POSTAL TLD AUDIT OF HETEROGENEITY CORRECTIONS IN RADIOTHERAPY IN THE CZECH REPUBLIC

In the Czech Republic, a more advanced version of postal audit in radiotherapy (RT) is available. It covers dose measurements with thermoluminescent dosemeters (TLD) in more complex conditions of irradiation, when dose distribution is affected by heterogeneities in the irradiated volume. Relative deviation between doses measured with TLDs and doses stated by RT centre should not exceed 3%. During 2015-2017, all Czech RT centres equipped with modern linear accelerators were subjected to this more advanced TLD audit. A total of  70% of participants complied with the limit of 3% in the first round of this audit.

ESTIMATION OF THE ANISOTROPY EMISSION OF LPN/CIEMAT NEUTRON SOURCES: EFFECT OF HEAVY CAPSULE HOLDERS

In order to estimate the anisotropy emission of 241Am-Be and 252Cf neutron sources from the Spanish Neutron Standards Laboratory (LPN/CIEMAT) detailed models of sources capsules and capsule holders were designed with the MCNPX code. Simulations of the sources inside the capsules without the capsule holders were done to validate the MC model by comparison with experimental results provided by other authors. After that, the capsule holders were incorporated to the simulation. In general, a good agreement has been found between measurements and our calculations. Results show the additional encapsulations have significant influence on anisotropy factors, energy spectra and dose rates.

Feasibility of novel in vivo EPID dosimetry system for linear accelerator quality control tests

The main aim was to validate the capability of a novel EPID-based in vivo dosimetry system for machine-specific quality control (QC) tests. In current study, two sets of measurements were performed in Elekta Versa HD linear accelerator using novel iViewDose™ in vivo dosimetry software. In the first part, measurements were carried out to evaluate the feasibility of novel in vivo system for daily dosimetric QC tests including output constancy, percentage depth dose (PDD) and beam profile measurements. In addition to daily QC tests, measured output factor as a function of field size, leaf transmission and tongue and groove effect were compared with calculated TPS data. In the second part of the measurements, detection capability of iViewDose software for basic mechanical QC tests were investigated for different setup conditions. In dosimetric QC tests, measured output factor with changing field size, PDD, beam profile and leaf transmission factors were found to be compatible with calculated TPS data. Additionally, the EPID-based system was capable to detect given dose calibration errors of 1% with ± 0.02% deviation. In mechanical QC tests, it was found that iViewDose software was sensitive for catching errors in collimator rotation (≥ 1°), changes in phantom thickness (≥ 1 cm) and major differences in irradiated field size down to 1 mm. In conclusion, iViewDose was proved to be as useful EPID-based software for daily monitoring of linear accelerator beam parameters and it provides extra safety net to prevent machine based radiation incidents.

Do all the linear accelerators comply with the ICRU 91's constraints for stereotactic body radiation therapy treatments?

Recent technological developments in linear accelerators (linacs) and their imaging systems have made it possible to routinely perform stereotactic radiotherapy (SRT) treatments. To ensure the security and quality of the treatments, national and international recommendations have been written. This review focuses on the recommendations of the report 91 of the International Commission on Radiation Units (ICRU) on stereotactic treatments with small photon beams and proposes to answer the question of the eligibility of the commercially available accelerators for the treatment of extra-cranial SRT (SBRT). The ICRU 91 report outlines important features needed to respect the constraints, which are high intensity photon beam, integrated image-guidance, high mechanical accuracy of the linac, multileaf collimator with reduced leaf width, bundled motion management and bundled 6 Dimensional "robotic" couch tabletop. Most of the contemporary linacs meet these recommendations, in particular, stereotactic dedicated linacs, or modern gantry-based linacs equipped with 3 dimensional cone-beam CT imaging and 2D-stereoscopic planar imaging. Commercially available ring-based linacs have some limitations: they offer only coplanar treatments, and couch movements are limited to translations and, some have limited imaging equipment and no ability to manage intrafraction motion. However, for performing SBRT, non-coplanar irradiations are not mandatory, contrarily to intracranial stereotactic irradiations. Furthermore, patients' rotations can be corrected, thanks to real-time adaptive radiotherapy available on MRI-linacs. Finally, significant improvements are expected in the short term to compensate the weaknesses of the current devices.

An Alternative Approach to Estimating Instrument Decision Thresholds for Clearance of Personal Property From Accelerator Facilities

Personal property exposed to particle beams and stray radiation at high-energy particle accelerators may contain induced volumetric radioactivity. At both the Brookhaven National Laboratory's (BNL) Collider-Accelerator and National Synchrotron Light Source II Facilities this radioactivity contains both gamma-emitting and beta-emitting radionuclides. The US Department of Energy (US DOE) recently published Technical Standard 6004-2016. This standard provides radionuclide-specific volumetric screening levels (Bq g) below which accelerator materials are eligible for clearance and release from radiological control. The standard also establishes several approaches for decision-making relative to the clearance process implemented, one of which is the "Indistinguishable from Background" (IFB) approach. BNL implements the IFB approach for survey of potentially activated accelerator materials. Radiological control technicians perform on-contact measurements using portable scintillators that are sensitive to gamma and x-ray radiation. Instrument decision thresholds are usually estimated by measuring total background counts over a pre-determined counting interval using an integrating count-rate instrument and then applying an appropriate confidence level factor. Measurement results obtained in a low background area are then directly compared to these detection thresholds. This paper presents an alternative statistical approach using logistic regression for estimating instrument decision thresholds for small-mass items using the IFB method and compares them to established release criteria. On-contact Micro-R meter measurements are correlated with analytical data obtained for activated materials weighing 0.3 kg to 3 kg. Analytical sample results show that Co and Na accounted for more than 90% of total sample radioactivity. Co and Na emit high-energy gamma rays and are both group one radionuclides as defined in DOE-6004-2016. For this size material the results show that the probability of detecting residual volumetric radioactivity at the Group One screening level concentration of 0.11 Bq g under normal field conditions is about 68%. This increases to 95% at 0.16 Bq g. The 95% confidence interval is 0.09 Bq g to 0.23 Bq g. Grouping low-mass items during the survey process could mitigate this concern if all items are expected to have similar activity concentrations.

Benchmarking of Monte Carlo model of Siemens Oncor® linear accelerator for 18MV photon beam: Determination of initial electron beam parameters

OBJECTIVE

This study aims to benchmark a Monte Carlo (MC) model of the 18 MV photon beam produced by the Siemens Oncor® linac using the BEAMnrc and DOSXYZnrc codes.

METHODS

By matching the percentage depth doses and beam profiles calculated by MC simulations with measurements, the initial electron beam parameters including electron energy, full width at half maximum (spatial FWHM), and mean angular spread were derived for the 10×10 cm2 and 20×20 cm2 field sizes. The MC model of the 18 MV photon beam was then validated against the measurements for different field sizes (5×5, 30×30 and 40×40 cm2) by gamma index analysis.

RESULTS

The optimum values for electron energy, spatial FWHM and mean angular spread were 14.2 MeV, 0.08 cm and 0.8 degree, respectively. The MC simulations yielded the comparable measurement results of these optimum parameters. The gamma passing rates (with acceptance criteria of 1% /1 mm) for percentage depth doses were found to be 100% for all field sizes. For cross-line profiles, the gamma passing rates were 100%, 97%, 95%, 96% and 95% for 5×5, 10×10, 20×20, 30×30 and 40×40 cm2 field sizes, respectively.

CONCLUSIONS

By validation of the MC model of Siemens Oncor® linac using various field sizes, it was found that both dose profiles of small and large field sizes were very sensitive to the changes in spatial FWHM and mean angular spread of the primary electron beam from the bending magnet. Hence, it is recommended that both small and large field sizes of the 18 MV photon beams should be considered in the Monte Carlo linac modeling.

Development of a dedicated phantom for multi-target single-isocentre stereotactic radiosurgery end to end testing

PURPOSE

The aim of this project was to design and manufacture a cost-effective end-to-end (E2E) phantom for quantifying the geometric and dosimetric accuracy of a linear accelerator based, multi-target single-isocenter (MTSI) frameless stereotactic radiosurgery (SRS) technique.

METHOD

A perspex Multi-Plug device from a Sun Nuclear ArcCheck phantom (Sun Nuclear, Melbourne, FL) was enhanced to make it more applicable for MTSI SRS E2E testing. The following steps in the SRS chain were then analysed using the phantom: magnetic resonance imaging (MRI) distortion, planning computed tomography (CT) scan and MRI image registration accuracy, phantom setup accuracy using CBCT, dosimetric accuracy using ion chamber, planar film dose measurements and coincidence of linear accelerator mega-voltage (MV), and kilo-voltage (kV) isocenters using Winston-Lutz testing (WLT).

RESULTS

The dedicated E2E phantom was able to successfully quantify the geometric and dosimetric accuracy of the MTSI SRS technique. MRI distortions were less than 0.5 mm, or half a voxel size. The average MRI-CT registration accuracy was 0.15 mm (±0.31 mm), 0.20 mm (±0.16 mm), and 0.39 mm (±0.11 mm) in the superior/inferior, left/right and, anterior/posterior directions, respectively. The phantom setup accuracy using CBCT was better than 0.2 mm and 0.1°. Point dose measurements were within 5% of the treatment planning system predicted dose. The comparison of planar film doses to the planning system dose distributions, performed using gamma analysis, resulted in pass rates greater than 97% for 3%/1 mm gamma criteria. Finally, off-axis WLT showed MV/kV coincidence to be within 1 mm for off-axis distances up to 60 mm.

CONCLUSION

A novel, versatile and cost-effective phantom for comprehensive E2E testing of MTSI SRS treatments was developed, incorporating multiple detector types and fiducial markers. The phantom is capable of quantifying the accuracy of each step in the MTSI SRS planning and treatment process.

[In-machine Fault Monitoring Mechanism and Maintainability Improvement of Varian Linear Accelerator]

Through repaired the RF driver momentary oscillation stop of Varian 2300CD linear accelerator, systematically and comprehensively expounds the three state machine mode of control system and the in-machine fault monitoring mechanism involved in maintainability of Varian high energy accelerator. It proposes an improved solution to bring RF driver output into interlock system, by doing so it can avoid the control computer breakdown and improve maintainability.

A management method for the statistical results of patient-specific quality assurance for intensity-modulated radiation therapy

There are many reports concerning patient-specific quality assurance (QA) for intensity-modulated radiation therapy (IMRT). However, reports about the statistical results of QA are lacking. Management methods for the results of the QA are needed, even though we have the ESTRO group recommendation that a tolerance limit of 1.96 standard deviation (SD) be established in each institution. The purpose of this study was to establish a management method for determining the tolerance limit and to report the statistical results of patient-specific QA. From April 2006 to March 2015, five linacs in the National Cancer Center, Tokyo, Japan, were used to treat 1185 patients with IMRT. Patient-specific QA was performed using an ion chamber, films, and some detectors. To establish a management method for the results, differences between the measured and calculated doses in the ion chamber were analyzed for each linac, each phantom, and each treatment site. The overall mean dose difference was 0.5 ± 1.3%, and the mean dose difference in each linac was 0.6 ± 1.2%, 0.9 ± 1.3%, -0.4 ± 1.4%, -0.1 ± 1.2% and -0.1 ± 0.9%. The difference between linacs and between treatment sites was significant (P < 0.001 and 0.01, respectively). The proportion of the dose difference within ±3% was 97.7%, and that was improved from 2006 to 2014. The results of the patient-specific QA should be managed for each linac and each treatment site in order to decide the suitable tolerance limit. Reports of statistical results will be helped if a new tolerance limit and action level will be considered.

Quantitative Approach to Failure Mode and Effect Analysis for Linear Accelerator Quality Assurance

PURPOSE

To determine clinic-specific linear accelerator quality assurance (QA) TG-142 test frequencies, to maximize physicist time efficiency and patient treatment quality.

METHODS AND MATERIALS

A novel quantitative approach to failure mode and effect analysis is proposed. Nine linear accelerator-years of QA records provided data on failure occurrence rates. The severity of test failure was modeled by introducing corresponding errors into head and neck intensity modulated radiation therapy treatment plans. The relative risk of daily linear accelerator QA was calculated as a function of frequency of test performance.

RESULTS

Although the failure severity was greatest for daily imaging QA (imaging vs treatment isocenter and imaging positioning/repositioning), the failure occurrence rate was greatest for output and laser testing. The composite ranking results suggest that performing output and lasers tests daily, imaging versus treatment isocenter and imaging positioning/repositioning tests weekly, and optical distance indicator and jaws versus light field tests biweekly would be acceptable for non-stereotactic radiosurgery/stereotactic body radiation therapy linear accelerators.

CONCLUSIONS

Failure mode and effect analysis is a useful tool to determine the relative importance of QA tests from TG-142. Because there are practical time limitations on how many QA tests can be performed, this analysis highlights which tests are the most important and suggests the frequency of testing based on each test's risk priority number.

Establishment of 6- to 7-MeV high-energy gamma-ray calibration fields produced using the 4-MV Van de Graaff accelerator at the Facility of Radiation Standards, Japan Atomic Energy Agency

A 6- to 7-MeV high-energy gamma-ray field, produced by the nuclear reaction of (19)F(p, αγ)(16)O, has been established at the Facility of Radiation Standards (FRS) in Japan Atomic Energy Agency for calibration purposes. Basic dosimetric quantities (i.e. averaged gamma-ray energy, air-kerma-to-dose equivalent conversion coefficients and air kerma rates at the point of test) have been precisely determined through a series of measurements using the NaI(Tl) spectrometer and an ionisation chamber coupled with an appropriate build-up material. The measurements obtained comply with values recommended by the International Organization for Standardization for an 'R-F field'. The neutron contamination component for the field has also been measured by means of a conventional neutron dose equivalent meter (the so-called neutron rem-counter) and determined to be ∼ 0.5 % of the total dose equivalent.

Optimisation of X-ray emission from a laser plasma source for the realisation of microbeam in sub-keV region

In this work, the X-ray emission generated from a plasma produced by focusing Nd-YAG laser beam on the Mylar and Yttrium targets will be characterised. The goal is to reach the best condition that optimises the X-ray conversion efficiency at 500 eV (pre-edge of the Oxigen K-shell), strongly absorbed by carbon-based structures. The characteristics of the microbeam optical system, the software/hardware control and the preliminary measurements of the X-ray fluence will be presented.

Comparison of the NMIJ and the ARPANSA standards for absorbed dose to water in high-energy photon beams

The authors report the results of an indirect comparison of the standards of absorbed dose to water in high-energy photon beams from a clinical linac and (60)Co radiation beam performed between the National Metrology Institute of Japan (NMIJ) and the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA). Three ionisation chambers were calibrated by the NMIJ in April and June 2013 and by the ARPANSA in May 2013. The average ratios of the calibration coefficients for the three ionisation chambers obtained by the NMIJ to those obtained by the ARPANSA were 0.9994, 1.0040 and 1.0045 for 6-, 10- and 15-MV (18 MV at the ARPANSA) high-energy photon beams, respectively. The relative standard uncertainty of the value was 7.2 × 10(-3). The ratio for (60)Co radiation was 0.9986(66), which is consistent with the results published in the key comparison of BIPM.RI(I)-K4.

Quality control of VMAT synchronization using portal imaging

For accurate delivery of volumetric-modulated arc therapy (VMAT), the gantry position should be synchronized with the multileaf collimator (MLC) leaf positions and the dose rate. This study, therefore, aims to implement quality control (QC) of VMAT synchronization, with as few arcs as possible and with minimal data handling time, using portal imaging. A steel bar of diameter 12 mm is accurately positioned in the G-T direction, 80 mm laterally from the isocenter. An arc prescription irradiates the bar with a 16 mm × 220 mm field during a complete 360° arc, so as to cast a shadow of the bar onto the portal imager. This results in a sinusoidal sweep of the field and shadow across the portal imager and back. The method is evaluated by simulating gantry position errors of 1°-9° at one control point, dose errors of 2 monitor units to 20 monitor units (MU) at one control point (0.3%-3% overall), and MLC leaf position errors of 1 mm - 6 mm at one control point. Inhomogeneity metrics are defined to characterize the synchronization of all leaves and of individual leaves with respect to the complete set. Typical behavior is also investigated for three models of accelerator. In the absence of simulated errors, the integrated images show uniformity, and with simulated delivery errors, irregular patterns appear. The inhomogeneity metrics increase by 67% due to a 4° gantry position error, 33% due to an 8 MU (1.25%) dose error, and 70% due to a 2 mm MLC leaf position error. The method is more sensitive to errors at gantry angle 90°/270° than at 0°/180° due to the geometry of the test. This method provides fast and effective VMAT QC suitable for inclusion in a monthly accelerator QC program. The test is able to detect errors in the delivery of individual control points, with the possibility of using movie images to further investigate suspicious image features.

Development and reproducibility evaluation of a Monte Carlo-based standard LINAC model for quality assurance of multi-institutional clinical trials

Technical developments in radiotherapy (RT) have created a need for systematic quality assurance (QA) to ensure that clinical institutions deliver prescribed radiation doses consistent with the requirements of clinical protocols. For QA, an ideal dose verification system should be independent of the treatment-planning system (TPS). This paper describes the development and reproducibility evaluation of a Monte Carlo (MC)-based standard LINAC model as a preliminary requirement for independent verification of dose distributions. The BEAMnrc MC code is used for characterization of the 6-, 10- and 15-MV photon beams for a wide range of field sizes. The modeling of the LINAC head components is based on the specifications provided by the manufacturer. MC dose distributions are tuned to match Varian Golden Beam Data (GBD). For reproducibility evaluation, calculated beam data is compared with beam data measured at individual institutions. For all energies and field sizes, the MC and GBD agreed to within 1.0% for percentage depth doses (PDDs), 1.5% for beam profiles and 1.2% for total scatter factors (Scps.). Reproducibility evaluation showed that the maximum average local differences were 1.3% and 2.5% for PDDs and beam profiles, respectively. MC and institutions' mean Scps agreed to within 2.0%. An MC-based standard LINAC model developed to independently verify dose distributions for QA of multi-institutional clinical trials and routine clinical practice has proven to be highly accurate and reproducible and can thus help ensure that prescribed doses delivered are consistent with the requirements of clinical protocols.

An analysis of radiation therapy medical events in New York State: the role of the state radiation programs in patient safety

From 2001 through 2009, the New York State Department of Health (NYSDOH) has documented 244 reports of radiation therapy events, of which 228 have resulted from the delivery of radiation beam therapy using linear accelerators (LINACs). Historically, radiation therapy events involving LINACs have not been uniformly reported across the country because LINACs are regulated by state radiation control programs, and reporting requirements vary among states. The Nuclear Regulatory Commission's Nuclear Material Events Database (NMED) only tracks events involving radioactive materials (RAM). Efforts to track medical events involving LINACs at a national level have begun only recently. This article highlights the importance of tracking and analyzing all medical radiation events in order to improve quality of care and patient safety. An analysis of a subset of the data collected by the NYSDOH from 2001-2009 is presented. This subset consists of only events arising from the use of LINACs in radiation therapy. There are very few publications on errors and error rates in the use of medical accelerators in radiation therapy. This analysis highlights the most common types of errors, causes and contributing factors, areas for improvement and actions taken to bring this information to the regulated community. An error rate of 0.07% per patient receiving radiation treatment is estimated using these data and the New York State Tumor Registry data for the same period. NY State Regulations governing the practice of Radiation Oncology have been revised recently to reflect the increased complexity in the delivery of therapeutic radiation. Collaboration and sharing of data such as those presented here, between federal, state and local regulators, professional organizations such as the Conference of Radiation Control Program Directors (CRCPD), American Society for Radiation Oncology (ASTRO), American Association of Physicists in Medicine (AAPM), American College of Radiology (ACR), American College of Radiation Oncology (ACRO), manufacturers of medical radiation equipment and software developers and the regulated community has begun and will contribute to improved quality of care and patient safety.

An improved method for calibrating the gantry angles of linear accelerators

Linear particle accelerators (linacs) are widely used in radiotherapy procedures; therefore, accurate calibrations of gantry angles must be performed to prevent the exposure of healthy tissue to excessive radiation. One of the common methods for calibrating these angles is the spirit level method. In this study, a new technique for calibrating the gantry angle of a linear accelerator was examined. A cubic phantom was constructed of Styrofoam with small lead balls, embedded at specific locations in this foam block. Several x-ray images were taken of this phantom at various gantry angles using an electronic portal imaging device on the linac. The deviation of the gantry angles were determined by analyzing the images using a customized computer program written in ImageJ (National Institutes of Health). Gantry angles of 0, 90, 180, and 270 degrees were chosen and the results of both calibration methods were compared for each of these angles. The results revealed that the image method was more precise than the spirit level method. For the image method, the average of the measured values for the selected angles of 0, 90, 180, and 270 degrees were found to be -0.086 ± 0.011, 90.018 ± 0.011, 180.178 ± 0.015, and 269.972 ± 0.006 degrees, respectively. The corresponding average values using the spirit level method were 0.2 ± 0.03, 90.2 ± 0.04, 180.1 ± 0.01, and 269.9 ± 0.05 degrees, respectively. Based on these findings, the new method was shown to be a reliable technique for calibrating the gantry angle.

Application of the National Ignition Facility distinguishable-from-background program to accelerator facilities at Lawrence Livermore National Laboratory

Lawrence Livermore National Laboratory must control potentially activated materials and equipment in accordance with U.S. Department of Energy (DOE) Order 458.1, Radiation Protection of the Public and the Environment, which requires DOE approval of the process used to release volumetrically contaminated personal property and establishes a dose constraint of 10 µSv y(-1) (1 mrem y(-1)) for clearance of such property. The National Ignition Facility at Lawrence Livermore National Laboratory developed a technical basis document and protocol for determining the radiological status of property that is potentially activated from exposure to neutron radiation produced via fusion of tritium and deuterium. The technical basis included assessment of the neutron energy, the type of materials potentially exposed and the likely activation products, and the sensitivity of radiation detectors used to survey the property. This paper evaluates the National Ignition Facility technical basis document for applicability to the release of property from Lawrence Livermore National Laboratory's various accelerator facilities considering the different types of particles accelerated, radiations produced, and resultant activation products. Extensive process knowledge regarding the accelerators' operations, accompanied by years of routine surveys, provides an excellent characterization of these facilities. Activation studies conducted at the Stanford Linear Accelerator and the High Energy Accelerator Research Organization in Japan corroborate that the long-lived radionuclides produced at accelerator facilities are of the same variety produced at the National Ignition Facility. Consequently, Lawrence Livermore National Laboratory concludes that the release protocol developed for the National Ignition Facility can be used appropriately at all its accelerator facilities.

Impact of machines on plan quality: volumetric modulated arc therapy and intensity modulated radiation therapy

PURPOSE

To evaluate the impact of different machines on plan quality using both intensity modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) techniques.

MATERIALS AND METHODS

Eight patients with squamous cell carcinoma of the oropharynx were selected at random. Plans were computed for IMRT and VMAT Smart Arc, using Pinnacle TPS for an Elekta (IMRT-E, VMAT-E) and Varian linac (IMRT-V, VMAT-V). A three-dose level prescription was used to deliver 70, 63 and 58.1 Gy to regions of macroscopic, microscopic high- and low-risk disease, respectively. All doses were given in 35 fractions. Comparisons were performed on dose-volume histogram data, monitor units (MU), and delivery time.

RESULTS

VMAT-E plans resulted slightly MU efficient (-24 % p < 0.05) compared to VMAT-V while IMRT-V shortened delivery time (-19 % p < 0.05) compared to IMRT-E. All the delivery techniques resulted in equivalent target coverage in terms of D(98) % and D(2) %. For VMAT technique, a significant improvement of 7 % in homogeneity index (HI) for PTV58.1 was observed for Varian machine. A slight improvement in OARs sparing was observed with Elekta machine both for IMRT and VMAT techniques.

CONCLUSION

Similar plan quality was observed for Elekta and Varian linacs, significant differences were observed in delivery efficiency, as MU number and delivery times, in favor of Elekta and Varian, respectively.

On the effect of an error in a standard D2O-moderated 252Cf energy spectrum

There appears to be an error in the neutron fluence for neutrons with energies between 9 and 10 MeV for the tabulated D2O-moderated Cf source in ISO 8529-1. If the referenced spectrum is used as tabulated, the error contributes a total error to neutron dose values from this source of approximately 3%.

Investigation of the neutron contribution in the 6 MeV to 7 MeV high energy photon reference field

In order to provide reference fields for the ionising radiation, PTB operates the ion accelerator facility. Referring to high energy photons, reference fields according to International Organization for Standardization 4037 series are produced. The neutron component of the 6-7 MeV photon field (R-F), which is produced by bombarding a CaF(2) target with protons with an energy of E(p) = 2.7 MeV, is investigated in detail for the first time. Two discriminative methods are used to determine the yield for neutrons produced in the CaF(2) target.

Neutron dose per fluence and weighting factors for use at high energy accelerators

In June 2007, the United States Department of Energy incorporated revised values of neutron weighting factors into its occupational radiation protection regulation Title 10, Code of Federal Regulations Part 835, as part of updating its radiation dosimetry system. This has led to a reassessment of neutron radiation fields at high energy accelerators such as those at the Fermi National Accelerator Laboratory (Fermilab) in the context of the amended regulation and contemporary guidance of the International Commission on Radiological Protection (ICRP). Values of dose per fluence factors appropriate for accelerator radiation fields calculated elsewhere are collated and radiation weighting factors compared. The results of this revision to the dosimetric system are applied to americium-beryllium neutron energy spectra commonly used for instrument calibrations. Also, a set of typical accelerator neutron energy spectra previously measured at Fermilab are reassessed in light of the new dosimetry system. The implications of this revision and of recent ICRP publications are found to be of moderate significance.

New technologies in radiation therapy: ensuring patient safety, radiation safety and regulatory issues in radiation oncology

New technologies such as intensity modulated and image guided radiation therapy, computer controlled linear accelerators, record and verify systems, electronic charts, and digital imaging have revolutionized radiation therapy over the past 10-15 y. Quality assurance (QA) as historically practiced and as recommended in reports such as American Association of Physicists in Medicine Task Groups 40 and 53 needs to be updated to address the increasing complexity and computerization of radiotherapy equipment, and the increased quantity of data defining a treatment plan and treatment delivery. While new technology has reduced the probability of many types of medical events, seeing new types of errors caused by improper use of new technology, communication failures between computers, corrupted or erroneous computer data files, and "software bugs" are now being seen. The increased use of computed tomography, magnetic resonance, and positron emission tomography imaging has become routine for many types of radiotherapy treatment planning, and QA for imaging modalities is beyond the expertise of most radiotherapy physicists. Errors in radiotherapy rarely result solely from hardware failures. More commonly they are a combination of computer and human errors. The increased use of radiosurgery, hypofractionation, more complex intensity modulated treatment plans, image guided radiation therapy, and increasing financial pressures to treat more patients in less time will continue to fuel this reliance on high technology and complex computer software. Clinical practitioners and regulatory agencies are beginning to realize that QA for new technologies is a major challenge and poses dangers different in nature than what are historically familiar.

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.

The impact of advanced technologies on treatment deviations in radiation treatment delivery

PURPOSE

To assess the impact of new technologies on deviation rates in radiation therapy (RT).

METHODS AND MATERIALS

Treatment delivery deviations in RT were prospectively monitored during a time of technology upgrade. In January 2003, our department had three accelerators, none with "modern" technologies (e.g., without multileaf collimators [MLC]). In 2003 to 2004, we upgraded to five new accelerators, four with MLC, and associated advanced capabilities. The deviation rates among patients treated on "high-technology" versus "low-technology" machines (defined as those with vs. without MLC) were compared over time using the two-tailed Fisher's exact test.

RESULTS

In 2003, there was no significant difference between the deviation rate in the "high-technology" versus "low-technology" groups (0.16% vs. 0.11%, p = 0.45). In 2005 to 2006, the deviation rate for the "high-technology" groups was lower than the "low-technology" (0.083% vs. 0.21%, p = 0.009). This difference was caused by a decline in deviations on the "high-technology" machines over time (p = 0.053), as well as an unexpected trend toward an increase in deviations over time on the "low-technology" machines (p = 0.15).

CONCLUSIONS

Advances in RT delivery systems appear to reduce the rate of treatment deviations. Deviation rates on "high-technology" machines with MLC decline over time, suggesting a learning curve after the introduction of new technologies. Associated with the adoption of "high-technology" was an unexpected increase in the deviation rate with "low-technology" approaches, which may reflect an over-reliance on tools inherent to "high-technology" machines. With the introduction of new technologies, continued diligence is needed to ensure that staff remain proficient with "low-technology" approaches.

Assessment of pre-operational radiological conditions at the Linac Centre of the Lagos University Teaching Hospital, Nigeria

BACKGROUND

The Lagos University Teaching Hospital, Nigeria has acquired a 6 - 15 MeV ELEKTA linear accelerator for clinical teletherapy. The 15 MeV is a sufficient energy to activate photonuclear interactions of the type (h(nu), n) and (h(nu), e), in some materials found within the locality, thereby enhancing the background radiation exposure levels in the environments.

OBJECTIVE

The objective of this study is to conduct a pre-operational assessment of the radiological conditions of the new LINAC Centre and its environments which is required for future uses in the assessment of the environmental impacts due to the applications of the facility. The results of this study are to provide a baseline data needed for such assessments and the impact of eventual LINAC accidents.

METHODS

Radiation exposure levels due to natural background radioactivity in the controlled and supervised areas and offices in the LINAC localities have been measured. The environmental radiation survey around the Centre was conducted according to the requirements prescribed by the ICRP, IAEA Basic Safety Standards etc. The specific radioactivity contents of 100 building, furna and flora material samples have been determined using a Gamma Well Scintillation Counter.

RESULTS

The mean exposure rates all over the LINAC Centre varied from 2.0 muSvh(-1) to 3.3 muSvh(-1). In the controlled areas, the mean value was 2.4 muSvh(-1) and it was 2.7 muSvh in the supervised areas, while in the adjacent offices, it was 2.5 muSvh(-1). The means of the measured specific activities in the cement concrete, soil, plant and grass samples were 42.7299 Bqkg(-1), 39.9592 Bqkg(-1), 23.9010 Bqkg(-1) and 20.8940 Bqkg(-1) respectively.

CONCLUSION

A database for the assessment of the environmental impacts of the clinical LINAC and the impacts of its eventual accidents has been generated for future periodic uses.

Commissioning and acceptance testing of a CyberKnife linear accelerator

Acceptance testing and commissioning of a CyberKnife robotic stereotactic radiosurgery system was performed in April 2006. The CyberKnife linear accelerator produces a photon beam of 6 MV nominal energy, without the use of a flattening filter. Clinically measured tissue-phantom ratios, off-center ratios, and output factors are presented and compared with similar data from other CyberKnife sites throughout the United States. In general, these values agreed to within 2%.

Commissioning experience with cone-beam computed tomography for image-guided radiation therapy

This paper reports on the commissioning of an Elekta cone‐beam computed tomography (CT) system at one of the first U.S. sites to install a “regular,” off‐the‐shelf Elekta Synergy (Elekta, Stockholm, Sweden) accelerator system. We present the quality assurance (QA) procedure as a guide for other users. The commissioning had six elements: (1) system safety, (2) geometric accuracy (agreement of megavoltage and kilovoltage beam isocenters), (3) image quality, (4) registration and correction accuracy, (5) dose to patient and dosimetric stability, and (6) QA procedures. The system passed the safety tests, and agreement of the isocenters was found to be within 1 mm. Using a precisely moved skull phantom, the reconstruction and alignment algorithm was found to be accurate within 1 mm and 1 degree in each dimension. Of 12 measurement points spanning a 9×9×15‐cm volume in a Rando phantom (The Phantom Laboratory, Salem, NY), the average agreement in the x, y, and z coordinates was 0.10 mm, −0.12 mm, and 0.22 mm [standard deviations (SDs): 0.21 mm, 0.55 mm, 0.21 mm; largest deviations: 0.6 mm, 1.0 mm, 0.5 mm] respectively. The larger deviation for the y component can be partly attributed to the CT slice thickness of 1 mm in that direction. Dose to the patient depends on the machine settings and patient geometry. To monitor dose consistency, air kerma (output) and half‐value layer (beam quality) are measured for a typical clinical setting. Air kerma was 6.3 cGy (120 kVp, 40 mA, 40 ms per frame, 360‐degree scan, S20 field of view); half value layer was 7.1 mm aluminum (120 kV, 40 mA).

We suggest performing items 1, 2, and 3 monthly, and 4 and 5 annually. In addition, we devised a daily QA procedure to verify agreement of the megavoltage and kilovoltage isocenters using a simple phantom containing three small steel balls. The frequency of all checks will be reevaluated based on data collected during about 1 year.

PACS number: 87.53.Xd

Particle therapy in prostate cancer: a review

While dose escalation is proving important to achieve satisfactory long-term outcomes in prostate cancer, the optimal radiation modality to deliver the treatment is still a topic of debate. Charged particle beams can offer improved dose distributions to the target volume as compared to conventional 3D-conformal radiotherapy, with better sparing of surrounding healthy tissues. Exquisite dose distributions, with the fulfillment of dose-volume constraints to normal tissues, however, can also be achieved with photon-based intensity-modulated techniques. This review summarizes the literature on the use of particle therapy in prostate cancer and attempts to put in perspective its relative merits compared to current photon-based radiotherapy.

Influence of monitor chamber calibration on virtual Wedge dosimetry

We have investigated the influence of the linear accelerator (LINAC) monitor chamber calibration on the dosimetry of Siemens Virtual Wedge (VW.) The doses delivered in the three phases of wedge delivery (initial gap, sweep portion, and open field) utilize the ionization current generated in two dose monitoring ion chambers (MONITOR 1 and MONITOR 2) in the LINAC to control the wedge delivery. We intentionally offset the calibration of each of these chambers by +/- 3% and observed up to a 13% change in the dose along the wedge profile for a 6 MV beam at a field size of 20 x 20 cm2. If the calibration of one of the two dose monitoring chambers changed independently then the relative dose at points along the wedge profile were affected. Furthermore, the percentage change in dose varied across the wedge profile thereby affecting the wedge angle as well as the central axis wedge factor. We also present equations for calculating the change in dose at a position along the wedge profile as a function of monitor chamber calibration. A comparison with measurement showed that our theoretical predictions were accurate to within +/- 1.7%. The equations have proven useful tools in evaluating periodic drifts in VW dosimetry.

[Quality control and quality assurance for the isocentre of the medical linear accelerator]

This article expounds the method of quality control and quality assurance for the isocenter of the medical linear accelerator and explains the content and standards of its regular examinations, in order to ensure the safety and efficiency in use.

The matching of wedge transmission factors across six multi-energy linear accelerators

Elekta Precise linear accelerators create a wedged isodose distribution using a single, fixed, motorized wedge with a nominal wedge angle of 60 degrees. Wedge angles of less than 60 degrees can be produced by varying the proportion of open and wedge monitor units for a given exposure. The fixed wedge can be replaced with a mobile wedge, the position of which can be moved in order to adjust the wedge transmission factor (WTF). Using the original fixed wedges installed in our fleet of six Elekta accelerators, we found a range of 4% in measured wedge transmission factor for 6 MV beams. Results are presented which demonstrate that by using the mobile wedge it is possible to match the wedge transmission factors to within 1% for the six linear accelerators over three energies.

Evaluation of the characteristics of the neutron reference field using D2O-moderated 252Cf source

The ambient/personal dose equivalent per fluence for D(2)O moderated (252)Cf neutron source was determined by measurement. An appropriate subtraction of the scattered neutrons is required for the accurate measurement of direct neutrons. A cubic shadow object was used for the subtraction of the scattered neutrons from the surroundings. The scattered neutrons to be subtracted vary with the position of the shadow object due to the large volume of the source. Using the Monte Carlo code MCNP-4C, the optimum positions of the shadow object were surveyed for subtracting the scattered neutrons. The energy spectra of direct neutrons were measured in the optimum position. The dosimetric parameters for the D(2)O moderated (252)Cf neutron source were reasonable, taking into account the uncertainties of the parameters.

Investigation of properties of the TIARA neutron beam facility of importance for calibration applications

Evaluation of the properties for quasi-monoenergetic neutron calibration fields of high energies more than 20 MeV at TIARA is proceeding for development of the field. Among the properties needed for the development as the standard calibration field, we report on measurement of the neutron beam profile using an imaging plate with a polyethylene converter and on estimation of the contribution of scattered neutrons into the irradiation field based on pulse height distribution at various off-beam positions measured using an organic liquid scintillation detector.

Development of the fast neutron standard using a Be({alpha},n) reaction at the National Metrology Institute of Japan

This paper describes the 8-MeV neutron field where the neutrons are generated in the (9)Be(alpha,n)(12)C reaction by bombardment of a beryllium target with a 2.4-MeV (4)He(+) beam from a Van de Graaff accelerator. The neutron field is being prepared for a new national standard on neutron fluence in Japan. Absolute measurement of the neutron fluence was taken using a proton recoil neutron detector, consisting of a silicon surface barrier detector with a polyethylene radiator. Neutron spectra were measured using a newly developed recoil proton spectrometer and a liquid organic scintillation detector. The gamma rays existing in the field were also characterised using a liquid organic scintillation detector. The ambient dose equivalents of the gamma rays were estimated to be <100 microSv at the neutron fluence of 10(7) neutrons cm(-2).

Intensity-modulated radiation therapy and image-guided radiation therapy: small clinic implementation

In a small clinic with a small patient base, the implementation of IMRT/IGRT should be slow, measured, and meticulous. Most radiation oncologists in the United States have had no formal training in IMRT/IGRT because the modalities are so new. Proper patient selection and a team effort among the clinician, physicist, dosimetrist, and therapist are thus all the more critical. The clinician in the small clinic can take comfort in remembering that the technologies are new, but the principles of good radiation medicine are not. With patient selection, a team approach, and publication of data and maturation of the literature, IMRT/IGRT will become the new standard of care in academic centers, large private clinics, and small clinics alike.

An empirical method for the determination of wall perturbation factors for parallel-plate chambers in high-energy electron beams

The calibration of ion chambers in high-energy electron beams in terms of absorbed dose to water at the National Physical Laboratory requires knowledge of the ratio of perturbation factors in graphite and water phantoms. During a review of data required for the NPL calibration procedure an empirical model was developed to calculate the perturbation due to the rear wall, pwall, of a well-guarded ion chamber in a high-energy electron beam. The overall uncertainty in this method is estimated to be 0.4%, which is the lowest value reported to date. The model reproduces measured data at the 0.1% level or better and indicates that the NACP ion chamber has a nonzero perturbation factor in electron beams due to backscatter from the rear wall. The effect is small (<0.5%) at high energies (R50>4 cm, E0>10 MeV) but becomes large at low energies-up to 1.4% at E0=4 MeV (R50=1.2 cm). The model indicates that there is a nonzero correction for the NACP chamber in both a graphite and water phantom and that material adjacent to the air cavity has a significant effect on the measured ionization. These values are consistent with previous measurements and recent Monte Carlo calculations. The model could be used in the design of ion chambers and in the estimation of corrections for non-homogeneous systems, especially in the absence of accurate Monte Carlo simulations.

Intraoperative radiation therapy using mobile electron linear accelerators: report of AAPM Radiation Therapy Committee Task Group No. 72

Intraoperative radiation therapy (IORT) has been customarily performed either in a shielded operating suite located in the operating room (OR) or in a shielded treatment room located within the Department of Radiation Oncology. In both cases, this cancer treatment modality uses stationary linear accelerators. With the development of new technology, mobile linear accelerators have recently become available for IORT. Mobility offers flexibility in treatment location and is leading to a renewed interest in IORT. These mobile accelerator units, which can be transported any day of use to almost any location within a hospital setting, are assembled in a nondedicated environment and used to deliver IORT. Numerous aspects of the design of these new units differ from that of conventional linear accelerators. The scope of this Task Group (TG-72) will focus on items that particularly apply to mobile IORT electron systems. More specifically, the charges to this Task Group are to (i) identify the key differences between stationary and mobile electron linear accelerators used for IORT, (ii) describe and recommend the implementation of an IORT program within the OR environment, (iii) present and discuss radiation protection issues and consequences of working within a nondedicated radiotherapy environment, (iv) describe and recommend the acceptance and machine commissioning of items that are specific to mobile electron linear accelerators, and (v) design and recommend an efficient quality assurance program for mobile systems.

Ion recombination correction in the Clatterbridge Centre of Oncology clinical proton beam

Most codes of practice for dosimetry of proton beams do not give a clear recommendation on the determination of recombination correction factors for ionization chambers. In this work, recombination corrections were measured in the low-energy clinical proton beam of the Clatterbridge Centre of Oncology (CCO) using data collected at different dose rates and different polarizing voltages. This approach allows the separation of contributions from initial and volume recombination and was compared with results from extrapolation and two-voltage methods. A modified formulation of the method is presented for a modulated beam in which the ionization current is time dependent. Using a set-up with two identical chambers placed face-to-face yielded highly accurate data for plane-parallel ionization chambers. This method may also be used for high-energy photon and electron beam dosimetry. At typical dose rates of 26 Gy min(-1) used clinically at the CCO, the recombination correction is 0.8% and thus is of importance for reference dosimetry. The proton beam should be treated as purely continuous given the high pulse repetition frequency of the cyclotron beam. The results show that the volume recombination parameter for protons is consistent with values measured for photon beams. Initial recombination was found to be independent of beam quality, except for a tendency to increase at the distal edge of the Bragg peak; this is only relevant for depth dose measurements. Using a general equation for recombination and generic values for the initial and volume recombination parameters (A = 0.25 V and m2 = 3.97 x 10(3) s cm(-1) nC(-1) V2), the experimental results are reproduced within 0.1% for all conditions met in this work. For the CCO beam and similar proton beams used for treating optical targets operating at high dose rates, the recombination correction factor can be overestimated by up to 2%, resulting in an overestimation of dose to water by the same amount, if the recommendation from IAEA TRS-398, which is only valid for pulsed beams, is followed without consideration.

Infrastructure of radiation oncology in France: a large survey of evolution of external beam radiotherapy practice

PURPOSE

To study the structural characteristics of radiation oncology facilities for France and to examine how technological evolutions had to be taken into account in terms of accessibility and costs. This study was initiated by the three health care financing administrations that cover health care costs for the French population. The needs of the population in terms of the geographic distribution of the facilities were also investigated. The endpoint was to make proposals to enable an evolution of the practice of radiotherapy (RT) in France.

METHODS AND MATERIALS

A survey designed by a multidisciplinary committee was distributed in all RT facilities to collect data on treatment machines, other equipment, personnel, new patients, and new treatments. Medical advisors ensured site visits in each facility. The data were validated at the regional level and aggregated at the national level for analysis.

RESULTS

A total of 357 machines had been installed in 179 facilities: 270 linear accelerators and 87 cobalt units. The distribution of facilities and megavoltage units per million inhabitants over the country was good, although some disparities existed between areas. It appeared that most megavoltage units had not benefited from technological innovation, because 25% of the cobalt units and 57% of the linear accelerators were between 6 and 15 years old. Computed tomography access for treatment preparation was not sufficient, and complete data management systems were scarce (15% of facilities). Seven centers had no treatment planning system. Electronic portal imaging devices were available in 44.7% of RT centers and in vivo dosimetry in 35%. A lack of physicians and medical physicists was observed; consequently, the workload exceeded the normal standard recommended by the French White Book. Discrepancies were found between the number of patients treated per machine per year in each area (range, 244.5-604). Most treatments were delivered in smaller facilities (61.6%).

CONCLUSION

On the basis of the findings of this study, measures were taken to update the infrastructure of RT in France. A first evaluation showed an improvement of care supply in RT in the country.

Commissioning of a medical accelerator photon beam Monte Carlo simulation using wide-field profiles

A method for commissioning an EGSnrc Monte Carlo simulation of medical linac photon beams through wide-field lateral profiles at moderate depth in a water phantom is presented. Although depth-dose profiles are commonly used for nominal energy determination, our study shows that they are quite insensitive to energy changes below 0.3 MeV (0.6 MeV) for a 6 MV (15 MV) photon beam. Also, the depth-dose profile dependence on beam radius adds an additional uncertainty in their use for tuning nominal energy. Simulated 40 cm x 40 cm lateral profiles at 5 cm depth in a water phantom show greater sensitivity to both nominal energy and radius. Beam parameters could be determined by comparing only these curves with measured data.

Proposed definitions for isodose flatness and symmetry in clinical radiotherapy beams

In clinical radiotherapy it is important that beam intensity be as homogeneous as possible to reduce the probability of treatment failure. As an extension of the well established concepts of beam flatness and symmetry for characterizing radiotherapy beams, the concepts of isodose flatness and symmetry are introduced. The definitions are tested with actual data obtained from a Co-60 unit and a linear accelerator and results are presented. The concepts as defined appear to provide a useful quantitative indication of beam homogeneity.

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.

Comparison of electron beam characteristics from multiple accelerators

PURPOSE

To determine the relationships between electron beam depth dose characteristics, depth of maximum dose (d(max)), depth of 80% dose (d(80)), and depth of 50% dose (d(50)), and the nominal energy designation of electron beams from multiple linear accelerators for the purpose of electron beam treatment planning and quality assurance.

METHODS

The Radiological Physics Center Staff, during its on-site dosimetry review visits to institutions participating in clinical trials, measured depth dose characteristics for more than 2000 electron beams. Measurements were performed on Varian, Siemens, and Elekta/Philips accelerators generating beams with nominal energy values ranging from 4-22 MeV. The depth dose data were determined at the nominal source-to-skin distance with the reference cone size in accordance with recommendation of the American Association of Physicists in Medicine Task Group 25 report.

RESULTS

The important depth dose characteristics d(max), d(80), and d(50) varied in a predictable fashion when plotted against the true beam quality indicator, R(50). However, d(80) and d(50) values overlapped considerably when plotted against the manufacturers' nominal electron energy values. For a specific nominal electron energy value, the values of d(max), d(80), and d(50) varied by as little as 3 mm for low energy levels to nearly as much as 20 mm for high energies.

CONCLUSIONS

The manufacturer's nominal electron energy value does not adequately describe the depth dose characteristics of an electron beam for treatment planning purposes. Clinicians and physicists should determine and use only the specific depth dose data for their clinical beams and not the manufacturer's nominal value.