian‐Trilogy accelerator. Images were acquired both in integrated and cine modes using a aS1000 EPID. The EPIDimages were analyzed using home‐grown software. Results: Weekly constancy checks such as leaf alignment and transmissions are performed using the integrated image for both with and without inter‐digitations. Monthly measurements of leaf speeds and position accuracy for four cardinal gantry angles are performed using the cine mode images. A positional variation larger than 0.5 mm can be detected. The leaf positions were measured at four locations with an error of ±0.038 cm. Annual tests such as MLC transmission for all energies and leaf position repeatability are performed using the cine‐mode images.Conclusion: The new pattern is sensitive to potential errors and is convenient to use. Multiple tests can be performed using a single pattern, and the testing time is short, e.g., 5 and 10 min for the weekly and monthly checks, respectively.

Dose assessment during the commissioning of flat detector imaging systems for cardiology

Incident air kerma (IAK) and entrance surface air kerma (ESAK) have been measured for a range of copper (Cu) absorbers (1-10 mm) and polymethylmethacrylate (PMMA) slabs (12-28 cm) with kilovolt values ranging from 61 to 120 during the commissioning of an X-ray system equipped with a flat detector used in interventional cardiology procedures. Numerical parameters on image quality have also been measured for different X-ray beam qualities using the plastic wall of the ionisation chamber. When moving from 1 to 10 mm of Cu, IAK per frame increased to a factor of 38 for cine and 27 for fluoroscopy. A cine frame requires 60-116 times more IAK than a fluoroscopy frame. As for PMMA, when the backscatter factor is included (simulating real conditions with patients), and when moving from 12 to 28 cm, the increases in ESAK are 16 times for cine and 10 times for fluoroscopy. Because of the differences in X-ray beam quality for cine and fluoroscopy modes, the Cu thicknesses necessary to drive the generator to equivalent kilovolts resulted in the following values (cine and fluoroscopy, respectively): 12 cm of PMMA (1 and 1.5 mm Cu), 20 PMMA (2.5 and 3.5 mm Cu) and 28 cm PMMA (4.5 and 8.5 mm Cu). With the analysis of IAK, ESAK and image quality, one can verify the appropriate settings of the X-ray system and obtain baseline information for constancy checks and help cardiologists in the management of patient doses by knowing the dose increase factors and image quality changes when increasing patient thickness or using different C-arm projections.

Electron beam quality control using an amorphous silicon EPID

An amorphous silicon EPID has been investigated to determine whether it is capable of quality control constancy measurements for linear accelerator electron beams. The EPID grayscale response was found to be extremely linear with dose over a wide dose range and, more specifically, for exposures of 95-100 MU. Small discrepancies of up to 0.8% in linearity were found at 6 MeV (8-15 MeV showed better agreement). The shape of the beam profile was found to be significantly altered by scatter in air over the approximately 60 cm gap between the end of the applicator and the EPID. Nevertheless, relative changes in EPID-measured profile flatness and symmetry were linearly related to changes in these parameters at 95 cm focus to surface distance (FSD) measured using a 2D diode array. Similar results were obtained at 90 degrees and 270 degrees gantry angles. Six months of daily images were acquired and analyzed to determine whether the device is suitable as a constancy checker. EPID output measurements agreed well with daily ion chamber measurements, with a 0.8% standard deviation in the difference between the two measurement sets. When compared to weekly parallel plate chamber measurements, this figure dropped to 0.5%. A Monte Carlo (MC) model of the EPID was created and demonstrated excellent agreement between MC-calculated profiles in water and the EPID at 95 and 157 cm FSD. Good agreement was also found with measured EPID profiles, demonstrating that the EPID provides an accurate measurement of electron profiles. The EPID was thus shown to be an effective method for performing electron beam daily constancy checks.

SU‐GG‐T‐545: Dosimetric Verification of RapidArc Treatment Delivery Using the Delta4 Phantom

Purpose: Recently, Varian Medical Systems have introduced a new treatment technique, in which dose is delivered over one gantry rotation with variable MLC positions, dose rate and gantry speed, RapidArc. A preclinical RapidArc installation was carried out on a Varian Clinac at our institution. We report dosimetric verification of reproducibility and consistency of RapidArc delivery using the Delta4 phantom (Scandidos, SE). Method and materials: 9 treatment plans generated in the Eclipse preclinical version of the RapidArc optimizer (H&N, prostate and lung) were delivered using a Clinac equipped with RadidArc delivery capability. The delivered dose was measured using the Scandidos Delta4 phantom. The measured dose distributions were compared with doses calculated in Eclipse. All plans were delivered three times consecutively, and the first run was used as reference for constancy checks. The temporal resolution of the delivery was analysed by investigating the arc segments between control points separately (177 control points per rotation). Gamma analysis (3%/3mmDTA) was used to quantify correspondence between dose distributions. Results: Overall, good agreement was observed between measured and calculated doses, generally with gamma values <1 in >95% of measured points. Comparison of the accumulated doses for two consecutive runs of a plan showed gamma values <1 in all measured points. The segment‐by‐segment analysis showed discrepancies between two consecutive runs, with >90% gamma values <1 in around 50–60% of the segments (gamma dose criterium 3%; <1–2 mGy for single segments). Segments with large dose deviations were typically counter‐balanced by following segments with large deviations of the opposite sign. The segment discrepancies cancelled out in the accumulated dose. Conclusion: The delivery of RapidArc beam delivery corresponded well with calculated dose distributions for different cases. The delivery was reproducible, and was carried out with high stability of the accelerator performance.Research sponsored by Varian Medical Systems, Inc.

Application of European protocol in the evaluation of contrast-to-noise ratio and mean glandular dose for two digital mammography systems

The performance of two digital mammography systems, Agfa CR75 and CRMM3 computed radiography (CR) and IMS Giotto MD direct digital radiography (DR), was assessed by applying a method recommended in the European protocol for quality control in mammography screening. The contrast-to-noise ratio (CNR) and mean glandular dose (MGD) values were measured and contrast detail (CD) analysis was performed. The CNRs for system CR were 21.9, 12.9, 9.5, 8.8, 7.4, 5.5 and 4.4 for 2, 3, 4, 4.5, 5, 6 and 7-cm polymenthylmethacrylate (PMMA) thickness, respectively. The respective CNRs for system DR were 10.4, 8.8, 6.3, 7.3, 7.2, 6.4 and 6.54. For the same phantom thickness sequence, the MGDs were 0.7, 1.1, 1.3, 1.6, 1.9, 2.5 and 3.4 mGy for system CR, whereas they were 0.7, 1.2, 1.1, 1.3, 1.8, 3.5 and 3.9 mGy for system DR. The CNR and MGD results satisfactorily correlate with CD analysis results. The MGD values compare well with the values recommended in the European protocol. Despite being simple, CNR and MGD can provide an effective system for performance assessment and constancy checks for related optimisations.

The influence of output measurement time interval and tolerance on treatment dose deviation in photon external beam radiotherapy

A method was developed to estimate the spread of treatment doses due to a given output measurement, time interval and action level in external photon beam radiotherapy. The method exploits empirical or simulated data of both output stability and the reproducibility of the absolute output measurements and relative constancy checks. The spread of an output and the probability that an output is outside the predefined tolerance limits were estimated for different quality control procedures of the output. The use of internationally recommended output measurement intervals of 3, 1 and 0.25 months together with an action level of ±2% results in the out-of-tolerance probability up to 49%, 26% and 3%, respectively, while the output is kept within up to ±5.5%, ±4.0% and ±2.0%, respectively. It was also found that the action level has more prominent relative influence than the measurement interval. The results suggest that similar treatment quality can be achieved by doubling the output measurement time interval and lowering the action level from ±2% to ±1.5% and the constancy check action level from ±3% to ±2%.

The use of a commercial QA device for daily output check of a helical tomotherapy unit

Helical tomotherapy radiation therapy units, due to their particular design and differences from a traditional linear accelerator, require different procedures by which to perform routine quality assurance (QA). One of the principal QA tasks that should be performed daily on any radiation therapy equipment is the output constancy check. The daily output check on a Hi-Art TomoTherapy unit is commonly performed utilizing ionization chambers placed inside a solid water phantom. This provides a good check of output at one point, but does not give any information on either energy or symmetry of the beam, unless more than one point is measured. This also has the added disadvantage that it has to be done by the physics staff. To address these issues, and to simplify the process, such that it can be performed by radiation therapists, we investigated the use of a commercially available daily QA device to perform this task. The use of this device simplifies the task of daily output constancy checks and eliminates the need for continued physics involvement. This device can also be used to monitor the constancy of beam energy and cone profile and can potentially be used to detect gross errors in the couch movement or laser alignment.

Analysis of quality control data of eight modern radiotherapy linear accelerators: the short- and long-term behaviours of the outputs and the reproducibility of quality control measurements

Quality control (QC) data of radiotherapy linear accelerators, collected by Helsinki University Central Hospital between the years 2000 and 2004, were analysed. The goal was to provide information for the evaluation and elaboration of QC of accelerator outputs and to propose a method for QC data analysis. Short- and long-term drifts in outputs were quantified by fitting empirical mathematical models to the QC measurements. Normally, long-term drifts were well (⩽1%) modelled by either a straight line or a single-exponential function. A drift of 2% occurred in 18 ± 12 months. The shortest drift times of only 2–3 months were observed for some new accelerators just after the commissioning but they stabilized during the first 2–3 years. The short-term reproducibility and the long-term stability of local constancy checks, carried out with a sealed plane parallel ion chamber, were also estimated by fitting empirical models to the QC measurements. The reproducibility was 0.2–0.5% depending on the positioning practice of a device. Long-term instabilities of about 0.3%/month were observed for some checking devices. The reproducibility of local absorbed dose measurements was estimated to be about 0.5%. The proposed empirical model fitting of QC data facilitates the recognition of erroneous QC measurements and abnormal output behaviour, caused by malfunctions, offering a tool to improve dose control.

459 Daily constancy checks of flatness, symmetry and output for photon beams using a commercial electronic portal image device (EPID)

459 Daily constancy checks of flatness, symmetry and output for photon beams using a commercial electronic portal image device (EPID)

SU‐FF‐T‐210: Performance Evaluation and Quality Assurance of Enhanced Dynamic Wedges

Purpose: Enhanced dynamic wedges have been used in clinical practice for many years. Obvious superiority of dynamic over physical wedges is accompanied by the increased overhead involved in verifying the accuracy and reliability of their use. Contrary to very limited quality assurance (QA) required to ensure proper functioning of the physical wedges, dynamic wedges, like any other dynamic treatment, require a robust quality assurance program. This work expands upon previous suggestions and describes a comprehensive QA program for Varian Enhanced Dynamic Wedges (EDW) and presents the results of a sixteen‐month evaluation of these wedges. Method and Materials: Daily, monthly, and annual QA procedures were devised for enhanced dynamic wedges on a dual energy Varian 21ex linear accelerator with seven wedge angles. These include daily constancy checks of the wedge performance as part of the morning QA procedure, monthly evaluation of wedge factors and profiles, and an analysis of dynamic log files generated by linac. Yearly performance evaluation includes measurement of a larger sample of wedge angles and profiles. In addition, individual treatment verification or “per patient” QA consists of visual inspection of the wedge direction at the conclusion of each treatment. Results: The daily constancy checks show a stable delivery of the wedged field with a variation of less than 1%. The monthly wedge factors measured have been within 1.5 % of the commissioning values. A comparison of beam profiles over the same time period shows a stable delivery of the wedged beams and good agreement with the expected segmented treatment tables (STT). Conclusion: An expended comprehensive QA of enhanced dynamic wedges is presented. Results of a sixteen‐month evaluation demonstrate stable and accurate delivery of these fields. Due to the dynamic nature of enhanced dynamic wedge deliveries, a comprehensive QA program is necessary to verify proper delivery of these fields.

Linear accelerator output variability

The Wellington Cancer Centre is equipped with two matched linear accelerators (Varian 2100CD) linear accelerators (identified as SN1027 and SN42). Each morning, before treatments commence, a radiation therapist carries out an output constancy check of the radiation output and every fortnight a physicist measures, in a phantom, the delivered radiation dose to check on the machine calibration. The daily output checks have been recorded into a database (Argus QA for Radiation Oncology) since August 1997 and in June 1998 the fortnightly calibration measurements were added. The information in the database, up to April 2003, has been analysed to consider the quality of the daily constancy checks as compared with the fortnightly calibration measurements and whether the data contains useful information on machine performance. After allowance for the effects of machine recalibration the fortnightly calibration measurements had an average standard deviation of 0.4% and the daily constancy checks 0.8%. The daily constancy checks had a greater number of large deviations than would be expected assuming a normal distribution and were not a good predictor of the need for a recalibration. The fortnightly calibration measurements with a much lower spread give a reliable indication of the need for a recalibration allowing the adoption of a +/- 1% tolerance. Over the period analysed one accelerator (SN42) was relatively stable with the output generally drifting between +/- 1% while the other (SN1027) had a consistent increase in the average output of about 2.5% per year.

Radiation exposure and image sharpness of mammographic units in bavaria with respect to the European guidelines

In a survey including all mammography units in Bavaria, entrance doses in air and other quality-relevant parameters were measured with standard settings and under comparable conditions. The study was aimed to discover discrepancies between practical mammographic imaging techniques and the demands established in the European Guidelines. Test images were made by the operators themselves with phantoms used for constancy checks. Entrance dose values were measured with official film dosimeters. The survey showed that more than 95 % of all mammographic units in Bavaria comply with the European guidelines with respect to radiation exposure and imaging sharpness.

Zum Stand der Qualitätssicherungsmaßnahmen an Röntgentherapieanlagen in der Schweiz

In Switzerland 57 X-ray therapy units are in operation at present. According to the Swiss Ordinance on Radiation Protection, a quality-assurance program must regularly be applied in to these units. However, as the Swiss X-ray Ordinance gives explicit control parameters only for diagnostic units, the present article issues proposals for the realization of a quality-assurance program for the therapy units. In this regard, it is distinguished between checks performed by technical personnel of the X-ray manufacturers and checks performed by a medical physicist with corresponding qualification, or under his supervision. The so-called mentor project for the performance of annual constancy checks in institutes without medical physicists is also taken into account. These proposals should be helpful for the discussion and clarification of competencies, hence contributing to standardization of control practices in Switzerland.

Treatment planning for the heavy-ion facility at GSI.

A new treatment planning program was developed for the heavy-ion therapy facility at GSI. In addition, a concise quality standard for treatment planning has been set up. It covers acceptance and constancy checks of all critical aspects in treatment planning. Dose verification measurements done during the commissioning phase show an overall good agreement with the treatment planning calculations.

Use of simple quality assurance procedures in the analysis of beam asymmetries on cobalt-60 treatment units.

Recently, the mechanical failure of one of the upper collimator mechanical trimmers on a cobalt-60 unit resulted in large beam asymmetries and unacceptable flatness characteristics. This malfunction was not detected using currently accepted schedules for quality assurance tests. The incident suggests that the frequency of routine beam profile constancy checks should be increased to weekly for cobalt-60 units.