Ralf Kollhoff

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.

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.

Einsatz eines zweidimensionalen Ionisationskammer-Arrays zur Qualitätssicherung an medizinischen Linearbeschleunigern

Two-dimensional dosimetry measurements are an important tool of Quality Assurance in modern radiotherapy techniques, such as the Intensity Modulated Radiation Therapy (IMRT). Common procedures are usually based on the use of films or semiconductor arrays as dosimeters. This paper presents our experience with a two-dimensional ionization-chamber array. The methods presented here allow the daily use of the array for a constancy check of the accelerator. It is also shown that the position of the individual leafs of the multi-leaf collimator (MLC) can be verified to within +/- 1 mm of their calibration. A procedure for the measurements is described and discussed.

The Schwarzschild effect of the dosimetry film Kodak EDR 2.

The magnitude of the Schwarzschild effect or failure of the reciprocity law has been experimentally investigated for the dosimetry film EDR 2 from Kodak. When the dose rate applied to achieve a given dose was reduced by a factor of 12, the net optical density was reduced by up to 5%. The clinical importance of this effect is negligible as long as the films are calibrated at a value of the dose rate approximately representative of the dose rates occurring in the target volume, but in target regions of strongly reduced dose rate the Schwarzschild effect should be allowed for by a correction of the net optical density.

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.

Über das Auflösungsvermögen und die Empfindlichkeit eines zweidimensionalen Ionisationskammer-Arrays (PTW: Typ 10024)

The two-dimensional verification of intensity-modulated radiation plans is one of the major requirements for the safe application of this technique. The present study examines the resolution and sensitivity of a two-dimensional ionisation-chamber array (PTW2D-Array, type 10024), which can be used for plan verification instead of films. According to the Shannon-Nyquist theorem, the resolution of the 2D-Array is sufficient for dose distributions with a minimal field size of 2 cm x 2 cm. The minimal field size can be reduced to 1 cm x 1 cm by shifting the array 5 mm in the direction of the MLC movement and by repeating the measurements. The high sensitivity against a monitor decalibration for a single field of a sequence is demonstrated on the basis of an individual case. The minimal threshold for MLC misalignment detected by a chamber of the array is less than 1 mm. Therefore, the resolution capabilities of the 2D-Array are sufficient for most intensity-modulated radiation therapy (IMRT)fields.

Implementation of a complete IMRT QA program and clinical experience with the DAVID chamber for in-vivo verification of IMRT deliveries

A complete IMRT QA program based on a combination of the 2D-ARRAY and the DAVID system was implemented in our clinic to verify all IMRT deliveries. The QA program covers the whole treatment process from pretreatment dosimetric plan verification to daily in-vivo verification of the treatment delivery. This enables us to detect any discrepancies that would occur before and also during the treatment. The DAVID system has been used regularly in our clinic to verify the daily treatment delivery and has been shown to be a convenient and fast tool to pin-point errors that might have occurred during each fraction. The IMRT program presented in this study is very reliable and adds only minimal workload to the clinical routine.

SU-FF-T-140: Dosimetric IMRT Plan Verification and Daily Quality Assurance with a Two-Dimensional Ionization Chamber Array

Purpose:IMRT requires a more specified quality assurance program than the conventional techniques. In this work we present our solution for a full IMRTquality assurance program with two‐dimensional ionization chamber arrays (2D‐ARRAY, PTW‐Freiburg) containing daily checks and individual dosimetric plan verifications. Method and Materials: The used array (type 10024) has 27 × 27 ionization chambers arranged in a plane, with an entrance window of 5 mm × 5 mm each. The centers of the 729 single chambers are positioned at 10 mm distance from each other. Results: Our quality assurance program is divided into two parts: On a daily basis, as a morning check, the dose at the central axis, the flatness and symmetry as well as the MLCcalibration and light/radiation field congruence are evaluated by a single measurement. For a patient specific IMRT plan verification, the calculated dose distribution of the patient is exported to a CT containing the phantom set‐up with the 2D‐ARRAY. The corresponding IMRT sequence is exported to the linear accelerator. The values calculated for the plane of the 2D‐ARRAY and the values measured with it are then compared. Conclusion: The daily QA program has been extensively tested. All important field parameters can be obtained in a single measurement per energy. Furthermore MLC misalignments can be detected with an accuracy of less then 1.0 mm, allowing an early warning for a necessary MLC recalibration. The described program for IMRT plan verification has been proved to be very useful for an easy and fast pre‐treatment quality assurance. The error detection capabilities will be discussed in detail and shown to be sufficient for standard IMRT plans. Conflict of Interest: This work was performed in collaboration with PTW‐Freiburg Dr. Pychlau GmbH, Freiburg, Germany.

TU-C-T-6E-09: The DAVID System — a Device for In-Vivo Verification of IMRT and Conformal Irradiation Techniques

Purpose: While dosimetric plan verification ensures consistency between planned and measured dose distributions in the pretreatment phase, a daily in‐vivo verification of the beam profiles by a radiation detector positioned at the 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 a daily in‐vivo verification of IMRT beams in front of the patient during the treatment.Method and Materials: The DAVID system is a flat, translucent multi‐wire ionization chamber. It is placed in the accessory holder of the linear accelerator. Each detection wire of the chamber is positioned exactly in the projection line of two opposing leafs of the MLC. The measurement signal of each detection wire is directly proportional to the opening of the leaf pair. Therefore the number of measurement channels equals the number of leaf pairs. After a successful 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. In case of a deviation beyond a threshold a warning occurs. Results: The error detection capability for a 1 cm × 1 cm field is a leaf position error of less than 0.5 mm. The inherent limit due to electronic noise of the chamber is 1mm for a 20 cm × 20 cm field (all values related to the isocenter). Conclusion: Clinical examples demonstrate that the DAVID system is a relevant tool to improve the reliability of IMRTtreatments. Because the DAVID system operates as an ionization chamber, disadvantages which might be observed in other devices, such as aging, are not to be expected. Conflict of Interest: This work was performed in collaboration with PTW‐Freiburg Dr. Pychlau GmbH, Freiburg, Germany.