Armand Djouguela

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.

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.

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.

WE-D-AUD B-02: On the Influences of the Detector Size and Sampling Frequency On IMRT Verifications with 2D Arrays

Purpose: The influences of single detector size and sampling frequency of detector arrays on IMRT verification are discussed. Method and Materials: The 2D‐ARRAY Type 10024 (PTW‐Freiburg, single chamber cross section 5×5 mm2, center‐to‐center distance between chambers 10 mm) is analyzed as an example. Due to scattering effects at the ridges between the chambers, the Full‐Width‐At‐Half‐Maximum of the lateral detector response function is approximately 7 mm. By shifting the array in 3 steps of 5 mm, re‐measuring the dose distribution and arranging the data in a matrix, the sampling rate of 0.1 mm−1 can be increased to 0.2 mm−1 (sampling distance of 5mm). Results: The measurement process with detector arrays can be described as a two step process: 1. Convolution of the dose distribution with the response function of a single chamber of the array. 2. Sampling of the convolved dose distribution with the chosen sampling rate. Step 1 results in a small deviation of measured and real doses in the region of steep dose gradients. A mathematical model is introduced to estimate the deviation by consideration of detector size and initial dose penumbra. For the chosen array, the deviation in the region of clinical relevant doses is shown to be approximately 1mm. In the Fourier space, step 2 leads to a periodic replication of the Fourier Transform of the convolved dose distribution in intervals of the sampling frequency 0.2 mm−1. For various IMRT distributions we can show that the maximum spatial frequency does not exceed 0.1mm−1. According means that the sampling frequency is sufficient. Conclusion: With a sampling distance of 5 mm, the measurement of typical IMRT dose distributions with the 2D‐ARRAY complies with the Nyquist criterion. The results can be generalized to other arrays to analyze the limits of applicability for IMRT verification measurements.

SU‐FF‐T‐414: The Dose Area Product in Radiation Therapy‐ a New Concept for the Parameterisation of Small Fields

Purpose: To establish a new parameterisation of small fields. Material and Methods: The traditional parameterisation of a narrow photon field via the central axis dose and the relative transversal dose profile has met considerable methodical complications. These difficulties are to find a sufficiently small detector, to adjust the detector accurately on the axis of the narrow beam and to find a detector material not responding to lateral changes of the electron spectrum within the small field. These obstacles can be avoided by reconsidering the parameterisation of the narrow‐field dose distribution. The new parameter recommended for characterising the absolute dose values in a plane perpendicular to the beam axis is the dose‐area product DAP (the area integral of the dose in this plane). It can be measured with a flat ionisation chamber of large cross section of the sensitive volume. Results and Conclusions: The radial adjustment of the large area chamber is by no means critical. The dose‐area product provides a simple normalisation of the relative transversal dose distribution which can be measured with radiochromic film. We have investigated the abilities of a large‐area flat ionisation chamber of PTW Freiburg (PTW TM 34070‐2,5) of 8,1 cm diameter and 2 mm thickness of the sensitive volume to measure the DAP of narrow photon beams with side lengths up to 5 cm. A modified output factor has been defined as the quotient of the DAP, measured at 5 cm phantom depth for SSD 100 cm distance of the phantom, and the monitor reading. Besides the useful feature of the DAP is its direct measurability during patient treatment by means of the DAVID chamber, an on‐line monitor arranged in the accessory holder, so that non‐negligible deviations of the actual from the chosen field size of narrow photon fields can be immediately detected.

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.