Biomedical subjects

J V Siebers

Publications and source records attributed to J V Siebers.

At least 19 recordsLinked to original sources

PubMed2006

Investigation of patient, tumour and treatment variables affecting residual motion for respiratory-gated radiotherapy.

Respiratory gating can reduce the apparent respiratory motion during imaging and treatment; however, residual motion within the gating window remains. Respiratory training can improve respiratory reproducibility and, therefore, the efficacy of respiratory-gated radiotherapy. This study was conducted to determine whether residual motion during respiratory gating is affected by patient, tumour or treatment characteristics. The specific aims of this study were to: (1) identify significant characteristics affecting residual motion, (2) investigate time trends of residual motion over a period of days (inter-session) and (3) investigate time trends of residual motion within the same day (intra-session). Twenty-four lung cancer patients were enrolled in an Institutional Review Board (IRB)-approved protocol. For approximately five sessions, 331 four-minute, respiratory motion traces were acquired with free breathing, audio instructions and audio-visual biofeedback for each patient. The residual motion was quantified by the standard deviation of the displacement within the gating window. The generalized linear model was used to obtain coefficients for each variable within the model and to evaluate the clinical and statistical significance. The statistical significance was determined by a p-value<0.05, while effect sizes of 0.1 cm (one standard deviation) were considered clinically significant. This data analysis was applied to patient, tumour and treatment variables. Inter- and intra-session variations were also investigated. The only variable that was significant for both inhale- and exhale-based gating was disease type. In addition, visual-training displacement, breathing type and Karnofsky performance status (KPS) values were significant for inhale-based gating, and dose-per-fraction was significant for exhale-based gating. Temporal respiratory variations within and between sessions were observed for individual patients. However inter- and intra-session analyses did not show significant time trends on average for any of the variables considered. The lack of significant time trends within and between sessions indicates that on average (1) there is no significant learning period for breathing training, (2) the patients did not experience training-related fatigue and (3) the margin component to account for residual motion during gated radiotherapy appears to remain constant throughout the treatment.

Computer Simulation↗

PubMed2006

Clinical comparison of head and neck and prostate IMRT plans using absorbed dose to medium and absorbed dose to water.

Conventional photon radiation therapy dose-calculation algorithms typically compute and report the absorbed dose to water (D(w)). Monte Carlo (MC) dose-calculation algorithms, however, generally compute and report the absorbed dose to the material (D(m)). As MC-calculation algorithms are being introduced into routine clinical usage, the question as to whether there is a clinically significant difference between D(w) and D(m) remains. The goal of the current study is to assess the differences between dose-volume indices for D(m) and D(w) MC-calculated IMRT plans. Ten head-and-neck (H&N) and ten prostate cancer patients were selected for this study. MC calculations were performed using an EGS4-based system. Converting D(m) to D(w) for MC-based calculations was accomplished as a post-MC calculation process. D(w) and D(m) results for target and critical structures were evaluated using the dose-volume-based indices. For H&N IMRT plans, systematic differences between dose-volume indices computed with D(w) and D(m) were up to 2.9% for the PTV prescription dose (D(98)), up to 5.8% for maximum (D(2)) dose to the PTV and up to 2.7% for the critical structure dose indices. For prostate IMRT plans, the systematic differences between D(w)- and D(m)-based computed indices were up to 3.5% for the prescription dose (D(98)) to the PTVs, up to 2.0% for the maximum (D(2)) dose to the PTVs and up to 8% for the femoral heads due to their higher water/bone mass stopping power ratio. This study showed that converting D(m) to D(w) in MC-calculated IMRT treatment plans introduces a systematic error in target and critical structure DVHs. In some cases, this systematic error may reach up to 5.8% for H&N and 8.0% for prostate cases when the hard-bone-containing structures such as femoral heads are present. Ignoring differences between D(m) and D(w) will result in systematic dose errors ranging from 0% to 8%.

Absorption↗

PubMed2006

Analytic IMRT dose calculations utilizing Monte Carlo to predict MLC fluence modulation.

A hybrid dose-computation method is designed which accurately accounts for multileaf collimator (MLC)-induced intensity modulation in intensity modulated radiation therapy (IMRT) dose calculations. The method employs Monte Carlo (MC) modeling to determine the fluence modulation caused by the delivery of dynamic or multisegmental (step-and-shoot) MLC fields, and a conventional dose-computation algorithm to estimate the delivered dose to a phantom or a patient. Thus, it determines the IMRT fluence prediction accuracy achievable by analytic methods in the limit that the analytic method includes all details of the MLC leaf transport and scatter. The hybrid method is validated and benchmarked by comparison with in-phantom film dose measurements, as well as dose calculations from two in-house, and two commercial treatment planning system analytic fluence estimation methods. All computation methods utilize the same dose algorithm to calculate dose to a phantom, varying only in the estimation of the MLC modulation of the incident photon energy fluence. Gamma analysis, with respect to measured two-dimensional (2D) dose planes, is used to benchmark each algorithm's performance. The analyzed fields include static and dynamic test patterns, as well as fields from ten DMLC IMRT treatment plans (79 fields) and five SMLC treatment plans (29 fields). The test fields (fully closed MLC, picket fence, sliding windows of different size, and leaf-tip profiles) cover the extremes of MLC usage during IMRT, while the patient fields represent realistic clinical conditions. Of the methods tested, the hybrid method most accurately reproduces measurements. For the hybrid method, 79 of 79 DMLC field calculations have gamma < 1 (3%/3 mm) for more than 95% of the points (per field) while for SMLC fields, 27 of 29 pass the same criteria. The analytic energy fluence estimation methods show inferior pass rates, with 76 of 79 DMLC and 24 of 29 SMLC fields having more than 95% of the test points with gamma < or = 1 (3%/3 mm). Paired one-way ANOVA tests of the gamma analysis results found that the hybrid method better predicts measurements in terms of both the fraction of points with gamma < or = 1 and the average gamma for both 2%/2 mm and 3%/3 mm criteria. These results quantify the enhancement in accuracy in IMRT dose calculations when MC is used to model the MLC field modulation.

Body Burden↗

PubMed2006

A dynamic compensation strategy to correct patient-positioning errors in conformal prostate radiotherapy.

Traditionally, pretreatment detected patient-positioning errors have been corrected by repositioning the couch to align the patient to the treatment beam. We investigated an alternative strategy: aligning the beam to the patient by repositioning the dynamic multileaf collimator and adjusting the beam weights, termed dynamic compensation. The purpose of this study was to determine the geometric range of positioning errors for which the dynamic compensation method is valid in prostate cancer patients treated with three-dimensional conformal radiotherapy. Twenty-five previously treated prostate cancer patients were replanned using a four-field technique to deliver 72 Gy to 95% of the planning target volume (PTV). Patient-positioning errors were introduced by shifting the patient reference frame with respect to the treatment isocenter. Thirty-six randomly selected isotropic displacements with magnitudes of 1.0, 2.0, 4.0, 6.0, 8.0, and 10.0 cm were sampled for each patient, for a total of 5400 errors. Dynamic compensation was used to correct each of these errors by conforming the beam apertures to the new target position and adjusting the monitor units using inverse-square and off-axis factor corrections. The dynamic compensation plans were then compared with the original treatment plans via dose-volume histogram (DVH) analysis. Changes of more than 5% of the prescription dose, 3.6 Gy, were deemed significant. Compared with the original treatment plans, dynamic compensation produced small discrepancies in isodose distributions and DVH analyses. These differences increased with the magnitudes of the initial patient-positioning errors. Coverage of the PTV was excellent: D95 and Dmean were not increased or decreased by more than 5% of the prescription dose, and D5 was not decreased by more than 5% of the prescription dose for any of the 5400 simulated positioning errors. D5 was increased by more than 5% of the prescription dose in only three of the 5400 positioning errors, all three occurring with a positioning error of 10.0 cm. Dose increases for adjacent organs at risk were more common. D33 of the rectum and the periprostatic rectum was increased by more than 5% of the prescription dose in 235 (4.4%) and 212 (3.9%) of the 5400 positioning errors, respectively. D10 of the right femoral head increased by more than 5% of the prescription dose in 444 (8.2%) positioning errors, and the degree of change was highly related to individual patient anatomy and simulation position. For the bladder D20, there were three increases of more than 5% of the prescription dose. These data demonstrate the robustness of dynamic compensation for correction of patient-positioning errors in four-field conformal prostate radiotherapy, with minimal deviation from the original treatment plans even for errors greatly exceeding those commonly encountered in the clinic. Dynamic compensation can be performed remotely, thus eliminating errors that may result from unnecessary increases in treatment time or from secondary patient motion induced by couch motion during the repositioning process. Further, the ability of dynamic compensation to correct large positioning errors has implications for the accuracy necessary during the initial patient setup and, hence, patient throughput for prostate radiotherapy.

Analysis of Variance↗

PubMed2005

A comparison of Monte Carlo dose calculation denoising techniques.

Recent studies have demonstrated that Monte Carlo (MC) denoising techniques can reduce MC radiotherapy dose computation time significantly by preferentially eliminating statistical fluctuations ('noise') through smoothing. In this study, we compare new and previously published approaches to MC denoising, including 3D wavelet threshold denoising with sub-band adaptive thresholding, content adaptive mean-median-hybrid (CAMH) filtering, locally adaptive Savitzky-Golay curve-fitting (LASG), anisotropic diffusion (AD) and an iterative reduction of noise (IRON) method formulated as an optimization problem. Several challenging phantom and computed-tomography-based MC dose distributions with varying levels of noise formed the test set. Denoising effectiveness was measured in three ways: by improvements in the mean-square-error (MSE) with respect to a reference (low noise) dose distribution; by the maximum difference from the reference distribution and by the 'Van Dyk' pass/fail criteria of either adequate agreement with the reference image in low-gradient regions (within 2% in our case) or, in high-gradient regions, a distance-to-agreement-within-2% of less than 2 mm. Results varied significantly based on the dose test case: greater reductions in MSE were observed for the relatively smoother phantom-based dose distribution (up to a factor of 16 for the LASG algorithm); smaller reductions were seen for an intensity modulated radiation therapy (IMRT) head and neck case (typically, factors of 2-4). Although several algorithms reduced statistical noise for all test geometries, the LASG method had the best MSE reduction for three of the four test geometries, and performed the best for the Van Dyk criteria. However, the wavelet thresholding method performed better for the head and neck IMRT geometry and also decreased the maximum error more effectively than LASG. In almost all cases, the evaluated methods provided acceleration of MC results towards statistically more accurate results.

Algorithms↗

PubMed2005

Determination of maximum leaf velocity and acceleration of a dynamic multileaf collimator: implications for 4D radiotherapy.

The dynamic multileaf collimator (MLC) can be used for four-dimensional (4D), or tumor tracking radiotherapy. However, the leaf velocity and acceleration limitations become a crucial factor as the MLC leaves need to respond in near real time to the incoming respiration signal. The aims of this paper are to measure maximum leaf velocity, acceleration, and deceleration to obtain the mechanical response times for the MLC, and determine whether the MLC is suitable for 4D radiotherapy. MLC leaf sequence files, requiring the leaves to reach maximum acceleration and velocity during motion, were written. The leaf positions were recorded every 50 ms, from which the maximum leaf velocity, acceleration, and deceleration were derived. The dependence on the velocity and acceleration of the following variables were studied: leaf banks, inner and outer leaves, MLC-MLC variations, gravity, friction, and the stability of measurements over time. Measurement results show that the two leaf banks of a MLC behave similarly, while the inner and outer leaves have significantly different maximum leaf velocities. The MLC-MLC variations and the dependence of gravity on maximum leaf velocity are statistically significant. The average maximum leaf velocity at the isocenter plane of the MLC ranged from 3.3 to 3.9 cm/s. The acceleration and deceleration at the isocenter plane of the MLC ranged from 50 to 69 cm/s2 and 46 to 52 cm/s2, respectively. Interleaf friction had a negligible effect on the results, and the MLC parameters remained stable with time. Equations of motion were derived to determine the ability of the MLC response to fluoroscopymeasured diaphragm motion. Given the present MLC mechanical characteristics, 4D radiotherapy is feasible for up to 97% of respiratory motion. For the largest respiratory motion velocities observed, beam delivery should be temporarily stopped (beam hold).

Algorithms↗

PubMed2004

Monte Carlo as a four-dimensional radiotherapy treatment-planning tool to account for respiratory motion.

Four-dimensional (4D) radiotherapy is the explicit inclusion of the temporal changes in anatomy during the imaging, planning and delivery of radiotherapy. Temporal anatomic changes can occur for many reasons, though the focus of the current investigation was respiration motion for lung tumours. The aims of the current research were first to develop a 4D Monte Carlo methodology and second to apply this technique to an existing 4D treatment plan. A 4D CT scan consisting of a series of 3D CT image sets acquired at different respiratory phases was used. Deformable image registration was performed to map each CT set from the end-inhale respiration phase to the CT image sets corresponding with subsequent respiration phases. This deformable registration allowed the contours drawn on the end-inhale CT to be automatically drawn on the other respiratory phase CT image sets. A treatment plan was created on the end-inhale CT image set and then automatically created on each of the 3D CT image sets corresponding with subsequent respiration phases, based on the beam arrangement and dose prescription in the end-inhale plan. Dose calculation using Monte Carlo was simultaneously performed on each of the N (=8) 3D image sets with 1/N fewer particles per calculation than for a 3D plan. The dose distribution from each respiratory phase CT image set was mapped back to the end-inhale CT image set for analysis. This use of deformable image registration to merge all the statistically noisy dose distributions back onto one CT image set effectively yielded a 4D Monte Carlo calculation with a statistical uncertainty equivalent to a 3D calculation, with a similar calculation time for the 3D and 4D methods. Monte Carlo as a dose calculation tool for 4D radiotherapy planning has two advantages: (1) higher accuracy for calculation in electronic disequilibrium conditions, such as those encountered during lung radiotherapy, and (2) if deformable image registration is used, the calculation time for Monte Carlo is independent of the number of 3D CT image sets constituting a 4D CT, unlike other algorithms for which the calculation time scales linearly with the number of 3D CT image sets constituting a 4D CT.

Algorithms↗

PubMed2004

On the use of EPID-based implanted marker tracking for 4D radiotherapy.

Four-dimensional (4D) radiotherapy delivery to dynamically moving tumors requires a real-time signal of the tumor position as a function of time so that the radiation beam can continuously track the tumor during the respiration cycle. The aim of this study was to develop and evaluate an electronic portal imaging device (EPID)-based marker-tracking system that can be used for real-time tumor targeting, or 4D radiotherapy. Three gold cylinders, 3 mm in length and 1 mm in diameter, were implanted in a dynamic lung phantom. The phantom range of motion was 4 cm with a 3-s "breathing" period. EPID image acquisition parameters were modified, allowing image acquisition in 0.1 s. Images of the stationary and moving phantom were acquired. Software was developed to segment automatically the marker positions from the EPID images. Images acquired in 0.1 s displayed higher noise and a lower signal-noise ratio than those obtained using regular (> 1 s) acquisition settings. However, the markers were still clearly visible on the 0.1-s images. The motion of the phantom blurred the images of the markers and further reduced the signal-noise ratio, though they could still be successfully segmented from the images in 10-30 ms of computation time. The positions of gold markers placed in the lung phantom were detected successfully, even for phantom velocities substantially higher than those observed for typical lung tumors. This study shows that using EPID-based marker tracking for 4D radiotherapy is feasible, however, changes in linear accelerator technology and EPID-based image acquisition as well as patient studies are required before this method can be implemented clinically.

Artifacts↗

PubMed2003

Quantifying the effect of intrafraction motion during breast IMRT planning and dose delivery.

Respiratory motion during intensity modulated radiation therapy (IMRT) causes two types of problems. First, the clinical target volume (CTV) to planning target volume (PTV) margin needed to account for respiratory motion means that the lung and heart dose is higher than would occur in the absence of such motion. Second, because respiratory motion is not synchronized with multileaf collimator (MLC) motion, the delivered dose is not the same as the planned dose. The aims of this work were to evaluate these problems to determine (a) the effects of respiratory motion and setup error during breast IMRT treatment planning, (b) the effects of the interplay between respiratory motion and multileaf collimator (MLC) motion during breast IMRT delivery, and (c) the potential benefits of breast IMRT using breath-hold, respiratory gated, and 4D techniques. Seven early stage breast cancer patient data sets were planned for IMRT delivered with a dynamic MLC (DMLC). For each patient case, eight IMRT plans with varying respiratory motion magnitudes and setup errors (and hence CTV to PTV margins) were created. The effects of respiratory motion and setup error on the treatment plan were determined by comparing the eight dose distributions. For each fraction of these plans, the effect of the interplay between respiratory motion and MLC motion during IMRT delivery was simulated by superimposing the respiratory trace on the planned DMLC leaf motion, facilitating comparisons between the planned and expected dose distributions. When considering respiratory motion in the CTV-PTV expansion during breast IMRT planning, our results show that PTV dose heterogeneity increases with respiratory motion. Lung and heart doses also increase with respiratory motion. Due to the interplay between respiratory motion and MLC motion during IMRT delivery, the planned and expected dose distributions differ. This difference increases with respiratory motion. The expected dose varies from fraction to fraction. However, for the seven patients studied and respiratory trace used, for no breathing, shallow breathing, and normal breathing, there were no statistically significant differences between the planned and expected dose distributions. Thus, for breast IMRT, intrafraction motion degrades treatment plans predominantly by the necessary addition of a larger CTV to PTV margin than would be required in the absence of such motion. This motion can be limited by breath-hold, respiratory gated, or 4D techniques.

Algorithms↗

PubMed2002

A fluence-convolution method to calculate radiation therapy dose distributions that incorporate random set-up error.

The International Commission on Radiation Units and Measurements Report 62 (ICRU 1999) introduced the concept of expanding the clinical target volume (CTV) to form the planning target volume by a two-step process. The first step is adding a clinically definable internal margin, which produces an internal target volume that accounts for the size, shape and position of the CTV in relation to anatomical reference points. The second is the use of a set-up margin (SM) that incorporates the uncertainties of patient beam positioning, i.e. systematic and random set-up errors. We propose to replace the random set-up error component of the SM by explicitly incorporating the random set-up error into the dose-calculation model by convolving the incident photon beam fluence with a Gaussian set-up error kernel. This fluence-convolution method was implemented into a Monte Carlo (MC) based treatment-planning system. Also implemented for comparison purposes was a dose-matrix-convolution algorithm similar to that described by Leong (1987 Phys. Med. Biol. 32 327-34). Fluence and dose-matrix-convolution agree in homogeneous media. However, for the heterogeneous phantom calculations, discrepancies of up to 5% in the dose profiles were observed with a 0.4 cm set-up error value. Fluence-convolution mimics reality more closely, as dose perturbations at interfaces are correctly predicted (Wang et al 1999 Med. Phys. 26 2626-34, Sauer 1995 Med. Phys. 22 1685-90). Fluence-convolution effectively decouples the treatment beams from the patient. and more closely resembles the reality of particle fluence distributions for many individual beam-patient set-ups. However, dose-matrix-convolution reduces the random statistical noise in MC calculations. Fluence-convolution can easily be applied to convolution/superposition based dose-calculation algorithms.

Algorithms↗

PubMed2001

Monte Carlo dose calculations for dynamic IMRT treatments.

Dose calculations for intensity modulated radiation therapy (IMRT) face new challenges due to the complex leaf geometry and time dependent nature of the delivery. A fast method of particle transport through a dynamic multileaf collimator (MLC) geometry that accounts for photon attenuation and first-scattered Compton photon production has been incorporated into an existing Monte Carlo code used for patient dose calculations. Dosimetric agreement between calculation and measurement for two photon energies and MLC types is within experimental error for the sliding window tests. For a patient IMRT field, the Monte Carlo calculations are closer to measured dose than similar superposition or pencil beam calculations.

Humans↗

PubMed2001

Acceleration of dose calculations for intensity-modulated radiotherapy.

The requirements and trade-offs between accuracy and speed for radiotherapy dose computations have been discussed for decades. Inverse planning used for intensity-modulated radiotherapy (IMRT) optimization imposes additional demands on dose calculation since it is an iterative process in which dose calculations might be repeated many (10's to 1000's) of times. This work discusses the accuracy and speed issues as related to IMRT dose calculations. A hybrid dose calculation method which accelerates the optimization process is proposed and applied in which a fast-pencil beam (PB) model is used for initial optimization iterations, followed by superposition/convolution (SC) calculations. Optimization dose results are compared for pure PB optimization, pure SC optimization, and PB optimization followed by SC optimization. Plans were evaluated in terms of isodose coverage, dose-volume histograms, and total dose calculation time for five head and neck cases with diverse locations, sizes, and shapes for tumors and critical structures. Patient plans were designed for nine equispaced beams. For one patient, an additional five-beam configuration was tested. We found that gross features of intensity distributions resulting from all schemes were similar, however there were differences in the fine detail. Differences were small between composite dose distributions optimized with PB and SC methods, yet differences in individual beam dose distributions were quite significant. When the SC method was used to compute dose following optimization with PB method, dose differences were reduced significantly both for composite plans and for individual beams. Substantial overall timesavings were observed, allowing IMRT dose planning to become a more interactive activity.

Algorithms↗

PubMed2001

A Monte Carlo study of radiation transport through multileaf collimators.

Due to the significant increase in the number of monitor units used to deliver a dynamic IMRT treatment, the total MLC leakage (transmission plus scatter) can exceed 10% of the maximum in-field dose. To avoid dosimetric errors, this leakage must be accurately accounted for in the dose calculation and conversion of optimized intensity patterns to MLC trajectories used for treatment delivery. In this study, we characterized the leaf end transmission and leakage radiation for Varian 80- and 120-leaf MLCs using Monte Carlo simulations. The complex geometry of the MLC, including the rounded leaf end, leaf edges (tongue-and-groove and offset notch), mounting slots, and holes was modeled using MCNP4b. Studies were undertaken to determine the leakage as a function of field size, components of the leakage, electron contamination, beam hardening and leaf tip effects. The leakage radiation with the MLC configured to fully block the field was determined. Dose for 6 and 18 MV beams was calculated at 5 cm depth in a water phantom located at 95 cm SSD, and normalized to the dose for an open field. Dose components were scored separately for radiation transmitted through and scattered from the MLC. For the 80-leaf MLC at 6 MV, the average leakage dose is 1.6%, 1.7%, 1.8%, and 1.9% for 5 x 5, 10 x 10, 15 x 15, and 20 x 20cm2 fields, respectively. For the 120-leaf MLC at 6 MV, the average leakage dose is 1.6%, 1.6%, 1.7%, and 1.9% for the same field sizes. Measured leakage values for the 120-leaf MLC agreed with calculated values to within 0.1% of the open field dose. The increased leakage with field size is attributed to MLC scattered radiation. The fractional electron contamination for a blocked MLC field is greater than that for an open field. The MLC attenuation significantly affects the photon spectrum, resulting in an increase in percent depth dose at 6 MV, however, little effect is observed at 18 MV. Both phantom scatter and the finite source size contribute to the leaf tip profile observed in phantom. The results of this paper can be applied to fluence-to-trajectory and trajectory-to-fluence calculations for IMRT.

Electrons↗

PubMed2000

Converting absorbed dose to medium to absorbed dose to water for Monte Carlo based photon beam dose calculations.

Current clinical experience in radiation therapy is based upon dose computations that report the absorbed dose to water, even though the patient is not made of water but of many different types of tissue. While Monte Carlo dose calculation algorithms have the potential for higher dose accuracy, they usually transport particles in and compute the absorbed dose to the patient media such as soft tissue, lung or bone. Therefore, for dose calculation algorithm comparisons, or to report dose to water or tissue contained within a bone matrix for example, a method to convert dose to the medium to dose to water is required. This conversion has been developed here by applying Bragg-Gray cavity theory. The dose ratio for 6 and 18 MV photon beams was determined by computing the average stopping power ratio for the primary electron spectrum in the transport media. For soft tissue, the difference between dose to medium and dose to water is approximately 1.0%, while for cortical bone the dose difference exceeds 10%. The variation in the dose ratio as a function of depth and position in the field indicates that for photon beams a single correction factor can be used for each particular material throughout the field for a given photon beam energy. The only exception to this would be for the clinically non-relevant dose to air. Pre-computed energy spectra for 60Co to 24 MV are used to compute the dose ratios for these photon beams and to determine an effective energy for evaluation of the dose ratio.

Air↗

PubMed2000

A method for determining multileaf collimator transmission and scatter for dynamic intensity modulated radiotherapy.

The main purpose of this work is to demonstrate a practical means of determining the leaf transmission and scatter characteristics of a multileaf collimator (MLC) pertinent to the commissioning of dynamic intensity modulated radiotherapy, especially for the sweeping window technique. The data are necessary for the conversion of intensity distributions produced by intensity-modulated radiotherapy optimization systems into trajectories of MLC leaves for dynamic delivery. Measurements are described for two, tungsten alloy MLCs: a Mark II 80-leaf MLC on a Varian 2100C accelerator and a Millenium 120-leaf MLC on a Varian 2100EX accelerator. MLC leakage was measured by film for a series of field sizes. Measured MLC leakage was 1.68% for a 10 x 10 cm2 field for both 6 and 18 MV for the 80-leaf MLC. For the 6 MV field, the 1.68% leakage consisted of 1.48% direct transmission and 0.20% leaf scatter. Direct transmission through the 80-leaf MLC, including the rounded leaf tip, was calculated analytically taking into account the detailed leaf geometry and a Monte Carlo-generated energy spectrum of the accelerator. The integrated fluence under the leaf tip was equivalent to an inward shift of 0.06 cm of a hypothetical leaf with a flat, focused tip. Monte Carlo calculations of the dose to phantom beyond a closed 80-leaf MLC showed excellent agreement with the analytic results. The transmission depends on the density of the MLC alloy, which may differ among individual MLCs. Thus, it is important to measure the transmission of any particular MLC. Calculated doses for a series of uniform fields produced by dynamic sweeping windows of various widths agree with measurements within 2%.

Humans↗

PubMed2000

The effect of dose calculation uncertainty on the evaluation of radiotherapy plans.

Monte Carlo dose calculations will potentially reduce systematic errors that may be present in currently used dose calculation algorithms. However, Monte Carlo calculations inherently contain random errors, or statistical uncertainty, the level of which decreases inversely with the square root of computation time. Our purpose in this study was to determine the level of uncertainty at which a lung treatment plan is clinically acceptable. The evaluation methods to decide acceptability were visual examination of both isodose lines on CT scans and dose volume histograms (DVHs), and reviewing calculated biological indices. To study the effect of systematic and/or random errors on treatment plan evaluation, a simulated "error-free" reference plan was used as a benchmark. The relationship between Monte Carlo statistical uncertainty and dose was found to be approximately proportional to the square root of the dose. Random and systematic errors were applied to a calculated lung plan, creating dose distributions with statistical uncertainties of between 0% and 16% (1 s.d.) at the maximum dose point and also distributions with systematic errors of -16% to 16% at the maximum dose point. Critical structure DVHs and biological indices are less sensitive to calculation uncertainty than those of the target. Systematic errors affect plan evaluation accuracy significantly more than random errors, suggesting that Monte Carlo dose calculation will improve outcomes in radiotherapy. A statistical uncertainty of 2% or less does not significantly affect isodose lines, DVHs, or biological indices.

Algorithms↗

PubMed1999

Nuclear data for radiotherapy: presentation of a new ICRU report and IAEA initiatives.

An ICRU report entitled "Nuclear Data for Neutron and Proton Radiotherapy and for Radiation Protection" is in preparation. The present paper presents an overview of this report, along with examples of some of the results obtained for evaluated nuclear cross sections and kerma coefficients. These cross sections are evaluated using a combination of measured data and the GNASH nuclear model code for elements of importance for biological, dosimetric, beam modification and shielding purposes. In the case of hydrogen both R-matrix and phase-shift scattering theories are used. In the report neutron cross sections and kerma coefficients will be presented up to 150 MeV and proton cross sections up to 250 MeV. An IAEA Consultants' Meeting was also convened to examine the "Status of Nuclear Data needed for Radiation Therapy and Existing Data Development Activities in Member States". Recommendations were made regarding future endeavours.

Models, Theoretical↗

PubMed1999

Comparison of EGS4 and MCNP4b Monte Carlo codes for generation of photon phase space distributions for a Varian 2100C.

Monte Carlo based dose calculation algorithms require input data or distributions describing the phase space of the photons and secondary electrons prior to the patient-dependent part of the beam-line geometry. The accuracy of the treatment plan itself is dependent upon the accuracy of this distribution. The purpose of this work is to compare phase space distributions (PSDs) generated with the MCNP4b and EGS4 Monte Carlo codes for the 6 and 18 MV photon modes of the Varian 2100C and determine if differences relevant to Monte Carlo based patient dose calculations exist. Calculations are performed with the same energy transport cut-off values. At 6 MV, target bremsstrahlung production for MCNP4b is approximately 10% less than for EGS4, while at 18 MV the difference is about 5%. These differences are due to the different bremsstrahlung cross sections used in the codes. Although the absolute bremsstrahlung production differs between MCNP4b and EGS4, normalized PSDs agree at the end of the patient-independent geometry (prior to the jaws), resulting in similar dose distributions in a homogeneous phantom. EGS4 and MCNP4b are equally suitable for the generation of PSDs for Monte Carlo based dose computations.

Algorithms↗