Dosimetry of rotational partial-skin electron irradiation.

BACKGROUND AND PURPOSE: Often, the most appropriate treatment for superficially and extensively spreading tumors of the skin is to use electron irradiation at enlarged distances. Rotational skin electron irradiation is a proven method for the treatment of the entire skin surface. We here report modifications of this technique in the set-up of partial-skin electron irradiation and the results of dosimetric examinations with regard to optimal shielding, dose profiles and depth dose curves under various irradiation conditions. MATERIALS AND METHODS: Irradiation was performed using electron beams with nominal energies of 6 MeV from a linear accelerator. The phantom was located on a rotating platform at a source-surface distance SSD=300 cm. A horizontal slit aperture (height: 32 cm) within a 2 cm thick polymethylmethacrylate (PMMA) shielding plate near the phantom was used to define the size of the irradiated region. Influences on dose distributions due to scattering processes on the PMMA edges were investigated using a flat ionization chamber and films. Absolute dose measurements and film calibration were made with the flat chamber. The quality of bremsstrahlung radiation behind the shielding was determined with a thimble ionization chamber in the phantom. RESULTS AND CONCLUSIONS: The results of rotational partial-skin electron irradiation reveal some of the investigated shielding geometries to be optimal. Depth dose distributions and dose rates correspond to the results obtained in total skin electron rotational irradiation. It is possible to apply the dose superficially in the first millimeters of the skin; the dose maximum is located at a depth of 0-2 mm, the 80% isodose at 9 mm. The amount of bremsstrahlung contamination is 2.5%. The local amount of absorbed dose per monitor unit depends strongly on patient/phantom cross-section geometry. At our institute, rotational partial-skin electron irradiation was implemented into clinical routine in 1997.

A simple and reliable method to simulate multileaf-plans.

To combine conformity of the irradiation with time effectiveness during treatment, the use of multileaf collimators has become more and more common. However, the simulation of the leaf positions is rather difficult compared with metal blocks. We developed a new method, utilizing an acrylic template in which the contour produced by the leaves is machined in the form of a 1 mm groove by a computerized numerically controlled milling machine. This template is then inserted into a mount attached to the simulator. The main advantages are the errorfree, direct communication from the therapy planning system to the milling machine via a network, the possibility to transfer the contour to the skin, and the documentation on the simulation film. The use of templates is reliable and, e.g. the costs of the materials are lower than for block simulation.

Comparison of calibration procedures for 192Ir high-dose-rate brachytherapy sources.

PURPOSE: To compare the efficacy of different calibration procedures for 192Ir high-dose-rate (HDR) brachytherapy sources and to determine their suitability in clinical practice. In addition the manufacturer's calibration is compared with our experimental measurements so that the accuracy of the source strength on the manufacturer certificate which is supplied with each new 192Ir source can be accessed. METHODS AND MATERIALS: We compared three types of calibration system: well-type chambers (HDR-1000 and SDS), cylindrical phantom, and plate phantom. The total number of measurements we obtained was 365. The number of sources used for the calibration procedure comparison was 20 and the number used for comparison with the manufacturer's calibration was 46. This study was made during the period 1989-1997. Also, Physikalisch-Technische Bundesanstalt (PTB) calibrated one of our sources using their PTB protocol so that the results could be compared with our own. RESULTS: The sensitivity of each system on scattering from the room walls was studied. It was found that different minimum lateral distances from the walls were required for the different systems tested: 15 cm and 25 cm for the well-type chambers, 75 cm for the cylindrical phantom, and 13 cm for the plate phantom. The minimum thickness required to reach phantom scattering saturation for the plate phantom setup is 24 cm. The influence of the applicator material used in the calibration setup was found to be 1.7% for the stainless steel dosimetry applicator compared to the plastic 5F applicator. The accuracy of source positioning within the applicator can lead to dosimetric errors of +/-1.2% for the radial distance of 8.0 cm used with both solid phantoms. The change in the response for both well-type chambers was only 0.1% for changes in the source position within +/-7.5 mm around the response peak. Good agreement was found between all dosimetry systems included in our study. Taking the HDR-1000 well-type chamber results as a reference, we observed percentage root mean square (RMS) values of 0.11% for the SDS well-type chamber, 0.44% for the cylindrical, and 0.60% for the plate phantom setup. A comparison of our results using the cylindrical phantom with those of the manufacturer showed a percentage RMS value of 3.3% with a percentage fractional error range of -13.0% to +6.0%. The comparison of our calibration results with those of PTB gave deviations less than 0.4% for all systems. CONCLUSIONS: Our results have shown that with careful use of all calibration system protocols an accurate determination of source strength can be obtained. However, the manufacturer's calibration is not accurate enough on its own, and it should be mandatory for clinics to always measure the source strength of newly delivered 192Ir brachytherapy sources. The influence of the applicator material, metal or plastic, should always be taken into account.

[A multileaf collimator: additional quality assurance in a GE linear accelerator]. / Multileafkollimator: erweiterte Qualitätssicherung an einem GE-Linearbeschleuniger.

BACKGROUND: In comparison to a conventional collimator, a multileaf collimator demands a great deal of quality assurance procedures due to its large number of leaves. A concept for daily quality assurance is presented, mainly concerning the positioning accuracy of the leaves. MATERIAL AND METHODS: Two leaf configurations including maximal opening as well as overtravel of single leaves, at a maximal opening of the jaws, are transmitted online in daily exchange from our record- and verify system to the linac. Aiming at a special test phantom a visual control of the positioning accuracy is performed. The leaf positioning is documented by an electronic portal imaging system and is compared with a reference shot by superposition of a grid. RESULTS: This method of quality assurance offers a fast and effective possibility to guarantee the proper function of the whole system by simulating the routine treatment situation. CONCLUSIONS: Compared to a conventional collimator only a slightly greater workload is needed for quality assurance of a multileaf collimator.

[A quality assurance program of simulators in radiation therapy. 2. Extent and results of long-term quality assurance test on a therapy simulator]. / Ein Qualitätssicherungsprogramm für Simulatoren in der Strahlentherapie. Teil 2: Umfang und Ergebnisse langjähriger Konstanzprüfungen an einem Therapiesimulator.

BACKGROUND: Since 1990 we follow a quality assurance program with periodical tests of functional performance values of a 16-year-old simulator. MATERIAL AND METHOD: For this purpose we adopted and modified German standards for quality assurance on linear accelerators and international standards elaborated for simulators (International Electrotechnical Commission). The tests are subdivided into daily visual checks (light field indication, optical distance indicator, isocentre-indicating devices, indication of gantry and collimator angles) and monthly and annually tests of relevant simulator parameters. Some important examples demonstrate the small variation of parameters over 6 years: position of the light field centre when rotating the collimator, diameter of the isocentre circle when rotating the gantry, accuracy of the isocentre indication device, and coincidence of light field and simulated radiation field. RESULTS: As an important result we can state, that by these rigid periodic tests it was possible to detect and compensate deteriorations of simulators quality rapidly. CONCLUSIONS: Technical improvements and specific calling-in of maintenance personnel whenever felt appropriate provided performance characteristics of our old simulator which are required by international recommendations as a basis for modern radiotherapy.

[A quality assurance program for simulators in radiotherapy. 1: The universal phantom for quality control of therapy simulators and teletherapy equipment]. / Ein Qualitätssicherungsprogramm für Simulatoren in der Strahlentherapie. Teil 1: Das Universalphantom zur Qualitätskontrolle an Therapiesimulatoren und Teletherapiegeräten.

A new phantom is described which shall be utilized to check geometric parameters of simulators and therapy units. Because of its mechanical assembly technique construction it is possible to use any part of the system fixed on a base plate which has to be adjusted only once. The application of the different parts of the phantom will be illustrated by some check examples on simulators and linacs. An intercomparison of quality control data referring to therapy simulators in various centres of South Germany has been started whereby the usability and reliability of the phantom also will be tested. The preliminary results give objective reasons for its suitability to study generally the performance tolerances of simulators with regard to quality assurance in radiotherapy.

[The surface dosage in the radiotherapy of a small breast carcinoma with 6-MV x-rays. A contribution to quality assurance in radiotherapy]. / Oberflächendosis bei Strahlentherapie des kleinen Mammakarzinoms mit 6-MV-Röntgenstrahlen. Ein Beitrag zur Qualitätssicherung in der Strahlentherapie.

Radiation treatment of breast cancer with high-energy X-rays in breast conserving therapy tends to an under-dosage of the skin region. Results of our measurements with patients and in water demonstrate that the skin receives only 40% of the reference dose applying tangential opposed wedged fields with 6 MV X-rays. Doing so we must accept a dose inhomogeneity of about 60% in the target volume. That may cause an increased recurrence rate especially for superficially situated tumors and/or in the case of intraoperatively disseminated tumor cells. We show that utilizing bolus material of only 5 mm water equivalence on the skin above the tumor bed reduces the dose inhomogeneity to 8%.

[Mounting devices for screening blocks on the linear accelerator--the drawbacks and modifications]. / Haltevorrichtungen für Abschirmblöcke am Linearbeschleuniger--Mängel und Modifikationen.

Block mounting devices for linacs--drawbacks and modifications: Shielding blocks are often used in radiotherapy. They are fixed on the linac's head by means of standard accessory mounting devices. The disadvantages of these parts are discussed and two modifications are presented: One device with adjustable focus-support distance for standard lead blocks and another with gliding planes to insert easily the heavy individually formed shielding blocks. Errors in mounting of all these devices are very dangerous, so we developed a simple locking lever providing a high security.

[A computer-supported method for the direct production of shielding blocks and compensators for radiotherapy]. / Computergestützte Methode zur unmittelbaren Herstellung von Abschirmblöcken und Kompensatoren für die Strahlentherapie.

The homogeneous irradiation of irregularly shaped targets with high doses and the simultaneous protection of the equally irregular surrounding high risk regions can be carried out with the help of shielding blocks and compensation filters specially cut for individual use. The machines offered on the market that help in their production have serious drawbacks in spite of the advanced technology used. These machines with limited usage are not only expensive but also require personnel to control the several steps the production of the blocks and filters involves. Safer radiotherapy however demands a more intensive use of these protective measures. However, it is now possible to overcome the drawbacks we have mentioned. The projection of the contours of the block marked on the X-ray on the shielding blocks can be carried out directly with the help of the computer that directs the radiotherapy. This computer could be connected to a multipurpose CNC milling machine. Similarly, one could, in the near future, use the data from computed tomograms in the computer used to plan the radiotherapy to mould three dimensional compensation filters. The cutting out of the negative moulds from Styrodur, the casting with fluid metal alloys, the imprecision that follows, as well as the health hazards and environmental pollution attached to his process could then be avoided, not to speak of the possibility of total reutilization of the rests. The positioning of the block ensemble can follow effortlessly and with precision on the lucite plates which have been drilled to accommodate the blocks.

Considerations on recombination losses in ionization chambers using pulsed electron beams with beam scanning.

Based on Boag's theory, referring to saturation effects in ionization chambers with pulsed electron beams, we developed a simple method of determining recombination losses. The correction factors for chamber readings result from slope and intercept of a regression line M1/3 = f(M/V) obtained with at least two different chamber voltages, V, and chamber readings, M. The deviation of these correction factors, compared with those of Boag's precise theory, does not exceed +/- 0.5%.

[Individual fixation during irradiation following breast-conserving surgery. Methodology and clinical results in 50 patients]. / Individuelle Fixation bei Bestrahlung nach brusterhaltender Operation. Methodik und erste klinische Ergebnisse an 50 Patientinnen.

From February, 1986 through October, 1987, fifty patients with mammary carcinomas of the stages T1N0M0, T2N0M0, T1N1M0 and T2N1M0 were irradiated after breast-preserving surgery. The irradiation conditions were standardized with cobalt-60 gamma radiation or photons emitted by a linear accelerator and fixed oblique position in a specially manufactured, individual rigid foam bed. The position was checked every week at the therapy simulator with an analog tomographic attachment. The method, positioning in a 30 degrees oblique position and immobilization in an individually moulded rigid foam bed is described, and the necessity of regular tomographic controls in treatment position is underlined. Because of the short follow-up time of eight months, clinical results can only be presented with regard to early side effects and cosmetic results. A final assessment of this method is therefore not yet possible.

[Dosimetry on a rotating phantom in remote electron irradiation]. / Zur Dosismessung im rotierenden Phantom bei Elektronen-Abstandsbestrahlung.

The Therac-20 Saturne produces a high dose rate electron radiation allowing a whole-skin electron irradiation. The problem of producing large fields enclosing the whole patient has been solved by superposition of two tilted stationary fields. The angle regulations are optimized by a computer program fed with the TLD values of transverse dose distributions. A homogeneous irradiation is obtained by the patient's rotation on a turntable within the radiation field. In case of a distance of 3 m between focus and skin, the irradiation times are only 5 min for a body surface dose of 1 Gy.

A new approach to overcome the inconveniences in electron dosimetry associated with the beam scanning technique in linacs.

The therapeutic advantages of scanned electron beams from medical linear accelerators are accompanied by difficulties in electron dosimetry. Owing to short-time dose inhomogeneities in the treatment field, implicit in the system, time-consuming point-by-point measurements of dose distributions are normally required. However, by triggering the measuring time synchronously with the slow scan frequency, the time necessary to record reliable dose distributions can be shortened considerably. The instrumentation for the computer-controlled dosimetry system developed for this task is described. The results of our measurements show that reproducibility in dose within +/- 1% is achievable in less than 6.3 s per point for scanned electron beams.

[First experience with the electron accelerator "Therac 20-Saturne" in clinical practice. 1st communication: technical data and dosimetry of photon radiation (author's transl)]. / Erste Erfahrungen mit dem Elektronen-Linearbeschleuniger Therac 20-Saturne im klinischen Betrieb. 1. Mitteilung: Technische Daten und Dosimetrie der Photonenstrahlung.

The fundamental principles of construction, the operational data, handling and safety devices of the first linear electron accelerator type Therac 20-Saturne (CGR MeV) installed in Germany are described. Relevant characteristics of the radiation field for therapy with 12 and 18-MV photons and also the dosimetric data are reported. Observance of the tolerances recommended by the IEC is guaranteed with this accelerator.

[Linear accelerators in oncoradiology. Comparison of the effciency of four clinically utilized electron linear accelerators with energies up to 20 MeV (author's transl)]. / Der Linearbeschleuniger in der Onkoradiologie. Ein Leistungsvergleich von vier medizinisch genutzten Elektronen-Linearbeschleunigern mit Energien bis 20 MeV.

Further development of linear accelerators for medical purposes results in leaving off the principle of circular acceleration. The present investigation, comparing four clinically utilized electron linear accelerators with different constructional systems, reveals a large scale of clinical applicability, advantages and disadvantages partly compensating one another. None of the four machines examined offers a total of decisive advantages as compared to the other ones. Some essential data from the side of radiation exit are tested: depth dose data and photon contamination during operation with electrons, field sizes and homogeneity, penumbral width and penetration power for electron and X radiations. The appreciation of electron therapy by the radiologist may be a factor as well the handling of the machine, but in many cases the space available in an existing locality or the confidence in the ready service of a contracting firm, considerations concerning failure periods, estimate of repair and subsequent costs are influencing the decision for a certain type of equipment.