Antje Rühmann

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.

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.

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.

Low-energy photons in high-energy photon fields--Monte Carlo generated spectra and a new descriptive parameter.

The varying low-energy contribution to the photon spectra at points within and around radiotherapy photon fields is associated with variations in the responses of non-water equivalent dosimeters and in the water-to-material dose conversion factors for tissues such as the red bone marrow. In addition, the presence of low-energy photons in the photon spectrum enhances the RBE in general and in particular for the induction of second malignancies. The present study discusses the general rules valid for the low-energy spectral component of radiotherapeutic photon beams at points within and in the periphery of the treatment field, taking as an example the Siemens Primus linear accelerator at 6 MV and 15 MV. The photon spectra at these points and their typical variations due to the target system, attenuation, single and multiple Compton scattering, are described by the Monte Carlo method, using the code BEAMnrc/EGSnrc. A survey of the role of low energy photons in the spectra within and around radiotherapy fields is presented. In addition to the spectra, some data compression has proven useful to support the overview of the behaviour of the low-energy component. A characteristic indicator of the presence of low-energy photons is the dose fraction attributable to photons with energies not exceeding 200 keV, termed P(D)(200 keV). Its values are calculated for different depths and lateral positions within a water phantom. For a pencil beam of 6 or 15 MV primary photons in water, the radial distribution of P(D)(200 keV) is bellshaped, with a wide-ranging exponential tail of half value 6 to 7 cm. The P(D)(200 keV) value obtained on the central axis of a photon field shows an approximately proportional increase with field size. Out-of-field P(D)(200 keV) values are up to an order of magnitude higher than on the central axis for the same irradiation depth. The 2D pattern of P(D)(200 keV) for a radiotherapy field visualizes the regions, e.g. at the field margin, where changes of detector responses and dose conversion factors, as well as increases of the RBE have to be anticipated. Parameter P(D)(200 keV) can also be used as a guidance supporting the selection of a calibration geometry suitable for radiation dosimeters to be used in small radiation fields.

Enhanced accuracy of the permanent surveillance of IMRT deliveries by iterative deconvolution of DAVID chamber signal profiles

In vivo dosimetry systems, capable of permanently monitoring IMRT treatment deliveries throughout all fractions, are increasingly used in clinical practice. The first of these solutions is the DAVID system, a translucent multiwire ionization chamber placed in the accessory holder of the treatment head below the MLC. Each wire is exactly adjusted along the midline of its associated leaf pair, thereby generating a signal correlated with the aperture of this leaf pair. However, there is some blurring of the profile of the wire signals across the beam due to the lateral transport of scattered secondary electrons in the air gap of the DAVID chamber. This paper deals with a numerical correction by which this effect is eliminated. The true photon fluence profile is calculated from the measured signal profile by an iterative deconvolution algorithm, based upon the convolution kernel formed by the lateral wire signal profile when only one leaf pair is opened. Lateral fluence profiles are thereby obtained with increased resolution, and errors in MLC positioning are revealed with enhanced sensitivity. The needed computational time of less than 1 s has made it feasible to implement the deconvolution algorithm into the daily routine for the accurate surveillance of IMRT deliveries.

A direction-selective flattening filter for clinical photon beams. Monte Carlo evaluation of a new concept

A new concept for the design of flattening filters applied in the generation of 6 and 15 MV photon beams by clinical linear accelerators is evaluated by Monte Carlo simulation. The beam head of the Siemens Primus accelerator has been taken as the starting point for the study of the conceived beam head modifications. The direction-selective filter (DSF) system developed in this work is midway between the classical flattening filter (FF) by which homogeneous transversal dose profiles have been established, and the flattening filter-free (FFF) design, by which advantages such as increased dose rate and reduced production of leakage photons and photoneutrons per Gy in the irradiated region have been achieved, whereas dose profile flatness was abandoned. The DSF concept is based on the selective attenuation of bremsstrahlung photons depending on their direction of emission from the bremsstrahlung target, accomplished by means of newly designed small conical filters arranged close to the target. This results in the capture of large-angle scattered Compton photons from the filter in the primary collimator. Beam flatness has been obtained up to any field cross section which does not exceed a circle of 15 cm diameter at 100 cm focal distance, such as 10 × 10 cm(2), 4 × 14.5 cm(2) or less. This flatness offers simplicity of dosimetric verifications, online controls and plausibility estimates of the dose to the target volume. The concept can be utilized when the application of small- and medium-sized homogeneous fields is sufficient, e.g. in the treatment of prostate, brain, salivary gland, larynx and pharynx as well as pediatric tumors and for cranial or extracranial stereotactic treatments. Significant dose rate enhancement has been achieved compared with the FF system, with enhancement factors 1.67 (DSF) and 2.08 (FFF) for 6 MV, and 2.54 (DSF) and 3.96 (FFF) for 15 MV. Shortening the delivery time per fraction matters with regard to workflow in a radiotherapy department, patient comfort, reduction of errors due to patient movement and a slight, probably just noticable improvement of the treatment outcome due to radiobiological reasons. In comparison with the FF system, the number of head leakage photons per Gy in the irradiated region has been reduced at 15 MV by factors 1/2.54 (DSF) and 1/3.96 (FFF), and the source strength of photoneutrons was reduced by factors 1/2.81 (DSF) and 1/3.49 (FFF).

Mapping radiation quality inside photon-irradiated absorbers by means of a twin-chamber method

In photon-beam radiotherapy, the absorbed dose in an irradiated object contains a contribution by energy-degraded photons originating from Compton scatter processes at parts of the treatment head and within the absorber itself. These low-energy spectral components may lead to changes in the response of non-ideally water-equivalent radiation detectors, such as Si diodes and radiographic films, in the water/tissue dose conversion factors and in the relative biological effectiveness (RBE). As a simple means of accounting for these changes in spectral quality, the Monte Carlo calculated fraction of the kerma or absorbed dose contributed by scattered photons with energies not exceeding a certain cut-off value has previously been proposed as a useful parameter. In this paper, we present an equivalent experimental approach, providing a means for the spatial mapping of radiation quality. Its applicability will be demonstrated for the case of (60)Co and 6 MV photons. A twin-chamber combination of a Farmer type ionization chamber, equipped with a graphited PMMA outer electrode, and a chamber of the same design, but with an outer electrode made from copper, has been developed. The measured quantity is the signal ratio (SR) of the copper wall and graphited wall chambers. A correlation between the SR and the fraction of the air kerma respectively of the absorbed dose to water, contributed by photons with energies not exceeding 200 keV, has been established at a Theratron 780-C (60)Co teletherapy unit and at a Siemens Primus 6 MV linear accelerator. We also describe a two-dimensional version of the twin-chamber method using the PTW 2D-Array 256. Typical trends of parameter SR with depth and off-axis distance in water-equivalent phantoms have been observed. Thereby, a simple experimental method for the space-resolved assessment of the dose fraction attributable to low-energy Compton scattered photons can be presented as an innovative instrument of describing radiation quality in radiotherapy.

Lorentz function convolution kernel of narrow photon beam profiles in homogeneous and inhomogeneous media

A successful method for the mathematical description of the lateral dose profiles of narrow photon beams for IMRT treatment is herewith presented. Based on the measurement of dose profiles in homogeneous water equivalent absorbers and on their analysis by deconvolution in Fourier space, the Lorentz function 1/(x2+λ 2) has been identified, in a good approximation, as the convolution kernel of narrow photon beam dose profiles [1,2,3]. Its full half width is 2λ and its lateral tails are more strongly expressed compared to Gaussian kernels. Convolutions of this kernel with rectangular and wedge-shaped photon fluence profiles have been performed analytically, and good agreement of the resulting lateral dose profiles with measured dose profiles has been obtained at accelerators of various manufacturers by adjusting the λ values. Complete knowledge of all measured profiles has been stored in condensed form by storing the λ values, which vary with the size of the effective photon source of the accelerator, with photon energy and with absorber thickness. Lateral dose profiles of photon beams have also been measured in inhomogeneous absorbers, e.g. in a water equivalent phantom with an air layer of variable thickness in between phantom plates. By Fourier analysis, each dose profile measured at the bottom of such air layer has been identified as the sum of two components, one due to the secondary electrons moving away from the upper phantom plate, the other due to backscattered secondary electrons originating from the lower phantom plate. The variation of these components as a function of air layer thickness has been quantitatively evaluated in order to facilitate dose calculations for points at the bottom of an air-filled cavity.