Deconvolution of detector size effect for output factor measurement for narrow Gamma Knife radiosurgery beams.

This paper presents the results of measurements of output factors (OFs) for a model U Gamma Knife collimator, with special emphasis on the accurate determination of the OF for the 4 mm collimator (OF4). In the past, the OF4 was set to 0.800 relative to the 18 mm collimator. Recently, the manufacturer has recommended a new value of 0.870 for OF4. However, most centres still use the old value of the OF4. In the present study, the Gamma Knife OFs were measured using a commercially available miniature diamond detector and a miniature 0.006 cc ion chamber, which was especially designed for the task. The measured OF4 were corrected for spatial averaging effects by measuring dose profiles for the 4 mm collimator with the same detectors and deconvolving their response from the measured profiles. A Gaussian kernel was used to describe the detector response. The relative OFs measured with the diamond detector/ion chamber were 0.986/0.982, 0.953/0.935 and 0.812/0.765 for the 14,8 and 4 mm collimators, respectively, as compared with the manufacturer's values of 0.984, 0.956 and 0.87. The corrected OF4 was 0.881 +/- 0.012 for the diamond detector and 0.851 +/- 0.012 for the ion chamber, supporting the manufacturer's revised value for this collimator.

XINPUT: a program to edit "DOSRZ" input files.

The DOSRZ user code, which is part of the EGS4 standard distribution, is widely used in medical physics for the calculation of dose deposition in cylindrical geometries. The code provides the use of advanced Monte-Carlo techniques (PRESTA) and variance reduction methods. In the case of complex cylinder geometries the input of coordinates and radii is not only tedious but also prone to a high error rate. Coordinates are to be stated in absolute numbers. A change of one number, e.g., the slab thickness, requires the change of all subsequent numbers. Furthermore, parameters are only stated as numbers with no indication of their meaning. Obviously, there is a need for a user interface to facilitate the input for DOSRZ and to largely reduce the possibility for errors. We, therefore, wrote a graphical user interface (GUI) consisting of an input mask, a coordinate input interpreter, and a two-dimensional and/or pseudo-three-dimensional display section. The GUI is based on the scripting language Tcl/Tk, which runs under various platforms such as UNIX (LINUX), w95, and WIN NT. It consists of a main window which provides common-style menus and buttons to navigate through the edit dialog boxes. The most important tools are the region input, which enables the user to create the simulation geometry, and the graphics section where the scaled output can be displayed. Different media are shown in different colors which are user defined. Furthermore, the program contains some tools to reduce the probability of an erroneous input in the EGS4 input file. Since Tcl/Tk is a modern scripting language, it offers advanced tools to create the GUI and to "glue" different applications to it. XINPUT may also be considered as a model program for the development of a more general interface to other input areas of the EGS4 simulation code.

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.

Notes on the legacy of the Röntgen rays.

The discovery of the Röntgen rays and the events connected to it are extraordinary in many respects. Röntgen never disclosed the full details of the experiment which led to the discovery on November 8, 1895. He observed the x-rays by chance. Neither he nor any other scientist had an idea that such radiation might exist. However, it needed a Röntgen to make the discovery, an experimenter of his superior capabilities. His achievement was the culmination point of the development of physics as an experimental science in the 19th Century. This development of physics is described in this report in some detail, together with the institutional structure of university physics in Germany and the status of technical achievements at the time of Röntgen. Röntgen was suspicious, if not disdainful, of theoretical physics which slowly had gained in importance and in institutional representation. Thus, Röntgen's famous discovery, possible only to a mind not prejudiced by theoretical considerations or expectations, happened at a point in the history of physics when predominantly theoretical concepts introduced the paradigm change from "classical" to "modern" physics: Maxwell's electromagnetic theory, Planck's quantum hypothesis, and Einstein's relativity principle and explanation of the photo-electric effect. The tremendous speed by which the news of the "new kind of rays" spread around the world and the sensation this news caused launched an intensity of research on x-rays unprecedented in other areas of research and is reflected in a description of the publication history. The traditional working style of the Institute Director Röntgen, the structure of his Institute, his lack of interest in theory, the burden of his sudden fame and other factors made it practically impossible for him to compete with the rapid development and to contribute substantially to the research on x-rays. Even the award of the first Nobel Prize in physics in 1901 did not stimulate him to undertake new research activities. He even succeeded in avoiding presentation of his Nobel Lecture. He took the role of the interested observer, sometimes diverted by priority disputes, mainly with Lenard, or by sensational press reports, and retreated increasingly into final solitude, possibly his answer also to the abundance of honors heaped upon him. The impact of his discovery was, however, enormous. In physics it gave impulses to the discovery of radioactivity, of the identification of the electron, and the development of the model of the atom; and in medicine in its immediate applications in diagnosis and therapy.

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.

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 technique for treating local breast cancer using a single set-up point and asymmetric collimation.

Using both pairs of asymmetric jaws of a linear accelerator local-regional breast cancer may be treated from a single set-up point. This point is placed at the abutment of the supraclavicular fields with the medial and lateral tangential fields. Positioning the jaws to create a half-beam superiorly permits treatment of the supraclavicular field. Positioning both jaws asymmetrically at midline to define a single beam in the inferoanterior quadrant permits treatment of the breast from medial and lateral tangents. The highest possible matching accuracy between the supraclavicular and tangential fields is inherently provided by this technique. For treatment of all fields at 100 cm source to axis distance (SAD) the lateral placement and depth of the set-up point may be determined by simulation and simple trigonometry. We elaborate on the clinical procedure. For the technologists treatment of all fields from a single set-up point is simple and efficient. Since the tissue at the superior border of the tangential fields is generally firmer than in mid-breast, greater accuracy in day-to-day set-up is permitted. This technique eliminates the need for table angles even when tangential fields only are planned. Because of half-beam collimation the limit to the tangential field length is 20 cm. Means will be suggested to overcome this limitation in the few cases where it occurs. Another modification is suggested for linear accelerators with only one independent pair of jaws.

Small-field electron dosimetry for the Philips SL25 linear accelerator.

Electron-beam characteristics of a Philips SL25 linear accelerator have been studied. Central-axis percentage depth doses, cross-beam profiles and beam output factors of 6-, 10-, and 20-MeV beams, selected from the available energy range of 4 to 22 MeV, are reported in this paper. The main thrust of this work is to determine the systematic variation of beam characteristics, especially the output factor, with standard cone sizes and cerrobend beam-shaping cutouts down to a field size of 2 X 2 cm Output factors for the standard cones (open field) are energy dependent in a complex manner, increasing with the cone size for the 6-MeV beam whereas decreasing for 10- and 20-MeV beams. The output factor falls below unity at lower energies (6 and 10 MeV) for fields with at least one side smaller than 6 cm, and stays nearly constant for the 20-MeV beam. Measured output factors of small fields are least squares fitted by a second-order polynomial function. Output factors for small rectangular fields have been derived from the one-dimensional and square-root formulas, and the equivalent-square method. Only the one-dimensional formula predicts the measured output factors of highly elongated fields to within +/- 1% experimental uncertainties. Different cones with the same size electron cutout show a varied dose response, primarily due to variation in scattered electron contamination from the cones.

[A rapid radiotherapy planning program in intracavitary afterloading therapy]. / Ein schnelles Bestrahlungsplanungsprogramm für die intrakavitäre Afterloading-Therapie.

A recently developed program for the irradiation planning of intracavitary afterloading applications in the treatment of gynecological diseases is presented. On the basis of measured data, a rapid algorithm for calculating the dose within the field near to ray emitters is introduced which avoids any uncertainties as to the greatest activity and the dose rate constant. Distance- and direction-dependent corrections of the inverse square law are performed by means of polynomials easy to calculate or by tables. The structure and performance of the program are described. Some examples are given in order to illustrate the possible applications of the irradiation planning program.

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.

125I interstitial brachytherapy for primary malignant brain tumors: technical aspects of treatment planning and implantation methods.

Use of interstitial radiation holds promise in the treatment of primary malignant brain tumors, but optimal technical factors have yet to be determined. We have developed a method of precise CT directed stereotactic placement of radioactive sources in a predetermined target volume. We use low activity (1-2 millicurie/speed) sources of 125I loaded in silastic catheters, which are positioned in a parallel array in the target. Positioning of such multiple sources toward the periphery of the volume enhances achievable dose homogeneity. Seeds of various activities can be differentially loaded into each catheter and the catheters can be positioned at various radii from the central target so that the treated volume corresponds to the identified (often irregular) target volume. Although the implant is designed to be permanent, the sources can be removed easily in a second procedure.

A block localizer for easy and reproducible transfer of shielding block positions from simulator to treatment unit.

For the localization of shielding blocks at the simulator a special tray called "block localizer" was constructed. It can be attached to the simulator head by means of magnets in the top frame. The "block localizer" provides a platform at a distance from the X-ray source corresponding to the source-to-block distance at the treatment unit. On the platform templates made from 0.5 mm brass to the size of the block bases can be positioned under fluoroscopy. Before taking the beam localization exposure a low sensitivity film is placed in a slit in the platform. On this film the template (block) position is recorded. For treatment the developed film is attached to the block tray and the shielding block inserted according to the light shadow of the template projected from the film onto the patient. An easy and reliable positioning of blocks during repeated irradiations is thus provided. The block (template) position is also recorded on the beam localization film of the patient as a transparent shadow.

[Revision of the radium isodose atlas of Göttingen with a new calculation method]. / Revision des Göttinger Radiumisodosenatlas mit einem neuen Rechenverfahren.

The Radium Isodose Atlas of Göttingen was recalculated after having established a new computer program for the determination of dose distributions around intracavitary radium inserts. The program takes into consideration the complicated structure of the radium applicators type Buchler. The exactness of the program was checked by computing known radium sources and by detailed measures. The calculated dose values were reduced by 30% on an average with respect to the original measurements. This difference is explained mainly by the dependence on energy of the Cds dosimeter used at that time (1959). The clinically evaluated dose has not been changed (identical milligram element hours), however, a comparison of radium doses with other hospitals and with an additional percutaneous irradiation has become possible by the recalculation. Point A has been redefined according to international practice.