Ulf F. Rosenow

Quality assurance in radiotherapy: the importance of medical physics staffing levels. Recommendations from an ESTRO/EFOMP joint task group

The safe application of ionising radiation for diagnosis and therapy requires a high level of knowledge of the underlying processes and of quality assurance. Sophisticated modern equipment can be used effectively for complicated diagnostic and therapeutic techniques only with adequate physics support. In the light of recent analyses and recommendations by national and international societies a joint working group of representatives from ESTRO (European Society for Therapeutic Radiology and Oncology) and from EFOMP (European Federation of Organisations for Medical Physics) was set up to assess the necessary staffing levels for physics support to radiotherapy. The method used to assess the staffing levels, the resulting recommendations and examples of their practical application are described.

The NCI-atlas of dose distributions for regular 125I brain implants

In connection with the development of an optimization method for 125I brain implants in irregularly shaped target volumes, a systematic study was conducted toward optimizing seed configurations for regular target volumes. The intention was to find basic rules for the positioning of strings of seeds in the cylindrical implant pattern of the stereotactic neurosurgical procedure in use, and in accordance with the following criteria: (i) steep dose fall-off outside the target volume; (ii) coverage of the target volume by making the prescribed dose surface coincident with the target volume surface within 1 mm; (iii) uniformity of the dose distribution in the target volume as far as achievable with a seed implant. As a result of this study, an atlas of optimized regular 125I brain implant configurations was compiled. Regular implants were understood as being cylindrical or spherical. Diameters and heights from 2 to 5 cm were covered.

Clinical implementation of stereotaxic brain implant optimization

This optimization method for stereotaxic brain implants is based on seed/strand configurations of the basic type developed for the National Cancer Institute (NCI) atlas of regular brain implants. Irregular target volume shapes are determined from delineation in a stack of contrast enhanced computed tomography scans. The neurosurgeon may then select up to ten directions, or entry points, of surgical approach of which the program finds the optimal one under the criterion of smallest target volume diameter. Target volume cross sections are then reconstructed in 5-mm-spaced planes perpendicular to the implantation direction defined by the entry point and the target volume center. This information is used to define a closed line in an implant cross section along which peripheral seed strands are positioned and which has now an irregular shape. Optimization points are defined opposite peripheral seeds on the target volume surface to which the treatment dose rate is prescribed. Three different optimization algorithms are available: linear least-squares programming, quadratic programming with constraints, and a simplex method. The optimization routine is implemented into a commercial treatment planning system. It generates coordinate and source strength information of the optimized seed configurations for further dose rate distribution calculation with the treatment planning system, and also the coordinate settings for the stereotaxic Brown-Roberts-Wells (BRW) implantation device.

Energy constancy checking for electron beams using a wedge‐shaped solid phantom combined with a beam profile scanner

An energy constancy checking method is presented which involves a specially designed wedge-shaped solid phantom in combination with a multiple channel ionization chamber array known as the Thebes device. Once the phantom/beam scanner combination is set up, measurements for all electron energies can be made and evaluated without re-entering the treatment room. This is also valid for the readjustment of beam energies which are found to deviate from required settings. The immediate presentation of the measurements is in the form of crossplots which resemble depth dose profiles. The evaluation of the measured data can be performed using a hand-held calculator, but processing of the measured signals through a PC-type computer is advisable. The method is insensitive to usual fluctuations in beam flatness. The sensitivity and reproducibility of the method are more than adequate. The method may also be used in modified form for photon beams.

A simple method of producing depth ionization data for electron energy constancy check

A simple method has been developed to reproduce depth ionization data of electron beams for energy determination. The method utilizes a simple set of equipment, a combination of a specially designed wedge-shaped polystyrene phantom and a linear array of detectors, to collect the necessary data. The wedge-shaped phantom provides varying depths to various detectors in the array. The ionization readings received from the detectors were corrected for off-axis ratio and plotted against corresponding ray-line depths to produce depth ionization curves. The instrument setup was fast and simple. The relevant data, for a high-energy linear accelerator with multiple electron energies, were collected in minutes. The depths of 80% and 50% ionization determined by this method were found to differ by 2 and 3 mm, respectively, at the most, with those determined by a conventional method.

ENERGY DEPOSITION OF ELECTRONS IN LOW-, MEDIUM- AND HIGH-Z MATERIAL : COMPARISON OF THE MONTE CARLO TRANSPORT CODE EGS4 WITH EXPERIMENT

Abstract The Monte Carlo electron transport code EGS4 was benchmark tested against early experimental results derived by Freyberger. These consist of absolute depth ionization and depth dose curves measured at a pencil beam with sharp energy definition of nominally 4, 10 and 20 MeV electrons extracted from a Betatron. The Freyberger precision measurements have been made with a wide plane-parallel ionization chamber in a slab phantom for the materials PMMA, C, Al, Cu, and Pb. The bremsstrahlung and, in some experiments, the forward and backward directed contributions had been determined separately. The pencil-beam/wide-chamber geometry is equivalent, in respect to the measurement of depth ionization and depth dose curves, to the more common wide-beam/point-detector geometry. However, it requires the simulation of merely one pencil beam position and practically all particle histories contribute to the ionization in the wide air cavity. Thus a considerable amount of computing time is saved. We applied a prototype of the new electron transport code PRESTA II. The results of our simulation generally agree very well, even in absolute terms, with experiment. Small deviations are found at lower energies and high-Z materials. For low energies they may in part be explained by contamination with bremsstrahlung from the beam guide system and an overestimation of bremsstrahlung-production in the experiment. The simulation of a gas-filled chamber within a high-Z absorber block seems to produce small errors in the calculation of ionization for electrons of approximately 4 MeV. Larger deviations for high-Z materials are attributed to the employment of screened Rutherford cross sections which lead to an underprediction of ionization from backscattered particles. Backscatter was found to be sufficiently accurate in the simulation of the PMMA absorber.

Exploration of the Monte Carlo Code EGS4 for the Simulation of Plane-Parallel Electron Chambers

Summary The response of a plane-parallel ionisation chamber for electron dosimetry is influenced by scatter from the chamber walls. Such wall scatter should be investigated most accurately by Monte Carlo calculations. However, since Monte Carlo codes average over a certain number of steps and incorporate approximations at boundaries it is necessary to evaluate the codes prior to their use. The small dimensions of plane-parallel chambers require the definition of extremely small dose scoring regions. In consequence, interaction points are always close to boundaries which makes the results questionable. In this work we study the feasibility of the Monte Carlo code EGS4 with its electron transport algorithm PRESTA under these conditions. As a result we give the smallest meaningful size of dose-score-regions in the vicinity of a PMMA-air-boundary. Furthermore, we investigate a new algorithm [6] in respect to the expected improvements.

64. State of Monte Carlo calculations in radiation treatment planning

Monte Carlo (MC) particle transport simulations are increasingly applied in treatment planning methods. This has become feasible through a number of adaptations of general MC codes, such as EGS4 or ETRAN, to the specific needs of treatment planning. The currently most advanced “conventional” planning methods, such as convolution or delta–volume algorithms still have serious limitations in terms of accuracy when tissue inhomogeneities, small and complex body shapes or high-density implants are involved. The Monte Carlo simulation mimics individual particle transport, in any applicable geometry, by applying first principles of radiation interaction with matter and random choice of collision parameters such as step length, type of interaction, energies and scattering angles. In principle, the accuracy of MC calculations is only limited the radiation beam quality definition and the interaction parameters and can be taken to below 12%. In practice, a very large number of particle “histories” have to be simulated to attain sufficient statistical accuracy, and various approximations (e. g. condensed history, variance reduction) have been introduced in the process of adaptation of MC codes to the special needs of treatment planning. Such codes have become known as V(oxel)MC and X(ray)VMC, M(acro)MC, S(uper)MC, MCPAT(ient…). These will be described in detail and performance characteristics as well as treatment planning examples given. While the general-purpose MC codes result in computing times per case of the order of several hours, the special treatment planning codes reduce this time to around an hour or even much less on modern workstations or Pentium-based PCs.

Bio-Photogrammetry (Demonstrations)

Various applications of X-ray and normal Stereophoto-grammetry are given, of which only three can be displayed here:(Figures (a) can in each case be viewed stereoscopically.)

Roentgen-Stereometry and Sectional Display of Computed Three-Dimensional Dose Distributions in Radio-Therapy

The introduction of the computer to radiation treatment planning problems, especially to the calculations of dose distributions in irradiated patients, has led to a critical investigation of the fundamental data fed into the computer. While the physical data of the radiation are known to an accuracy of one or two percent, the data to be obtained from the patient are still subject to major inaccuracies. These mostly anatomical data are body outlines, sizes and positions of tumors in the patient, sizes and positions of tissue-inhomogenities as lung, bone, and gas in the bowel, the position of healthy organs which are sensitive to radiation and in proximity to the treatment area, and the exact position of interstitial or intracavitary sealed radioactive sources such as radium, cobalt, or caesium applicators, radio-gold or radon seeds, iridium wires, and others. In all cases a three-dimensional localization, that means a three-dimensional measurement of position and size, of certain structures at or inside the patient is necessary. A roentgen-stereoscopic method, described in the paper and in the demonstrations of Leydolph and Rosenow, was found to be best fitted to these problems of localization, because it anables a very accurate, and as well as a very fast and simple estimation of stereometric x-ray films.