Quality Assurance Device Research Articles

Spill‐to‐spill and daily proton energy consistency with a new accelerator control systema)

The Loma Linda University proton accelerator has had several upgrades installed including synchrotron dipole power supplies and a system for monitoring the beam energy. The consistency of the energy from spill-to-spill has been tested by measuring the depth ionization at the distal edge as a function of time. These measurements were made with a minimally equipped beamline to reduce interference from confounding factors. The consistency of the energy over several months was measured in a treatment room beamline using an ionization chamber based daily quality assurance device. The results showed that the energy of protons delivered from the accelerator (in terms of water equivalent range) was consistent from spill-to-spill to better than +/-0.03 mm at 70, 155, and 250 MeV and that the energy check performed each day in the treatment room over a several month period was within +/-0.11 mm (+/-0.06 MeV) at 149 MeV. These results are within the tolerances required for the energy stacking technique.

Quality assurance device for four‐dimensional IMRT or SBRT and respiratory gating using patient‐specific intrafraction motion kernels

Emerging technologies such as four‐dimensional computed tomography (4D CT) and implanted beacons are expected to allow clinicians to accurately model intrafraction motion and to quantitatively estimate internal target volumes (ITVs) for radiation therapy involving moving targets. In the case of intensity‐modulated (IMRT) and stereotactic body radiation therapy (SBRT) delivery, clinicians must consider the interplay between the temporal nature of the modulation and the target motion within the ITV. A need exists for a 4D IMRT/SBRT quality assurance (QA) device that can incorporate and analyze customized intrafraction motion as it relates to dose delivery and respiratory gating. We built a 4D IMRT/SBRT prototype device and entered (X, Y, Z)(T) coordinates representing a motion kernel into a software application that transformed the kernel into beam‐specific two‐dimensional (2D) motion “projections,”previewed the motion in real time, anddrove a precision X–Y motorized device that had, atop it, a mounted planar IMRT QA measurement device. The detectors that intersected the target in the beam's‐eye‐view of any single phase of the breathing cycle (a small subset of all the detectors) were defined as “target detectors” to be analyzed for dose uniformity between multiple fractions. Data regarding the use of this device to quantify dose variation fraction‐to‐fraction resulting from target motion (for several delivery modalities and with and without gating) have been recently published. A combined software and hardware solution for patient‐customized 4D IMRT/ SBRT QA is an effective tool for assessing IMRT delivery under conditions of intrafraction motion. The 4D IMRT QA device accurately reproduced the projected motion kernels for all beam's‐eye‐view motion kernels. This device has been proved to• effectively quantify the degradation in dose uniformity resulting from a moving target within a static planning target volume, and• integrate with a commercial respiratory gating system to ensure that the system is working effectively.Such a device is discussed as a potential tool to optimize the gating duty cycle to maximize delivery efficiency while minimizing dose variability.PACS numbers: 87.50.Gi, 87.53.Dq, 87.53.Kn, 87.53.Mr, 87.53.Tf, 87.56.Fc, 87.58.Sp

Daily quality assurance phantom for ultrasound image guided radiation therapy

A simple phantom was designed, constructed, tested, and clinically implemented for daily quality assurance (QA) of an ultrasound‐image‐guided radiation therapy (US‐IGRT) system, the Restitu Ultrasound system (Resonant Medical, Montreal, QC). The phantom consists of a high signal echogenic background gel surrounding a low signal hypoechoic egg‐shaped target. Daily QA checks involve ultrasound imaging of the phantom and segmenting of the embedded target using the automated tools available on the US‐IGRT system. This process serves to confirm system hardware and software functions and, in particular, accurate determination of the target position. Experiments were conducted to test the stability of the phantom at room temperature, its tissue‐mimicking properties, the reproducibility of target position measurements, and the usefulness of the phantom as a daily QA device. The phantom proved stable at room temperature, exhibited no evidence of bacterial or fungal invasion in 9 months, and showed limited desiccation (resulting in a monthly reduction in ultrasound‐measured volume of approximately 0.2 cm3). Furthermore, the phantom was shown to be nearly tissue‐mimicking, with speed of sound in the phantom estimated to be 0.8% higher than that assumed by the scanner calibration. The phantom performs well in a clinical setting, owing to its light weight and ease of operation. It provides reproducible measures of target position even with multiple users. At our center, the phantom is being used for daily QA of the US‐IGRT system with clinically acceptable tolerances of ±1 cm3 on target volume and ±2 mm on target position. For routine daily QA, this phantom is a good alternative to the manufacturer‐supplied calibration phantom, and we recommended that that larger phantom be reserved for less frequent, more detailed QA checks and system calibration.PACS numbers: 87.66.Xa, 87.63.Df

SU‐FF‐J‐17: Evaluation of Quality Assurance Phantom for Electronic Portal Imaging Device (EPID)

Electronic portal imaging device (EPID) has become an essential and integral tool in setup and treatment field verification for precision radiation therapy. Along with geometric verification, EPID is also being used for dosimetric verification. Consistent image quality and stable radiation response is necessary for proper utilization that requires routine quality assurance (QA). A commercial ‘EPID QC’ phantom from PTW is evaluated for EPID QA. The device is a solid 25×25×4.8cm3 phantom and weighs 3.8kg. It consists of 5 essential tools to measure the geometric accuracy, signal to noise ratio (SNR), dose linearity, the low and the high contrast resolutions and are aligned with divergence to measure the imaging and geometric parameters in both x, and y directions. The low contrast tool consists of a series of holes with various diameters and depths in aluminum slab, very similar to the Las Vegas phantom. The high resolution contrast tool provides the modulation transfer function (MTF) in both the x‐ and y‐dimensions to measure the focal spot of linear accelerator. The EPID QC device is tested on the latest and modern amorphous silicon imagers on Elekta, Siemens and Varian linear accelerators. Images were acquired in the anterior and lateral orientations for a 26×26 cm2 field for 1–15 MU at 95.2 cm. The analysis of the images was performed with the EPIDsoft software for automated analysis. The results indicates that all imagers have very good low contrast resolution and MTF(0.5) in the range of 0.3–0.4 lp/mm. All imagers also exhibit satisfactory geometric accuracy, dose linearity and SNR. Our analysis indicated that the imaging parameters are independent of MU and spatial orientations. The software maintains a record for the QA and verifies the parameters for temporal variation. The EPID QA device is easy to use, and allows readily assessment of the performance of any imager.

Dosimetric evaluation of a 2D pixel ionization chamber for implementation in clinical routine

In this paper we present the results of a dosimetric evaluation of a 2D ionization chamber array with the objective of its implementation for quality assurance in clinical routine. The pixel ionization chamber MatriXX (Scanditronix Wellhofer, Germany) consists of 32 × 32 chambers with a distance of 7.6 mm between chamber centres. The effective depth of measurement under the surface of the detector was determined. The dose and energy dependence, the behaviour of the device during its initial phase and its time stability as well as the lateral response of a single chamber of the detector in cross-plane and diagonal directions were analysed. It could be shown, that the detector's response is linear with dose and energy independent. Taking the lateral response into account, two different dose profiles, for a pyramidal and an IMRT dose distribution, were applied to compare the data generated by a treatment planning system with measurements. From these investigations it can be concluded that the detector is a suitable device for quality assurance and 2D dose verifications.

Investigation of an amorphous silicon EPID for measurement and quality assurance of enhanced dynamic wedge

This work assessed the dosimetric performance of an amorphous silicon electronic portal imaging device (EPID) for measurement and quality assurance of enhanced dynamic wedge (EDW) profile and wedge factor. EPID measurements of EDW profiles were corrected for pixel sensitivity variation and spectral over-response relative to ion chamber and compared to ion-chamber and diode-array measurements. The dependence of EPID measurements on wedge direction and source to EPID distance was assessed. The long-term stability was investigated by weekly measurement of EDW profiles and wedge factors over a seven month interval. An empirical correction method was developed to improve EPID profile agreement with diode-array measurements. The EPID profiles differed from conventional measurements by up to 5%. Backscatter from the EPID housing was also found to affect measurements by up to 4%, resulting in changes in EPID measured profile with the direction of the moving jaw and source to EPID distance. EPID profile measurements varied by a maximum of 0.3% (1 SD) within the umbra, and wedge factors varied by 0.3% (1 SD) over the seven month interval. The correction function improved agreement between EPID and diode array to within 2% for all wedge angles and energies. Due to the ease of use and reproducibility of the EPID-measured EDW profiles the device is highly suited to regular measurement of EDW.

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.

SU‐FF‐T‐428: 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 currently 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 check and allows for this measurement to be performed by the therapists, rather than the physicist(s). This device can also be used to monitor the beam energy and lateral symmetry and can potentially be used to detect errors in the couch movement or laser misalignment.

Quality Assurance and Safety of New Technologies for Radiation Oncology.

This article reviews suggested recommendations and procedures for quality assurance and safety tests of computer-controlled medical accelerators and the associated high-tech ancillary devices such as asymmetric collimators, dynamic wedge, multileaf collimators, and on-line electronic portal imaging devices for treatment verification, and technologies such as dynamic therapy and beam intensity modulation that are just coming on line. These new technologies are likely to lead to improved therapeutic ratios through the use of conformal physical dose distributions that cannot be achieved using traditional planning, delivery, and verification methods and, at the same time, provide more complete and thorough safety systems. However, these devices are not yet fully developed and thus come with some limitations and an increased potential for patient treatment error. To insure optimum use, a rigorous quality assurance program specifically designed for these technologies must be in place and the radiation oncology team should be constantly vigilant. In addition, provision should be made to provide for increased continuing staff education in the use of these technologies and complementary safety and quality assurance devices such as a record and verify system should be considered essential.

Photothermal characterization of natural gas automotive fuel

In this paper, we describe a new, simple and fast photothermal method for characterizing the quality of natural gas used as automobile fuel. The method consists essentially in measuring the thermal properties of a sequence of natural gas diluted in nitrogen with a photothermal gas analyser time especially designed for this purpose. It is shown that the proposed method is capable of measuring the thermal properties with an accuracy of 3%. It is also shown that the proposed method is capable of detecting traces of H2S greater than 7.8 mg m−3 in the natural gas. This good accuracy, together with its fast response and portability, suggests it as a simple routine, local, checking device for natural gas quality assurance at retailers' stations.

TH-C-T-6E-09: Multi-Leaf-Collimator Quality Assurance Using the Electronic Portal Imaging Device

Purpose: To test the amorphous silicon (aSi) electronic portal imaging device (EPID) for quality assurance (QA) of the multi-leaf collimators (MLC) and to validate its use as a dynamic intensity-modulated radiation therapy (IMRT) QA device. Method and Materials: Established MLC QA for IMRT utilizes dosimetric outputs from a single point in phantom with a chamber, and radiographic film to capture the density-pattern of the MLC. We propose using the aSi EPID 2D-density distribution to replace the chamber/film for MLC QA, and to verify the dosimetry and mechanics of the MLC for IMRT. A protocol was developed to acquire QA data from the aSi EPID to provide tests for high dose gradients, average output doses, mechanical leaf stability, speed and positioning. The effects of various EPID exposed areas (2×2, 4×4, 6×6 cm2) were used for intercomparison measurements with ion chamber and film. Results: Radiographic film output at gantry angle 270° relative to 0° varies by less than 2%, while the film measurements at other gantry/collimator angles agree within 1% with the ionization chamber. For the same conditions, EPID outputs vary by 1%–5% depending upon exposed area. Output variations may be due to mechanical shifts, gantry and EPID sag, or MLC drifts due to gravity. Minimizing these effects is under further investigation. Conclusion: A method has been developed to utilize the aSi EPID for MLC QA. The 2D density distributions of the EPID offer the potential for more quantitative analysis, and this procedure can be integrated into a routine program for comprehensive dynamic MLC-based IMRT and EPID QA. Conflict of Interest: This work supported in-part by Varian Medical Systems.

SU‐FF‐T‐174: A New Device for the Verification of Temporal Function of the 4DCT and Gating Delivery System

Purpose: A quality assurance device was developed for 4DCT (GE Medical System, Waukesha, Wisconsin) and the Real‐Time Position Management Respiratory Gating System (Varian Medical Systems, Palo Alto, CA). It verifies the temporal‐phase function of the 4DCT and the gated delivery function of the Linac. Method and Materials: The QA device is composed of a mobile phantom, static phantom, acrylic lobe, AC motor, power transformer and transmission mechanism. There are eight radio‐opaque lines embedded in the static phantom, which are used as reference indicators. A “Z” shape radio‐opaque marker is fixed on the top of the mobile. This phantom moves periodically with a maximum displacement of 3 cm in a horizontal direction. It was scanned under our 4DCT at a period of 5 seconds. The collected images were divided into 10 groups representing 10 phases of the moving cycle. The digitally reconstructed radiograph of each phase was generated for the verification of the temporal‐phase function of the 4DCT. To verify the gated delivery function of the Linac, a radiographic film was attached to the surface of the mobile phantom and a 0.5 cm by 10 cm X‐ray field was used to expose the film at certain phases. The expected position of 0% phase was marked by punching 2 small holes on the film. Results: The “Z” mark was correctly shown and aligned with the reference lines at each phase. The film exposed under the Linac has shown that the radiation had been delivered to the correct position. Conclusion: A reliable quality assurance device for 4DCT and respiratory gating system is necessary to ensure that the radiation dose is accurately delivered to the patient's target volume. This QA device is effective and convenient for checking temporal‐phase function on the 4DCT and RPM gating system.

SU‐FF‐T‐155: Monte Carlo Dosimetric Verification for IMRT QA Using MLC Log Files and EPID

Purpose: EPID is a useful tool for pre‐treatment patient setup and target localization and post‐treatment dosimetry verification. However, EPID images are significantly affected by photon attenuation and scattering in the patient and the detector and therefore cannot be used directly for dose reconstruction. This work investigates a Monte Carlo dosimetric verification method based on MLC log files and the electronic portal imaging device (EPID) for IMRT quality assurance (QA). Method and Materials: We have developed Monte Carlo based software to derive accurate intensity‐modulated fluence maps behind the patient using MLC log files, which take into account the accelerator head geometry, the MLC leaf movement accuracy and the patient attenuation and scatter. Patient initial simulation and pre‐treatment CT data were used to simulate the patient anatomy for interfraction dose comparison. The phase space data behind the patient were used in the EPID response simulation and compared with measured EPID images to investigate patient setup accuracy and dose reconstruction uncertainty. Results: Ten previously treated prostate IMRT plans and patient CT data are included in this study. The MLC leaf position accuracy of the dynalog files from a Varian 21Ex accelerator is verified to within 1mm. The dose distributions based on the leaf sequences from the treatment planning system and the fluence maps rebuilt from the dynalog files are consistent to within 2%, validating our software implementation. The primary photons and scatters are recorded separately behind the patient for different applications as described above. Energy spectrum, fluence distribution and angular distribution are derived to facilitate dose calculation in the EPID. Conclusion: We have developed a log file based Monte Carlo method to generate phase space and fluence maps for pre‐treatment patient setup and post‐treatment dose verification with EPID. This work is implemented as part of our IMRT QA procedure.

WE‐D‐T‐617‐07: Development of An Independent Audit Device for Remote Verification of 4D Radiotherapy

Purpose: To develop a 4D quality assurance device for remote verification of dose delivery at institutions participating in NCI‐sponsored clinical trials that require the use of respiratory motion mitigation techniques. Method and Materials: A plastic reciprocating platform was designed and constructed to simulate respiratory motion in two dimensions and to evaluate the reproducibility of 4D image acquisition and dose delivery. An anthropomorphic lung phantom was used with the platform to assess imaging characteristics, to provide CT data for radiotherapy treatment planning and for dosimetric evaluation. The lung phantom used radiochromic film in three major planes and TLD centered in the GTV. Static, simulated free‐breathing, breath‐hold, 4D phase‐sorted and respiratory‐gated CT image sets were acquired using the programmable moving platform. A volumetric comparison of internal phantom structures assessed imaging reproducibility. Conformal 6 MV photon treatment plans were created for free‐breathing ITV and respiratory‐gated techniques using Pinnacle. The reproducibility of the 4D irradiation techniques was analyzed through comparisons to calculated treatment planning dose data. Criteria for evaluation were derived from TG‐53 with a range of 5%/3mm to 7%/7mm. Results: CT image analysis showed mean(SD) tumor volumes [cm3] of 24.5(0.4), 23.1(1.6), 24.1(1.6) for static, 4D phase‐sorted and free‐breathing techniques, respectively. Initial evaluation of the ITV technique showed that the ratio of TLD/Pinnacle dose was 0.972. Axial film analysis with binary agreement maps showed ranges of 85.5% to 96.4% and 81.0% to 95.4% agreement within the 5%/3mm to 7%/7mm criteria range for gated and ITV techniques, respectively. Axial profile analyses through the tumor center indicated regions of disagreement between the measured dose data and calculated data within the 5% criterion. Conclusion: The 4D phantom QA system can be used as an independent audit device for dosimetric evaluation of 4D radiotherapy techniques. Work supported by PHS grant CA10953 and CA81647 awarded by the NCI.

A feasibility study of the Dynamic Phantom scanner for quality assurance of beam profiles at various gantry angles

The effect of gantry rotation on beam profiles of photon and electron beams is an important issue in quality assurance for radiotherapy. To address variations in the profiles of photon and electron beams at different gantry angles, a Dynamic Phantom scanner composed of a 20×12×6 cm3 scanning Lucite block was designed as a cross‐beam‐profile scanner. To our knowledge, differences between scanned profiles acquired at different gantry angles with a small size Lucite block and those acquired a full‐size (60×60×50 cm3) water phantom have not been previously investigated. We therefore performed a feasibility study for a first prototype Dynamic Phantom scanner without a gantry attachment mount. Radiation beams from a Varian LINAC 21EX and 2100C were used. Photon beams (6 MV and 18 MV) were shaped by either collimator jaws or a Varian 120 Multileaf (MLC) collimator, and electron beams (6 MeV, 12 MeV, and 20 MeV) were shaped by a treatment cone. To investigate the effect on profiles by using a Lucite block, a quantitative comparison of scanned profiles with the Dynamic Phantom and a full‐size water phantom was first performed at a 0° gantry angle for both photon and electron beams. For photon beam profiles defined by jaws at 1.0 cm and 5.0 cm depths of Lucite (i.e., at 1.1 cm and 5.7 cm depth of water), a good agreement (less than 1% variation) inside the field edge was observed between profiles scanned with the Dynamic Phantom and with a water phantom. The use of Lucite in the Dynamic Phantom resulted in reduced penumbra width (about 0.5 mm out of 5 mm to 8 mm) and reduced (1% to 2%) scatter dose beyond the field edges for both 6 MV and 18 MV beams, compared with the water phantom scanner. For profiles of the MLC‐shaped 6 MV photon beam, a similar agreement was observed. For profiles of electron beams scanned at 2.9 cm depth of Lucite (i.e., at 3.3 cm depth of water), larger disagreements in profiles (3% to 4%) and penumbra width (3 mm to 4 mm out of 12 mm) were observed. Additional profiles with the gantry at 90° and 270° were performed for both MLC‐ and jaw‐shaped photon beams and electron beams to evaluate the effect of gantry rotation. General good agreement is seen (less than 1 % variation) at all field sizes for collimator‐shaped 6 MV and 18 MV photon beams. Similar variations observed for MLC‐shaped photon beams indicate that the uncertainty in MLC position is similar to that for the collimator jaws. We conclude that the Dynamic Phantom scanner is a useful device for the routine quality assurance on beam profiles of photon beams and for constancy check on electron beams at various gantry angles. Caution should be taken when using this device to acquire basic electron dosimetry data.PACS number: 87.53.‐j

Alignment between Quality Enforcement Devices and Governance Structures in the Agro-food Vertical Chains

Using a Transaction Cost Economics framework, this paper studies how supply chain governance are aligned with the type of quality enforcement mechanisms used in transactions with consumers. Specifically we compare governance of supply chains when private brand assures quality to chains where official certification assures products’ quality for customers. To test our proposition, we conducted a set of 42 case studies in 3 agro-food sectors (processed meat, cheese, fruits and vegetable) from 7 European countries. We found that supply chain governance is closer to hierarchy-like modes of organization in cases where reputational capital is the main quality assurance device, whereas market-like governance is more prevalent in cases with public certification.

Quality assurance devices for dynamic conformal radiotherapy

Two quality control devices, a light‐field device and a radiation‐field device, have been specially designed to assist the clinical implementation of conformal dynamic arc treatment (CDAT) and intensity‐modulated radiation therapy (IMRT). With these devices, the light field as well as the radiation field projected from the individual beams at any treatment position (i.e., arbitrary gantry angle) can be evaluated. The devices are attached at the front end of the couch and placed at the isocenter of the linear accelerator treatment system (LINAC). The devices are designed to be able to rotate parallel to the gantry head so that the light field and the radiation field projected from a direct beam can be assessed. The aim of this study was to evaluate the geometric precision of the beam placement and the dosimetric accuracy performed in CDAT and IMRT with the aid of these devices. The devices are placed separately from the LINAC during use and provide an independent check on the quality performance of the LINAC in three dimensions. The condition of gantry sagging and any mechanical displacement resulting in field shift can be observed and traced during gantry rotation. Mistakes that occur during the isocenter calibration can lead to significant displacement in the field projection, which would not be revealed with the conventional quality control setting (i.e., gantry 0°). This was demonstrated with the aid of the two quality control devices in the study. The influence of gravitational acceleration in the multi‐leaf collimator (MLC) leaf positioning error, which would consequently lead to inaccurate dose delivery, was investigated. The results of our study show that the existence of a gravitational influence is statistically significant, although the magnitude of the dose inaccuracy is small.PACS numbers: 87.53.Kn, 87.56.Fc, 87.66.Cd

Quality assurance devices for dynamic conformal radiotherapy

Two quality control devices, a light-field device and a radiation-field device, have been specially designed to assist the clinical implementation of conformal dynamic arc treatment (CDAT) and intensity-modulated radiation therapy (IMRT). With these devices, the light field as well as the radiation field projected from the individual beams at any treatment position (i.e., arbitrary gantry angle) can be evaluated. The devices are attached at the front end of the couch and placed at the isocenter of the linear accelerator treatment system (LINAC). The devices are designed to be able to rotate parallel to the gantry head so that the light field and the radiation field projected from a direct beam can be assessed. The aim of this study was to evaluate the geometric precision of the beam placement and the dosimetric accuracy performed in CDAT and IMRT with the aid of these devices. The devices are placed separately from the LINAC during use and provide an independent check on the quality performance of the LINAC in three dimensions. The condition of gantry sagging and any mechanical displacement resulting in field shift can be observed and traced during gantry rotation. Mistakes that occur during the isocenter calibration can lead to significant displacement in the field projection, which would not be revealed with the conventional quality control setting (i.e., gantry 0°). This was demonstrated with the aid of the two quality control devices in the study. The influence of gravitational acceleration in the multi-leaf collimator (MLC) leaf positioning error, which would consequently lead to inaccurate dose delivery, was investigated. The results of our study show that the existence of a gravitational influence is statistically significant, although the magnitude of the dose inaccuracy is small. PACS numbers: 87.53.Kn, 87.56.Fc, 87.66.Cd

Quality assurance devices for dynamic conformal radiotherapy

Two quality control devices, a light-field device and a radiation-field device, have been specially designed to assist the clinical implementation of conformal dynamic arc treatment (CDAT) and intensity-modulated radiation therapy (IMRT). With these devices, the light field as well as the radiation field projected from the individual beams at any treatment position (i.e., arbitrary gantry angle) can be evaluated. The devices are attached at the front end of the couch and placed at the isocenter of the linear accelerator treatment system (LINAC). The devices are designed to be able to rotate parallel to the gantry head so that the light field and the radiation field projected from a direct beam can be assessed. The aim of this study was to evaluate the geometric precision of the beam placement and the dosimetric accuracy performed in CDAT and IMRT with the aid of these devices. The devices are placed separately from the LINAC during use and provide an independent check on the quality performance of the LINAC in three dimensions. The condition of gantry sagging and any mechanical displacement resulting in field shift can be observed and traced during gantry rotation. Mistakes that occur during the isocenter calibration can lead to significant displacement in the field projection, which would not be revealed with the conventional quality control setting (i.e., gantry 0°). This was demonstrated with the aid of the two quality control devices in the study. The influence of gravitational acceleration in the multi-leaf collimator (MLC) leaf positioning error, which would consequently lead to inaccurate dose delivery, was investigated. The results of our study show that the existence of a gravitational influence is statistically significant, although the magnitude of the dose inaccuracy is small. PACS numbers: 87.53.Kn, 87.56.Fc, 87.66.Cd

Evidence of the extreme oxidative state of soybean oil-filled (trilucent) breast implants raises concerns

To the Editor: About 9000 Trilucent soybean oil–filled breast implants (SOBIs) (LipoMatrix Inc, Neuchâtel, Switzerland) were implanted in about 5000 patients in Britain and 700 in Switzerland. In the United States, about 200 women were enrolled in a clinical study approved by the Food and Drug Administration to evaluate the safety and effectiveness of these implants. Reports indicating a high rupture rate of SOBIs and possible long-term adverse effects resulting from chemical breakdown of the filler prompted the British Medical Devices Agency (MDA) to advise against the further use of SOBIs in March 1999. In June 2000, after new data regarding high concentrations of potentially genotoxic and carcinogenic components from the breakdown of soybean oil filler, the MDA issued a hazard notice recommending that all women with SOBIs have them removed. This recommendation was adopted by the European Committee on Quality Assurance and Medical Devices in Plastic …