Dietrich Harder

Two‐dimensional ionization chamber arrays for IMRT plan verification

In this paper we describe a concept for dosimetric treatment plan verification using two-dimensional ionization chamber arrays. Two different versions of the 2D-ARRAY (PTW-Freiburg, Germany) will be presented, a matrix of 16 x 16 chambers (chamber cross section 8 mm x 8 mm; the distance between chamber centers, 16 mm) and a matrix of 27 x 27 chambers (chamber cross section 5 mm x 5 mm; the distance between chamber centers is 10 mm). The two-dimensional response function of a single chamber is experimentally determined by scanning it with a slit beam. For dosimetric plan verification, the expected two-dimensional distribution of the array signals is calculated via convolution of the planned dose distribution, obtained from the treatment planning system, with the two-dimensional response function of a single chamber. By comparing the measured two-dimensional distribution of the array signals with the expected one, a distribution of deviations is obtained that can be subjected to verification criteria, such as the gamma index criterion. As an example, this verification method is discussed for one sequence of an IMRT plan. The error detection capability is demonstrated in a case study. Both versions of two-dimensional ionization chamber arrays, together with the developed treatment plan verification strategy, have been found to provide a suitable and easy-to-handle quality assurance instrument for IMRT.

A Triple Gaussian Pencil beam Model for Photon beam Treatment Planning

Abstract The transverse profiles of pencil beams are often represented by Gaussian functions in order to speed up electron beam treatment planning algorithms, because convolutions of Gaussions with most beam fluence profiles can be performed analytically. We extend this approach to high-energy photon radiations. Monte-Carlo generated transverse profiles of photon pencil beams are adequately represented by a sum of three Gaussian functions, whose coefficients and parameters have been optimized using Fourier transform methods. The axial profile of the pencil beam is determined by the depth-dependent surface integral of the dose in the transverse plane. As a first application, the triple Gaussian pencil beam algorithm is used to demonstrate the field size dependence of the photon beam depth dose curve. Photon beams modified by wege filters or shielding blocks will be treated in a second communication.

Dosimetric characteristics of a new unshielded silicon diode and its application in clinical photon and electron beams

Shielded p-silicon diodes, frequently applied in general photon-beam dosimetry, show certain imperfections when applied in the small photon fields occurring in stereotactic or intensity modulated radiotherapy (IMRT), in electron beams and in the buildup region of photon beam dose distributions. Using as a study object the shielded p-silicon diode PTW 60008, well known for its reliable performance in general photon dosimetry, we have identified these imperfections as effects of electron scattering at the metallic parts of the shielding. In order to overcome these difficulties a new, unshielded diode PTW 60012 has been designed and manufactured by PTW Freiburg. By comparison with reference detectors, such as thimble and plane-parallel ionization chambers and a diamond detector, we could show the absence of these imperfections. An excellent performance of the new unshielded diode for the special dosimetric tasks in small photon fields, electron beams and build-up regions of photon beams has been observed. The new diode also has an improved angular response. However, due to its over-response to low-energy scattered photons, its recommended range of use does not include output factor measurements in large photon fields, although this effect can be compensated by a thin auxiliary lead shield.

Applications of a Triple Gaussian Pencil Beam Model for Photon Beam Treatment Planning

Abstract A new method for the calculation of dose distributions is applied, in which the transverse dose profile of the photon pencil beam is represented by the sum of three Gaussian functions with different r.m.s. widths. Thereby it is possible to perform the convolutions needed for treatment planning by analytical calculation. The analytical results are here given for Co-60, 6 MV, 8 MV, 16 MV and 18 MV photon beams with rectangular cross section, modified by penumbra, central fluence depression, wedge filters or rectangular satellite shielding blocks. The calculated dose values never show deviations from experimental data exceeding 2 % of the dose maximum. This consistent analytical approach provides computer economy along with reliability and accuracy, and further applications will be studied.

DAVID - a translucent multi-wire transmission ionization chamber for in-vivo verification of IMRT and conformal irradiation techniques

Permanent in vivo verification of IMRT photon beam profiles by a radiation detector with spatial resolution, positioned on the radiation entrance side of the patient, has not been clinically available so far. In this work we present the DAVID system, which is able to perform this quality assurance measurement while the patient is treated. The DAVID system is a flat, multi-wire transmission-type ionization chamber, placed in the accessory holder of the linear accelerator and constructed from translucent materials in order not to interfere with the light field. Each detection wire of the chamber is positioned exactly in the projection line of a MLC leaf pair, and the signal of each wire is proportional to the line integral of the ionization density along this wire. Thereby, each measurement channel essentially presents the line integral of the ionization density over the opening width of the associated leaf pair. The sum of all wire signals is a measure of the dose-area product of the transmitted photon beam and of the total radiant energy administered to the patient. After the dosimetric verification of an IMRT plan, the values measured by the DAVID system are stored as reference values. During daily treatment the signals are re-measured and compared to the reference values. A warning is output if there is a deviation beyond a threshold. The error detection capability is a leaf position error of less than 1 mm for an isocentric 1 cm x 1 cm field, and of 1 mm for an isocentric 20 cm x 20 cm field.

Polyethylene-based water-equivalent phantom material for X-ray dosimetry at tube voltages from 10 to 100 kV

A water-equivalent plastic material RW-1, in the form of thin foils as well as of thicker sheets, has been produced by melting powdered polyethylene together with CaCO3 and MgO. The mixture has been developed in a three-step procedure, starting with provisional mixtures and using data from their measured attenuation curves to determine the final composition. The attenuation curves of RW-1 and of water, as well as their backscatter factors, are in excellent agreement for x-ray tube voltages from 10 to 100 kV.

The Dose Dependence of Cyclobutane Dimer Induction and Repair in UVB-irradiated Human Keratinocytes¶

Abstract UVB and UVA components of the solar spectrum or from artificial UV-sources might be important etiological factors for the induction and development of skin cancer. In particular, deficiencies in the capacity to repair UV-induced DNA-lesions have been linked to this phenomenon. However, until now only limited data are available on the biological and physical parameters governing repair capacity. We have, therefore, developed a flowcytometric assay using fluorescence-labeled monoclonal antibodies to study the dose-dependence of induction and repair of UVB-induced cyclobutane pyrimidine dimers in a spontaneously immortalized keratinocytic cell line (HaCaT). Our results show that the kinetics of recognition and incision of UVB-induced DNA lesions slows down by a factor of about 3 in a dose range of 100–800 J m−2. Furthermore, a thorough analysis of repair kinetics indicates that this reduction in repair capacity might not be dependent on saturation of enzymatic repair capacity (Michalis–Menten) but may be caused by a UV-induced impairment of enzymes involved in DNA repair. Because this effect is evident in vitro at doses comparable to the minimal erythemal dose in vivo, our results might have significant impact on risk assessment for UV-induced carcinogenesis.

Clinical performance of a transmission detector array for the permanent supervision of IMRT deliveries.

BACKGROUND AND PURPOSE Clinical evaluation of a novel dosimetric accessory serving the permanent supervision of MLC function. MATERIALS AND METHODS The DAVID system (PTW-Freiburg, Germany) is a transparent, multi-wire transmission ionization chamber, placed in the accessory holder of the treatment head. Since each of the 37 individual wires is positioned exactly below the associated leaf pair of the MLC, its signal records the opening of this leaf pair during patient treatment. RESULTS The DAVID system closes a gap in the quality assurance program, permitting the permanent in-vivo verification of IMRT plans. During dosimetric plan verification with the 2D-ARRAY (PTW-Freiburg, Germany), reference values of the 37 DAVID signals are collected, with which the DAVID readings recorded during daily patient treatment are compared. This comparison is visually displayed in the control room, and warning and alarm levels of any discrepancies can be defined. The properties of the DAVID system as a transmission device, its sensitivity to beam delivery and leaflet errors as well as its stability have been analyzed for clinically relevant examples. In a recent version, the DAVID system has been equipped with 80 wires. CONCLUSIONS The DAVID system permits the on-line detection of clinically relevant MLC discrepancies in IMRT deliveries.

Internal scatter, the unavoidable major component of the peripheral dose in photon-beam radiotherapy

In clinical photon beams, the dose outside the geometrical field limits is produced by photons originating from (i) head leakage, (ii) scattering at the beam collimators and the flattening filter (head scatter) and (iii) scattering from the directly irradiated region of the patient or phantom (internal scatter). While the first two components can be modified, e.g. by reinforcement of shielding components or by re-modeling the filter system, internal scatter remains an unavoidable contributor to the peripheral dose. Its relative magnitude compared to the other components, its numerical variation with beam energy, field size and off-axis distance as well as its spectral distribution are evaluated in this study. We applied a detailed Monte Carlo (MC) model of our 6/15 MV Siemens Primus linear accelerator beam head, provided with ideal head leakage shielding conditions (multi-leaf collimator without gaps) to assess the head scatter contribution. Experimental values obtained under real shielding conditions were used to evaluate the head leakage contribution. It was found that the MC-computed internal scatter doses agree with the results of our previous measurements, that internal scatter is the major contributor to the peripheral dose in the near periphery while head leakage prevails in the far periphery, and that the lateral decline of the internal scatter dose can be represented by the sum of two exponentials, with an asymptotic tenth value of 18 to 19 cm. Internal scatter peripheral doses from various elementary beams are additive, so that their sum increases approximately in proportion with field size. The ratio between normalized internal scatter doses at 6 and 15 MV is approximately 2:1. The energy fluence spectra of the internal scatter component at all points of interest outside the field have peaks near 500 keV. The fact that the energy-shifted internal scatter constitutes the major contributor to the dose in the near periphery has a general bearing for dosimetry, i.e. for energy-dependent detector responses and dose conversion factors, for the relative biological effectiveness and for second primary malignancy risk estimates in the peripheral region.

Einfluß der Vielfachstreuung von Elektronen auf die Ionisation in gasgefüllten Hohlräumen

Beim Einbringen eines gasgefullten Hohlraums in ein elektronenbestrahltes, festes oder flussiges Material erhoht sich die Teilchenflusdichte im Volumenbereich des Hohlraums infolge geringerer Elektronenstreuung durch das Gas. Die im Gas erzeugte Ionisation ubertrifft daher den Wert, der dem ungestorten Strahlungsfeld vor Einbringen des Hohlraums entspricht. Nach der Theorie der Kleinwinkel-Vielfachstreuung wird der zusatzliche im Gasvolumen liegende Weg von Elektronen und die daraus resultierende zusatzliche Ionisation fur verschiedene Hohlraumformen berechnet. Die abgeleiteten Gesetzmasigkeiten ermoglichen ein quantitatives Verstandnis der von anderen Autoren experimentell beobachteten Streueffekte und die Abschatzung von Fehlereinflussen der Vielfachstreuung bei luftgefullten Ionisationskammern in der Elektronendosimetrie.