Biomedical subjects

H Huizenga

Publications and source records attributed to H Huizenga.

At least 19 recordsLinked to original sources

PubMed2006

The curvature of sensitometric curves for Kodak XV-2 film irradiated with photon and electron beams.

Sensitometric curves of Kodak XV-2 film, obtained in a time period of ten years with various types of equipment, have been analyzed both for photon and electron beams. The sensitometric slope in the dataset varies more than a factor of 2, which is attributed mainly to variations in developer conditions. In the literature, the single hit equation has been proposed as a model for the sensitometric curve, as with the parameters of the sensitivity and maximum optical density. In this work, the single hit equation has been translated into a polynomial like function as with the parameters of the sensitometric slope and curvature. The model has been applied to fit the sensitometric data. If the dataset is fitted for each single sensitometric curve separately, a large variation is observed for both fit parameters. When sensitometric curves are fitted simultaneously it appears that all curves can be fitted adequately with a sensitometric curvature that is related to the sensitometric slope. When fitting each curve separately, apparently measurement uncertainty hides this relation. This relation appears to be dependent only on the type of densitometer used. No significant differences between beam energies or beam modalities are observed. Using the intrinsic relation between slope and curvature in fitting sensitometric data, e.g., for pretreatment verification of intensity-modulated radiotherapy, will increase the accuracy of the sensitometric curve. A calibration at a single dose point, together with a predetermined densitometer-dependent parameter ODmax will be adequate to find the actual relation between optical density and dose.

Calibration↗

PubMed2003

Scattered radiation from applicators in clinical electron beams.

In radiotherapy with high-energy (4-25 MeV) electron beams, scattered radiation from the electron applicator influences the dose distribution in the patient. In most currently available treatment planning systems for radiotherapy this component is not explicitly included and handled only by a slight change of the intensity of the primary beam. The scattered radiation from an applicator changes with the field size and distance from the applicator. The amount of scattered radiation is dependent on the applicator design and on the formation of the electron beam in the treatment head. Electron applicators currently applied in most treatment machines are essentially a set of diaphragms, but still do produce scattered radiation. This paper investigates the present level of scattered dose from electron applicators, and as such provides an extensive set of measured data. The data provided could for instance serve as example input data or benchmark data for advanced treatment planning algorithms which employ a parametrized initial phase space to characterize the clinical electron beam. Central axis depth dose curves of the electron beams have been measured with and without applicators in place, for various applicator sizes and energies, for a Siemens Primus, a Varian 2300 C/D and an Elekta SLi accelerator. Scattered radiation generated by the applicator has been found by subtraction of the central axis depth dose curves, obtained with and without applicator. Scattered radiation from Siemens, Varian and Elekta electron applicators is still significant and cannot be neglected in advanced treatment planning. Scattered radiation at the surface of a water phantom can be as high as 12%. Scattered radiation decreases almost linearly with depth. Scattered radiation from Varian applicators shows clear dependence on beam energy. The Elekta applicators produce less scattered radiation than those of Varian and Siemens, but feature a higher effective angular variance. The scattered radiation decreases somewhat with increasing field size and is spread uniformly over the aperture. Experimental results comply with the results of simulations of the treatment head and electron applicator, using the BEAM Monte Carlo code, and Siemens, but feature a higher effective angular variance. The scattered radiation decreases somewhat with increasing field size and is spread uniformly over the aperture. Experimental results comply with the results of simulations of the treatment head and electron applicator, using the BEAM Monte Carlo code.

Algorithms↗

PubMed2001

Effectiveness of couch height-based patient set-up and an off-line correction protocol in prostate cancer radiotherapy.

PURPOSE: To investigate set-up improvement caused by applying a couch height-based patient set-up method in combination with a technologist-driven off-line correction protocol in nonimmobilized radiotherapy of prostate patients. METHODS AND MATERIALS: A three-dimensional shrinking action level correction protocol is applied in two consecutive patient cohorts with different set-up methods: the traditional "laser set-up" group (n = 43) and the "couch height set-up" group (n = 112). For all directions, left-right, ventro-dorsal, and cranio-caudal, random and systematic set-up deviations were measured. RESULTS: The couch height set-up method improves the patient positioning compared to the laser set-up method. Without application of the correction protocol, both systematic and random errors reduced to 2.2-2.4 mm (1 SD) and 1.7-2.2 mm (1 SD), respectively. By using the correction protocol, systematic errors reduced further to 1.3-1.6 mm (1 SD). One-dimensional deviations were within 5 mm for >90% of the measured fractions. The required number of corrections per patient in the off-line correction protocol was reduced significantly during the course of treatment from 1.1 to 0.6 by the couch height set-up method. The treatment time was not prolonged by application of the correction protocol. CONCLUSIONS: The couch height set-up method improves the set-up significantly, especially in the ventro-dorsal direction. Combination of this set-up method with an off-line correction strategy, executed by technologists, reduces the number of set-up corrections required.

Humans↗

PubMed2001

A model to determine the initial phase space of a clinical electron beam from measured beam data.

Advanced electron beam dose calculation models for radiation oncology require as input an initial phase space (IPS) that describes a clinical electron beam. The IPS is a distribution in position, energy and direction of electrons and photons in a plane in front of the patient. A method is presented to derive the IPS of a clinical electron beam from a limited set of measured beam data. The electron beam is modelled by a sum of four beam components: a main diverging beam, applicator edge scatter, applicator transmission and a second diverging beam. The two diverging beam components are described by weighted sums of monoenergetic diverging electron and photon beams. The weight factors of these monoenergetic beams are determined by the method of simulated annealing such that a best fit is obtained with depth-dose curves measured for several field sizes at two source-surface distances. The resulting IPSs are applied by the phase-space evolution electron beam dose calculation model to calculate absolute 3D dose distributions. The accuracy of the calculated results is in general within 1.5% or 1.5 mm; worst cases show differences of up to 3% or 3 mm. The method presented here to describe clinical electron beams yields accurate results, requires only a limited set of measurements and might be considered as an alternative to the use of Monte Carlo methods to generate full initial phase spaces.

Biophysical Phenomena↗

PubMed2000

Accuracy of the phase space evolution dose calculation model for clinical 25 MeV electron beams.

The phase space evolution (PSE) model is a dose calculation model for electron beams in radiation oncology developed with the aim of a higher accuracy than the commonly used pencil beam (PB) models and with shorter calculation times than needed for Monte Carlo (MC) calculations. In this paper the accuracy of the PSE model has been investigated for 25 MeV electron beams of a MM50 racetrack microtron (Scanditronix Medical AB, Sweden) and compared with the results of a PB model. Measurements have been performed for tests like non-standard SSD, irregularly shaped fields, oblique incidence and in phantoms with heterogeneities of air, bone and lung. MC calculations have been performed as well, to reveal possible errors in the measurements and/or possible inaccuracies in the interaction data used for the bone and lung substitute materials. Results show a good agreement between PSE calculated dose distributions and measurements. For all points the differences--in absolute dose--were generally well within 3% and 3 mm. However, the PSE model was found to be less accurate in large regions of low-density material and errors of up to 6% were found for the lung phantom. Results of the PB model show larger deviations, with differences of up to 6% and 6 mm and of up to 10% for the lung phantom; at shortened SSDs the dose was overestimated by up to 6%. The agreement between MC calculations and measurement was good. For the bone and the lung phantom maximum deviations of 4% and 3% were found, caused by uncertainties about the actual interaction data. In conclusion, using the phase space evolution model, absolute 3D dose distributions of 25 MeV electron beams can be calculated with sufficient accuracy in most cases. The accuracy is significantly better than for a pencil beam model. In regions of lung tissue, a Monte Carlo model yields more accurate results than the current implementation of the PSE model.

Air↗

PubMed2000

Optimization of multileaf collimator settings for radiotherapy treatment planning.

Multileaf collimators have become available in many radiotherapy treatment centres. The cross section of a beam can be shaped to a projection of the target area by moving the leaves of the multileaf collimator into the beam. In this paper, a method is described to optimize the positions of the individual leaves automatically, once the beam directions and weights have been chosen. The individual positions of the leaves are optimized using the variable metric method. Changes in dose resulting from small leaf movements are computed efficiently using a special method. The optimization method was tested on a treatment plan for a phantom patient. It was found that the unnecessary edges of the beams were trimmed efficiently.

Algorithms↗

PubMed1999

Inclusion of geometrical uncertainties in radiotherapy treatment planning by means of coverage probability.

PURPOSE: Following the ICRU-50 recommendations, geometrical uncertainties in tumor position during radiotherapy treatments are generally included in the treatment planning by adding a margin to the clinical target volume (CTV) to yield the planning target volume (PTV). We have developed a method for automatic calculation of this margin. METHODS AND MATERIALS: Geometrical uncertainties of a specific patient group can normally be characterized by the standard deviation of the distribution of systematic deviations in the patient group (Sigma) and by the average standard deviation of the distribution of random deviations (sigma). The CTV of a patient to be planned can be represented in a 3D matrix in the treatment room coordinate system with voxel values one inside and zero outside the CTV. Convolution of this matrix with the appropriate probability distributions for translations and rotations yields a matrix with coverage probabilities (CPs) which is defined as the probability for each point to be covered by the CTV. The PTV can then be chosen as a volume corresponding to a certain iso-probability level. Separate calculations are performed for systematic and random deviations. Iso-probability volumes are selected in such a way that a high percentage of the CTV volume (on average > 99%) receives a high dose (> 95%). The consequences of systematic deviations on the dose distribution in the CTV can be estimated by calculation of dose histograms of the CP matrix for systematic deviations, resulting in a so-called dose probability histogram (DPH). A DPH represents the average dose volume histogram (DVH) for all systematic deviations in the patient group. The consequences of random deviations can be calculated by convolution of the dose distribution with the probability distributions for random deviations. Using the convolved dose matrix in the DPH calculation yields full information about the influence of geometrical uncertainties on the dose in the CTV. RESULTS: The model is demonstrated to be fast and accurate for a prostate, cervix, and lung cancer case. A CTV-to-PTV margin size which ensures at least 95% dose to (on average) 99% of the CTV, appears to be equal to about 2Sigma + 0.7sigma for three all cases. Because rotational deviations are included, the resulting margins can be anisotropic, as shown for the prostate cancer case. CONCLUSION: A method has been developed for calculation of CTV-to-PTV margins based on the assumption that the CTV should be adequately irradiated with a high probability.

Female↗

PubMed1999

On the initial angular variances of clinical electron beams.

Electron beam radiotherapy treatment planning systems need to be fed with the characteristics of the high-energy electron beams (4-50 MeV) from the specifically applied accelerator. Beams can be characterized by their mean initial energy, effective initial angular variance, virtual source position and the resulting central axis depth dose distribution in water. This information is the only input to pencil beam dose calculation models. Newer calculation models like macro Monte Carlo, voxel Monte Carlo and phase space evolution require as input the full initial phase space or a parametrization of that initial phase space, generally consisting of a primary beam component and one or more scatter components. This primary beam component is often characterized by initial energy, primary beam initial angular variance and virtual source distance. The purpose of the present investigation was to investigate to what extent standard values can be used both for the effective initial angular variance as input to pencil beam models and for the primary beam initial angular variance. Comprehensive benchmark data were obtained on the initial angular variance of various types of accelerator, for various energies and field sizes. The initial angular variance sigma2theta(x) has been derived from penumbra measurements in air by means of film dosimetry at various distances from the lower collimator. For the types of accelerator used in radiotherapy nowadays the measurements show values for sigma2theta(x)/T(E) of around 13 cm where T(E) is the ICRU-35 linear angular scattering power in air. This value can be chosen as standard value for the primary beam initial angular variance, only slightly compromising the dose calculation accuracy. As input to pencil beam models, an effective sigma2theta(x)/T(E) should be used incorporating the scatter from the lower collimator. For the case that the air gaps between lower collimator and patient are small (5-10 cm) an effective sigma2theata(x)/T(E) of 20 cm has been found and is recommended as the standard input for pencil beam models. Of the accelerators investigated, a different value was found only for the Elekta SL15, i.e. 50% higher for the effective sigma2theta(x)/T(E).

Electrons↗

PubMed1999

Mixing intensity modulated electron and photon beams: combining a steep dose fall-off at depth with sharp and depth-independent penumbras and flat beam profiles.

For application in radiotherapy, intensity modulated high-energy electron and photon beams were mixed to create dose distributions that feature: (a) a steep dose fall-off at larger depths, similar to pure electron beams, (b) flat beam profiles and sharp and depth-independent beam penumbras, as in photon beams, and (c) a selectable skin dose that is lower than for pure electron beams. To determine the required electron and photon beam fluence profiles, an inverse treatment planning algorithm was used. Mixed beams were realized at a MM50 racetrack microtron (Scanditronix Medical AB, Sweden), and evaluated by the dose distributions measured in a water phantom. The multileaf collimator of the MM50 was used in a static mode to shape overlapping electron beam segments, and the dynamic multileaf collimation mode was used to realize the intensity modulated photon beam profiles. Examples of mixed beams were generated at electron energies of up to 40 MeV. The intensity modulated electron beam component consists of two overlapping concentric fields with optimized field sizes, yielding broad, fairly depth-independent overall beam penumbras. The matched intensity modulated photon beam component has high fluence peaks at the field edges to sharpen this penumbra. The combination of the electron and the photon beams yields dose distributions with the characteristics (a)-(c) mentioned above.

Electrons↗

PubMed1998

Effect of set-up uncertainties on the dose distribution in the match region of supraclavicular and tangential breast fields.

BACKGROUND: The objective of this paper is to illustrate the effect of a gap of 5 mm, an overlap of 5 mm and a perfect match on the dose distribution across the junction of tangential breast fields and adjacent supraclavicular and axillary fields. MATERIALS AND METHODS: For this purpose film dosimetry was applied to measure relative dose distributions in two sagittal planes in an anthropomorphic breast phantom having cork lungs, simulating a radiation therapy treatment of the breast and adjacent supraclavicular lymph nodes. Two different treatment techniques, an SSD match technique and a geometrically exact isocentric match technique, as routinely applied in the two institutes were examined. The three-dimensional treatment planning system of each institute was used to calculate the dose distribution in the match region of the supraclavicular fields and the two opposing tangential fields. The measured and calculated dose distributions were evaluated and compared along lines in two sagittal planes from the supraclavicular fields down to the tangential fields crossing the match planes. These dose distributions in the match region were extremely dependent on the set-up of the fields. RESULTS: Although the reproducibility of the film measurements was within 2%, it became clear that the set-up of the fields to achieve a gap of 5 mm, a perfect match or an overlap of 5 mm required a lot of attention, even when using a phantom. CONCLUSIONS: It can be concluded that in clinical practice, these set-up difficulties do influence the dose distribution in the match region much more than the systematic uncertainties in the dose calculation algorithms of the treatment planning systems and the type of treatment technique.

Breast Neoplasms↗

PubMed1998

Sharpening the penumbra of high energy electron beams with low weight narrow photon beams.

BACKGROUND AND PURPOSE: High energy (20-50 MeV) electron beams, available from the MM50 Racetrack Microtron, can be used for the treatment of deep-seated tumors. A disadvantage is the increasing penumbra width as a function of depth. By the addition of a narrow (typically 1 cm wide) photon beam near the field edge, the 50-90% penumbra width of the electron beam is reduced, yielding a significantly increased effective field size. MATERIALS AND METHODS: For rectangular electron beams in a water phantom (energies 25 and 40 MeV, field sizes 5 x 5-15 x 15 cm2) a computer program was used to optimize the photon beam parameters (position, weight and width) to obtain a combined beam with the sharpest penumbra at the optimization depth and a beam flatness within certain constraints. The study furthermore included penumbra sharpening of an irregular multileaf collimator-shaped field. RESULTS AND CONCLUSION: At optimization depths near R90, photon beam addition reduces the penumbra width by 40-50% (from 15-20 mm to 8-10 mm). Beam flatness at the optimization depth is within +/-5% and hot-spots are < or =120% for all depths. By the addition of narrow photon beams around the rectangular or irregular field, the electron field width can be reduced by 1-3 cm, while the effective field size is maintained.

Electrons↗

PubMed1997

Numerical calculation of energy deposition by high-energy electron beams: III-B. Improvements to the 6D phase space evolution model.

The phase space evolution model of Huizenga and Storchi, Morawska-Kaczyńska and Huizenga and Janssen et al has been modified to (i) allow application on currently available computer equipment with limited memory (128 Megabytes) and (ii) allow 3D dose calculations based on 3D computer tomographic patient data. This is a further development aimed at the use of the phase space evolution model in radiotherapy electrons beam treatment planning. The first modification regards the application of depth evolution of the phase space state combined with an alternative method to transport back-scattered electrons. This depth evolution method requires of the order of 15 times less computer memory than the energy evolution method. Results of previous and new electron transport methods are compared and show that the new electron transport method for back-scattered electrons hardly affects the accuracy of the calculated dose distributions. The second modification regards the simulation of electron transport through tissues with varying densities by applying distributed electron transport through similarly composed media with a limited number of fixed densities. Results of non-distributed and distributed electron transport are compared and show that the distributed electron transport method hardly affects the accuracy of the calculated dose distributions. It is also shown that the results of the new dose distribution calculations are still in good agreement with and require significantly less computation time than results obtained with the EGS4 Monte Carlo method.

Electrons↗

PubMed1996

A quality control study of the accuracy of patient positioning in irradiation of pelvic fields.

PURPOSE: Determining and improving the accuracy of patient positioning in pelvic fields. METHODS AND MATERIALS: Small pelvic fields were studied in 16 patients treated for urological cancers using a three-field isocentric technique. Large pelvic fields were studied in 17 gynecological cancer patients treated with anterior and posterior (AP-PA) parallel opposed fields. Quantitative analysis of 645 megavolt images and comparison to 82 simulation images were carried out. RESULTS: Small pelvic fields: for the position of the patient in the field, standard deviations of the difference between simulation (SIM) and treatment (MV) images were 3.4 mm in the lateral direction, 5.3 mm in the cranio-caudal direction, and 4.8 mm in the ventro-dorsal direction. Alterations in the positioning technique were made and tested. Large pelvic fields: differences between simulation and treatment images for the position of the patient in the field were 4 mm [1 standard deviation (SD)] in the lateral direction and 6.5 mm in the cranio-caudal direction. A systematic shift of the treatment field in the cranial direction had occurred in the majority of patients. A positioning technique using laser lines and marking of the caudal field border was shown to be more accurate. CONCLUSIONS: Studies of positioning accuracy in routine irradiation techniques are needed to obtain data for definition of the margins for each treatment site at each institution. Random variations should be kept at a minimum by monitoring and improving positioning techniques. Treatment verification by megavolt imaging or film should be used to detect and correct systematic variations early in the treatment series.

Endometrial Neoplasms↗

PubMed1996

Phase space evolution distribution functions for high energy electron beams.

The phase space evolution (PSE) model is a 3D electron beam dose calculation model for radiation oncology. The PSE model is based upon the transport of electrons with a specific energy and direction over short distances (typically 0.3-1 cm). The result of the transport of these electrons is described by an energy and direction distribution of the electrons, which is stored in a database. The database is used by the PSE model at the time of the actual electron transport simulation. A good agreement between dose distributions calculated by the PSE model and EGS4 Monte Carlo code for mono-energetic, mono-directional electron beams was found. The differences in point dose are within 1-2% of the maximum dose. These differences can be caused by errors in the database used, or by assumptions made in the PSE model. The aim of this paper is to get more insight into the possible errors introduced by the database. Results show that the data in the database are in good agreement with EGS4 calculated data. Also the influence of the database on a PSE calculated dose distribution has been investigated. The differences between a PSE calculated dose distribution and an EGS4 calculated dose distribution can be reduced to < 0.5% if the database is replaced by a database partly created by EGS4. This shows that small errors in the database have a distinct effect on the dose distribution, and that this dose distribution can be calculated accurately by the PSE model if the right database is used.

Computer Simulation↗

PubMed1996

Physical characteristics of a commercial electronic portal imaging device.

An electronic portal imaging device (EPID) for use in radiotherapy with high energy photons has been under development since 1985 and has been in clinical use since 1988. The x-ray detector consists of a metal plate/fluorescent screen combination, which is monitored by a charge-coupled device (CDD)-camera. This paper discusses the physical quantities governing image quality. A model which describes the signal and noise propagation through the detector is presented. The predicted contrasts and signal-to-noise ratios are found to be in agreement with measurements based on the EPID images. Based on this agreement the visibility of low contrast structures in clinical images has been calculated with the model. Sufficient visibility of relevant structures (4-10 mm water-equivalent thickness) has been obtained down to a delivered dose of 4 cGy at dose maximum. It is found that the described system is not limited by quantum noise but by camera read-out noise. In addition we predict that with a new type of CCD sensor the signal-to-noise ratio can be increased by a factor of 5 at small doses, enabling high quality imaging, for most relevant clinical situations, with a patient dose smaller than 4 cGy. The latter system would be quantum noise limited.

Equipment Design↗

PubMed1995

Portal dose measurement in radiotherapy using an electronic portal imaging device (EPID).

Physical characteristics of a commercially available electronic portal imaging device (EPID), relevant to dosimetric applications in high-energy photon beams, have been investigated. The EPID basically consists of a fluorescent screen, mirrors and a CCD camera. Image acquisition for portal dose measurement has been performed with a special procedure, written in the command language that comes with the system. The observed day-to-day variation in local EPID responses, i.e. measured grey scale value (EPID signal) per unit of delivered portal dose, is 0.4% (1 SD); day-to-day variation in relative EPID responses (e.g. normalized to the on-axis response) are within 0.2% (1 SD). Measured grey scale values are linearly proportional to transmitted portal doses with a proportionality constant which is independent of the thickness of a flat, water-equivalent absorber in the beam, but which does significantly depend on the size of the applied x-ray beam. It is shown that the observed increased in EPID response with increasing field size is mainly due to contributions to the EPID signals from scattered light: visible photons produced by the x-ray beam in a point of the fluorescent screen not only generate a grey scale value in the corresponding point of the EPID image, but also lead (due to scatter from components of the EPID structure onto the CCD chip) to an increased grey scale value at all other points of the image. A point spread function, derived from measured data and describing the increase in EPID response at the beam axis due to off-axis irradiation of the fluorescent screen, has been successfully applied to connect portal doses with grey scale values measured with the EPID.

Analysis of Variance↗

PubMed1994

In vivo determination of the accuracy of field matching in breast cancer irradiation using an electronic portal imaging device.

The purpose of this study was to investigate the accuracy of field matching in patients treated by irradiation of the breast and adjacent lymph nodes. Field matching is performed by the radiographers during each session on a match line drawn on the patient's skin. Field edge positions were assessed in the cranial match plane of tangential breast fields and supraclavicular-axillary fields using an electronic portal imaging device and match line markers placed on the skin of the patients. The mean gap/overlap of the four fields for individual patients during each treatment session, derived from 374 marker projections, was +0.5 mm indicating that no systematic gap or overlap was observed. The uncertainty in the position of the four fields with respect to the match plane ranges from 3.1 to 5.1 mm (1 SD) for the individual patients. Gaps and overlaps between fields were also related to an absolute match line position, found by comparison of simulator and portal images, showing a small systematic uncertainty of 2.4 mm and a standard deviation of 3.3 mm. It can be concluded that the use of an electronic portal imaging device in combination with match line markers is a good method to quantify the accuracy of field matching in vivo. The results showed good stability and reproducibility in the field matching region for this treatment technique of breast cancer irradiation.

Breast Neoplasms↗

PubMed1994

Film dosimetry for radiotherapy treatment planning verification of a 6 MV tangential breast irradiation.

Film dosimetry has been applied to measure relative dose distributions in an anthropomorphic polystyrene breast phantom having cork lungs, simulating a radiation therapy treatment with 6 MV opposing tangential beams. Measured relative dose distributions showed good reproducibility (about 1.5%, 1 SD) and good agreement (< 2%) with calculations performed with a three-dimensional treatment planning system. These results demonstrate that film dosimetry is a useful tool for treatment planning verification.

Breast Neoplasms↗