Björn Poppe

On the sensitivity of common gamma-index evaluation methods to MLC misalignments in Rapidarc quality assurance

PURPOSE In this study the effects of small systematic MLC misalignments and gravitational errors on the quality of Rapidarc treatment plan delivery are investigated with respect to verification measurements with two detector arrays and the evaluation of clinical significance of the error-induced deviations. METHODS Five prostate and six head and neck plans were modified by means of three error types: (1) both MLC banks are opened, respectively, in opposing directions, resulting in larger fields; (2) both MLC banks are closed, resulting in smaller fields; and (3) both MLC banks are shifted for lateral gantry angles, respectively, in the same direction to simulate the effects of gravity on the leaves. Measurements were evaluated with respect to a gamma-index of 3%/3 mm and 2%/2 mm. Dose in the modified plans was recalculated and the resulting dose volume histograms for target and critical structures were compared to those of the unaltered plans. RESULTS The smallest introduced leaf position deviations which fail the >90% criterion for a gamma-index of 2%/2 mm are: (1) 1 mm; (2) 0.5 mm for prostate and 1.0 mm for head and neck cases; and (3) 3 mm corresponding to the error types, respectively. These errors would lead to significant changes in mean PTV dose and would not be detected with the more commonly used 3%/3 mm gamma-index criterion. CONCLUSIONS A stricter gamma-index (2%/2 mm) is necessary in order to detect positional errors of the MLC. Nevertheless, the quality assurance procedure of Rapidarc treatment plans must include a thorough examination of where dose discrepancies occur, and professional judgment is needed when interpreting the gamma-index analysis, since even a >90% passing rate using the 2%/2 mm gamma-index criterion does not guarantee the absence of clinically significance dose deviation.

The effect of a carbon-fiber couch on the depth-dose curves and transmission properties for megavoltage photon beams.

Purpose:To investigate the attenuation of a carbon-fiber tabletop and a combiboard, alongside with the depth-dose profile in a solid-water phantom.Material and Methods:Depth-dose measurements were performed with a Roos chamber for 6- and 10-MV beams for a typical field size (15 cm × 15 cm, SSD [source-surface distance] 100 cm). A rigid-stem ionization chamber was used to measure transmission factors.Results:Transmission factors varied between 93.6% and 97.3% for the 6-MV beam, and 95.1% and 97.7% for the 10-MV photon beam. The lowest transmission factors were observed for the oblique gantry angle of 150° with the table-combiboard combination. The surface dose normalized to a depth of 5 cm increased from 59.4% (without table, 0° gantry), to 108.6% (tabletop present, 180° gantry), and further to 120% (table-combiboard combination) for 6-MV photon beam. For 10 MV, the increase was from 39.6% (without table), to 88.9% (with table), and to 105.6% (table-combiboard combination). For the 150° angle (tablecombiboard combination), the dose increased from 59.4% to 120% (6 MV) and from 39% to 108.1% (10 MV).Conclusion:Transmission factors for tabletops and accessories directly interfering with the treatment beam should be measured and implemented into the treatment-planning process. The increased surface dose to the skin should be considered.Ziel:In dieser Arbeit werden die Absorptionseigenschaften sowie der Dosisaufbaueffekt eines neuen Bestrahlungstisches aus Carbon in Kombination mit einer Lagerungshilfe aus demselben Material (Combiboard) analysiert.Material und Methodik:Mit einer Roos-Kammer wurden Tiefendosiskurven für ein Bestrahlungsfeld typischer Größe (15 cm × 15 cm, SSD [Oberflächen-Haut-Abstand] 100 cm) für 6 MV und 10 MV untersucht. Die Transmission wurde mit Hilfe einer Stielionisationskammer gemessen.Ergebnisse:Die ermittelten Transmissionswerte variierten zwischen 93,6% und 97,3% für 6 MV und zwischen 95,1% und 97,7% für 10 MV. Die niedrigsten Transmissionswerte wurden für die schräge Einstrahlung von 150° durch Tisch und Combiboard gefunden. Die Oberflächendosis, bezogen auf eine Tiefe von 5 cm, erhöhte sich für 6 MV von 59,4% (ohne Tisch, 0°-Gantry) auf 108,6% (Tisch, 180°-Gantry) und weiter auf 120% (Tisch-Combiboard-Kombination, 180°-Gantry). Für 10 MV wurden Oberflächendosen von 39,6% (ohne Tisch), 88,9% (mit Tisch) und 105,6% (Tisch-Combiboard-Kombination) ermittelt. Für den schrägen Einstrahlwinkel erhöhte sich die Dosis auf 120% (Tisch-Combiboard-Kombination) für 6 MV bzw. auf 108,1% für 10 MV.Schlussfolgerung:Tische und Hilfsmittel aus Carbon können die dosimetrischen Eigenschaften des Strahlenbündels merklich beeinflussen und sollten für jeden Tisch individuell untersucht werden. Eine mögliche Erhöhung der Hautoberflächendosis sollte berücksichtigt werden.

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.

The dose response functions of ionization chambers in photon dosimetry – Gaussian or non-Gaussian?

This study is concerned with the spatial resolution of air-filled ionization chambers in photon-beam dosimetry, i.e. with their dose response functions. These act as convolution kernels K(x,y), transforming true dose profiles D(x,y) into the measured signal profiles M(x,y). One-dimensional dose response functions have been experimentally determined for nine types of cylindrical ionization chambers both in their lateral and longitudinal directions, as well as across two plane-parallel chambers and for the single chambers of two 2D arrays. All these 1D dose response functions are closely described by Gaussian functions. The associated energy-dependent values of the standard deviations σ have been measured for 6 and 15 MV photons with an uncertainty of 0.02mm. At depths beyond secondary electron fluence build-up, there was no detectable depth dependence of the σ values. The general occurrence of Gaussian dose response functions, their extension beyond the geometrical boundaries of the chambers, and the energy dependence of their standard deviations can be understood by considering the underlying system of convolutions, which is the origin of the influences of secondary electron transport. Monte-Carlo simulations of the convolution kernels for a cylindrical, a square, and a flat ionization chamber and their Fourier analysis have been employed to show that the Gaussian convolution kernels are approximations to the true dose response functions, valid in the clinically relevant domain of the spatial frequency. This paper is conceived as the starting point for the deconvolution methods to be described in a further publication.

Performance parameters of a liquid filled ionization chamber array

PURPOSE In this work, the properties of the two-dimensional liquid filled ionization chamber array Octavius 1000SRS (PTW-Freiburg, Germany) for use in clinical photon-beam dosimetry are investigated. METHODS Measurements were carried out at an Elekta Synergy and Siemens Primus accelerator. For measurements of stability, linearity, and saturation effects of the 1000SRS array a Semiflex 31013 ionization chamber (PTW-Freiburg, Germany) was used as a reference. The effective point of measurement was determined by TPR measurements of the array in comparison with a Roos chamber (type 31004, PTW-Freiburg, Germany). The response of the array with varying field size and depth of measurement was evaluated using a Semiflex 31010 ionization chamber as a reference. Output factor measurements were carried out with a Semiflex 31010 ionization chamber, a diode (type 60012, PTW-Freiburg, Germany), and the detector array under investigation. The dose response function for a single detector of the array was determined by measuring 1 cm wide slit-beam dose profiles and comparing them against diode-measured profiles. Theoretical aspects of the low pass properties and of the sampling frequency of the detector array were evaluated. Dose profiles measured with the array and the diode detector were compared, and an intensity modulated radiation therapy (IMRT) field was verified using the Gamma-Index method and the visualization of line dose profiles. RESULTS The array showed a short and long term stability better than 0.1% and 0.2%, respectively. Fluctuations in linearity were found to be within ±0.2% for the vendor specified dose range. Saturation effects were found to be similar to those reported in other studies for liquid-filled ionization chambers. The detectors relative response varied with field size and depth of measurement, showing a small energy dependence accounting for maximum signal deviations of ±2.6% from the reference condition for the setup used. The σ-values of the Gaussian dose response function for a single detector of the array were found to be (0.72±0.25) mm at 6 MV and (0.74±0.25) mm at 15 MV and the corresponding low pass cutoff frequencies are 0.22 and 0.21 mm(-1), respectively. For the inner 5×5 cm2 region and the outer 11×11 cm2 region of the array the Nyquist theorem is fulfilled for maximum sampling frequencies of 0.2 and 0.1 mm(-1), respectively. An IMRT field verification with a Gamma-Index analysis yielded a passing rate of 95.2% for a 3 mm∕3% criterion with a TPS calculation as reference. CONCLUSIONS This study shows the applicability of the Octavius 1000SRS in modern dosimetry. Output factor and dose profile measurements illustrated the applicability of the array in small field and stereotactic dosimetry. The high spatial resolution ensures adequate measurements of dose profiles in regular and intensity modulated photon-beam fields.

Comparison of activation products and induced dose rates in different high-energy medical linear accelerators.

Sequences of in situ gamma spectra, accompanied by continuous dose rate measurements, have been obtained at the isocenters of four different brands of high-energy medical linear accelerators shortly after beam-off in order to study the effects of radioactivation. Spectral analysis revealed up to 20 different radionuclides per machine, with a total of 21 found isotopes having half-lives between 2.3 min and 5.3 y. Important isotopes as judged from activity, dose rate, and half-life were 28Al, 54Mn, 56Mn, 57Ni, 60Co, 62Cu, 64Cu, 124Sb, 187W, and 196Au. Dose rates at isocenter calculated from the results of spectrum analysis ranged between 0.78 and 3.16 &mgr;Sv h−1 after beam-off, decaying to values between 0.18 and 0.54 &mgr;Sv h−1 within 30 min. Measured dose rates were systematically higher by up to a factor of 2, which is attributed mainly to the effect of beta radiation. No systematic dependence on machine properties or manufacturer could be identified. Assuming realistic working scenarios, absorbed dose values for the radiotherapy technologist staff range between 0.62 and 2.53 mSv y−1.

Activation processes in a medical linear accelerator and spatial distribution of activation products

Activation products have been identified by in situ gamma spectroscopy at the isocentre of a medical linear accelerator shortly after termination of a high energy photon beam irradiation with 15 x 15 cm field size. Spectra have been recorded either with an open or with a closed collimator. Whilst some activation products disappear from the spectrum with closed collimator or exhibit reduced count rates, others remain with identical intensity. The former isotopes are neutron-deficient and mostly decay by positron emission or electron capture; the latter have neutron excess and decay by beta(-) emission. This new finding is consistent with the assumption that photons in the primary beam produce activation products by (gamma, n) reactions in the treatment head and subsequently the neutrons created in these processes undergo (n, gamma) reactions creating activation products in a much larger area. These findings are expected to be generally applicable to all medical high energy linear accelerators.

Conversion coefficients for the estimation of effective doses in intraoral and panoramic dental radiology from dose-area product values

Conversion coefficients for the estimation of effective doses in intraoral and panoramic dental radiology from dose-area product (DAP) values were determined by measuring organ-absorbed doses and the corresponding DAP values. Measurements were performed for all standard intraoral radiological projections and standard panoramic examination at different exposure parameters. Organ-absorbed doses were measured using thermoluminescent detectors and an adult anthropomorphic phantom specially designed for dosimetric study in dental radiology. Different techniques for the calculation of effective doses were evaluated. Conversion coefficients derived from this study range from 0.008 to 0.132 microSv mGy(-1) cm(-2) for intraoral radiography and 0.055 to 0.238 microSv mGy(-1) cm(-2) for panoramic radiography.

Experimental study on photon-beam peripheral doses, their components and some possibilities for their reduction

The component analysis of the peripheral doses produced at typical accelerators such as the Siemens Primus 6/15 is regarded as an approach enabling technical strategies towards the reduction of second malignancies associated with photon beam radiotherapy. Suitable phantom and detector arrangements have been applied to show that the unavoidable peripheral dose contribution due to photon scattering from the directly irradiated part of the body or phantom does not constitute the entirety of the peripheral doses. Rather, there are peripheral dose contributions due to beam head leakage and to extrafocal radiation which can be regarded as partly avoidable. Simple methods of reducing beam head leakage from the Siemens Primus 6/15 linac are, for the crossplane direction, to install a pair of adjustable shielding blocks in the accessory holder and, for the inplane direction, to close all out-of-field leaf pairs of the multileaf collimator via the treatment planning system software. The relative efficiency of these shielding measures is largest in the case of small unavoidable dose contributions, i.e. for small fields and small depths. Methods of avoiding doses coming from extrafocal radiation are also envisaged for future research.