PubMed Health⌕ Search

Biomedical subjects

D Georg

Publications and source records attributed to D Georg.

17 recordsLinked to original sources
PubMed2001

Quality control in interstitial brachytherapy of the breast using pulsed dose rate: treatment planning and dose delivery with an Ir-192 afterloading system.

BACKGROUND AND PURPOSE: In the Radiotherapy Department of Leuven, about 20% of all breast cancer patients treated with breast conserving surgery and external radiotherapy receive an additional boost with pulsed dose rate (PDR) Ir-192 brachytherapy. An investigation was performed to assess the accuracy of the delivered PDR brachytherapy treatment. Secondly, the feasibility of in vivo measurements during PDR dose delivery was investigated. MATERIALS AND METHODS: Two phantoms are manufactured to mimic a breast, one for thermoluminescent dosimetry (TLD) measurements, and one for dosimetry using radiochromic films. The TLD phantom allows measurements at 34 dose points in three planes including the basal dose points. The film phantom is designed in such a way that films can be positioned in a plane parallel and orthogonal to the needles. RESULTS: The dose distributions calculated with the TPS are in good agreement with both TLD and radiochromic film measurements (average deviations of point doses <+/-5%). However, close to the interface tissue-air the dose is overestimated by the TPS since it neglects the finite size of a breast and the associated lack of backscatter (average deviations of point doses -14%). CONCLUSION: Most deviations between measured and calculated doses, are in the order of magnitude of the uncertainty associated with the source strength specification, except for the point doses measured close to the skin. In vivo dosimetry during PDR brachytherapy treatment was found to be a valuable procedure to detect large errors, e.g. errors caused by an incorrect data transfer.

Brachytherapy↗
PubMed2001

Prescribing, recording, and reporting in endovascular brachytherapy. Quality assurance, equipment, personnel and education.

Endovascular brachytherapy is a new, rapidly growing field of interest in radiotherapy for the prevention of neointimal hyperplasia after angioplasty in both coronary and peripheral arteries. Many physics aspects of these treatments have already been addressed in the report of the American Association of Physicists in Medicine task group on 'Intravascular brachytherapy', but up to now there are no generally accepted recommendations for recording and reporting radiation doses and volumes. The terminology to be used by all individuals involved in such treatments (radiation oncologists, physicists, and interventionalists) is not clearly defined. The Endovascular Groupe Européen de Curiethérapie/European Society for Therapeutic Radiology and Oncology Working Group in this document presents recommendations for a common language for general use in endovascular brachytherapy. This proposal addresses general terms and concepts for target and dose specification as well as detailed recommendations for dose prescription, recording and reporting in endovascular brachytherapy for both peripheral and coronary arteries. Additionally, quality assurance and radiation safety aspects are briefly addressed, as are aspects related to equipment, personnel, and training and education related to endovascular brachytherapy.

Brachytherapy↗
PubMed2001

Formalisms for MU calculations, ESTRO booklet 3 versus NCS report 12.

Although the relevance and importance of quality assurance and quality control in radiotherapy is generally accepted, only recently, methods for monitor unit (MU) calculation and verification have been addressed in recognized recommendations, published by the European Society of Therapeutic Radiation Oncology (ESTRO) and by the Netherlands Commission on Radiation Dosimetry (Dutreix A, Bjärngard BE, Bridier A, Mijnheer B, Shaw JE, Svensson H. Monitor unit calculation for high-energy photon beams. Physics for clinical radiotherapy. ESTRO Booklet No. 3. Leuven: Garant, 1997; Netherlands Commission on Radiation Dosimetry (NCS). Determination and use of scatter correction factors of megavoltage photon beams. NCS report 12. Deift: NCS, 1998). Both documents are based on the same principles: (i) the separation of the output factor into a head and a volume (or phantom) scatter component; (ii) the use of a so-called mini-phantom to measure and verify the head scatter component; and (iii) the recommendation to use a single reference depth of 10 cm for all photon beam qualities. However, there are substantial differences between the approach developed in the IAEA-ESTRO task group and the NCS approach for MU calculations, which might lead to confusion and/or misinterpretation if both reports are used simultaneously or if data from the NCS report is applied in the algorithms of the ESTRO report without careful consideration. The aim of the present paper is to discuss and to clearly point out these differences (e.g. field size definitions, phantom scatter parameters, etc.). Additionally, corresponding quantities in the two reports are related where possible and several aspects concerning the use of a mini-phantom (e.g. size, detector position, composition) are addressed.

Humans↗
PubMed2001

A linac-based stereotactic irradiation technique of uveal melanoma.

PURPOSE: To describe a stereotactic irradiation technique for uveal melanomas performed at a linac, based on a non-invasive eye fixation and eye monitoring system. METHODS: For eye immobilization a light source system is integrated in a standard stereotactic mask system in front of the healthy eye: During treatment preparation (computed tomography/magnetic resonance imaging) as well as for treatment delivery, patients are instructed to gaze at the fixation light source. A mini-video camera monitors the pupil center position of the diseased eye. For treatment planning and beam delivery standard stereotactic radiotherapy equipment is used. If the pupil center deviation from a predefined 'zero-position' exceeds 1 mm (for more than 2 s), treatment delivery is interrupted. Between 1996 and 1999 60 patients with uveal melanomas, where (i) tumor height exceeded 7 mm, or (ii) tumor height was more than 3 mm, and the central tumor distance to the optic disc and/or the macula was less than 3 mm, have been treated. A total dose of 60 or 70 Gy has been given in 5 fractions within 10 days. RESULTS: The repositioning accuracy in the mask system is 0.47+/-0.36 mm in rostral-occipital direction, 0.75+/-0.52 mm laterally, and 1.12+/-0.96 mm in vertical direction. An eye movement analysis performed for 23 patients shows a pupil center deviation from the 'zero' position<1 mm in 91% of all cases investigated. In a theoretical analysis, pupil center deviations are correlated with GTV 'movements'. For a pupil center deviation of 1 mm (rotation of the globe of 5 degrees ) the GTV is still encompassed by the 80% isodose in 94%. CONCLUSION: For treatments of uveal melanomas, linac-based stereotactic radiotherapy combined with a non-invasive eye immobilization and monitoring system represents a feasible, accurate and reproducible method. Besides considerable technical requirements, the complexity of the treatment technique demands an interdisciplinary team continuously dedicated to this task.

Eye Movements↗
PubMed1999

Build-up modification of commercial diodes for entrance dose measurements in 'higher energy' photon beams.

BACKGROUND AND PURPOSE: Several commercially available p-type diodes do not provide sufficient build-up for in-vivo dosimetry in 'higher' energy photon beams, and only limited information could be found in the literature describing the correction factor variation and/or the achievable accuracy for in-vivo dosimetry methods in this energy range. The first aim of this study is to assess and analyze the variation of diode correction factors for entrance dose measurements at higher photon energies. In a second step the total build up thickness of the diode has been modified in order to minimize the correction factor variation. MATERIALS AND METHODS: Diode correction factors accounting for non-reference conditions (field size, source surface distance, tray, wedge, and block) are determined in 18-25 MV photon beams provided by different treatment units for Scanditronix p-type diodes recommended for higher energy photon beams: old type and new type EDP-20, and EDP-30 diodes. Hemispherical build-up caps of different materials (copper, iron, lead) are used to increase the total build-up thickness. Perturbation effects with and without additional build-up caps are assessed for the three diode types. RESULTS: For unmodified diodes field size correction factors (C(FS)) vary between 1.7% and 6%, dependent on diode type and treatment unit. For example, for an old type EDP-20 the C(FS) variation at 18 MV is much higher on a GE linac (5%) as compared to the Philips machine (1.7%). Depending on diode type, this variation can be reduced to 1-2% when adding additional build-up. The variation of source to surface distance correction factors is almost independent of build-up thickness. By adding additional build-up the influence of trays and blocks can be almost eliminated. CONCLUSIONS: The correction factor variation of unmodified diodes reflects the variation of the electron contamination with treatment geometry. A total build-up thickness of 30 mm is found to be the 'best compromise' for the three types of diodes investigated when measuring entrance doses in the energy range between 18 and 25 MV.

Dose-Response Relationship, Radiation↗
PubMed1999

Monitor unit calculation on the beam axis of open and wedged asymmetric high-energy photon beams.

An ESTRO booklet and a report of the Netherlands Commission on Radiation Dosimetry have been published recently describing empirical methods for monitor unit (MU) calculations in symmetrical high-energy photon beams. Both documents support the same basic ideas; firstly the separation of head scatter and volume scatter components and secondly the determination of head scatter quantities in a mini-phantom. Based on these ideas the methods previously described for MU calculations in symmetrical beams are extended to asymmetrical open and wedged beams in isocentric treatment conditions. All required dosimetric parameters (normalized head scatter factors, phantom scatter correction factors, wedge factors, off-axis ratios, quality index, and depth dose parameters) are determined as a function of beam axis position in order to study their off-axis dependence. Measurements are performed for 6 MV and 18 MV photon beams provided by two different dual-energy linear accelerators, a GE Saturne 42 and a Varian 2100 CD linac.

Models, Statistical↗
PubMed1999

Methods for beam data acquisition offered by a mini-phantom.

Mini-phantoms are an important tool for measurement of basic head scatter parameters in high-energy photon beams, and recently they have also been used for beam quality specification. Therefore the feasibility and reliability of basic beam parameter acquisition using only a mini-phantom is checked in 6, 18 and 25 MV photon beams. These parameters include head scatter correction factors, phantom scatter correction factors, total scatter correction factors, wedge factors, off-axis ratios, as well as beam attenuation coefficients and beam hardening coefficients. In order to specify beam quality variations and beam quality modifications by a wedge, two different methods are compared: the first method uses a constant source to chamber distance of 1 m, the second method refers to narrow beam geometry. Mu values derived with two different beam quality specification methods show a systematic deviation. However, relative variations of the attenuation coefficient within the beam and the associated beam quality modifications observed with the two methods show good agreement in open and wedged beams. Phantom scatter correction factors are calculated from measured head scatter correction factors and total scatter correction factors as well as from attenuation coefficients. Measured and calculated phantom scatter correction factors agree within 1% with the values given in literature. For 18 and 25 MV photon beam, wedge factors measured in water or in the mini-phantom agree within 0.5%, but maximum deviations of approximately 1.5% are observed at 6 MV for the largest field sizes. It is demonstrated that the determination of several beam data related to full scatter conditions does not necessarily require the availability of a full scatter phantom. The mini-phantom is a reliable but very cheap and simple tool. It offers versatile possibilities to measure, check and verify basic beam parameters in high-energy photon beams.

Light↗
PubMed1997

Dosimetric comparison of an integrated multileaf-collimator versus a conventional collimator.

The dosimetric characteristics of both a conventional GE collimator (CC) and a GE multileaf collimator (MLC) are compared for different photon beam energies. The integrated GE MLC consists of 32 pairs of tungsten leaves, replacing the lower pair of jaws of the conventional collimator. Measurements were performed with the conventional collimator before this collimator was replaced by the MLC. All parts of the accelerator except the collimator remained the same. Leakage and transmission measurements show good agreement with the manufacturer's specification, stating a leakage between leaves of less than 1% for all energies and a transmission through leaves of less than 0.5%. The dosimetric characteristics of both collimators are very similar for square and rectangular fields. No significant change in beam quality, beam attenuation and depth of maximum dose could be detected within the measurement accuracy. The MLC output ratio variation is smaller than the one measured with the CC. The penumbra difference in the Y direction is less than 0.5 mm at a depth of 5 cm in phantom; in the X direction the penumbra is 1 mm larger for the MLC due to the rounded leaf fronts. As the two leaf banks replace the lower pair of collimator jaws the distance from the collimator end to the isocentre is similar for the two collimators, therefore the MLC does not reduce the flexibility of the treatment unit. For symmetrical and regular collimator settings the MLC can be treated as the CC.

Biophysical Phenomena↗
PubMed1997

Output ratios in a miniphantom for asymmetric fields shaped by a multileaf collimator.

The integrated GE multileaf collimator (MLC) provides the ability to achieve 'double' asymmetric fields: each of the 64 leaves allow an over-axis travel of 10 cm and the Y-jaws allow 20 cm. A formalism has recently been proposed by the authors to calculate the output ratio in a miniphantom for this type of MLC by the product of independent leaf and jaw correction factors. The original proposed formalism was restricted to regular or irregular fields including the collimator rotational axis. Introducing 'reduced coordinates' for the correction factors in the present work this formalism is extended to asymmetric fields where central leaves or jaws overlap the collimator axis. The extended formalism is applied to asymmetric square, rectangular and irregular fields. For all fields checked at a given off-axis position, measured and calculated output ratios agree within 1% for 6, 18 and 25 MV photon beams. To relate output ratios normalized to off-axis points with output ratios on-axis, off-axis ratios are derived from film and miniphantom measurements. Both off-axis ratios agree to within 1% for 6 and 25 MV photon beams; a maximum deviation of 1.3% is observed at 18 MV. Calculated products of output ratios and off-axis ratios derived from films are compared with measurements for asymmetric square, rectangular and irregular fields, and agree mostly within 1% for all energies checked; maximum deviations of 1.3 and 1.6% are observed for 6 and 18 MV photon beams.

Biophysical Phenomena↗
PubMed1997

A formalism to calculate the output ratio in a mini-phantom for a GE multileaf collimator.

A GE multileaf collimator (MLC) has been recently installed on a Saturne 43 and is used with 6, 18 and 25 MV photon beams. In the integrated GE MLC the lower pair of jaws in the X direction is replaced by 32 pairs of computer controlled opposed tungsten leaves. The influence of each set of leaves on the output ratio is smaller than the influence of the jaws it replaces (8% instead of 10%). For irregular fields it is necessary to evaluate the influence on the output ratio of each independent leaf. It is assumed that each leaf and each jaw have an independent influence on the output ratio. According to this assumption leaf correction functions are derived from measurements as a function of their X position. A second-order correction (less than 1%) has to be applied for the jaw positions. The output ratio in a mini-phantom for a given irregular field can be calculated by the product of the 64 leaf and two jaw correction factors. The formalism is applied to symmetric square and rectangular fields, asymmetric and irregular fields. For all fields checked, the calculated and the measured output ratios agree within 1%. Furthermore the simple formula suggested by Vadash and Bjărngard for square field sizes equivalent to rectangular fields can be used with a good accuracy with an A value of 1.6 for the three energies used. The proposed formalism to calculate the output ratio in a mini-phantom is restricted to fields including the collimator axis with a minimum distance of 2 cm between any leaf and the collimator axis.

Algorithms↗
PubMed1997

Measurements of basic parameters in wedged high-energy photon beams using a mini-phantom.

Basic dosimetric quantities necessary to specify wedged beans (beam quality, wedge factors, output ratios) are obtained by measurements performed in a narrow coaxial mini-phantom for 6, 18 and 15 MV photon beams. To express beam quality, an attenuation coefficient mu is derived from measurements in a mini-phantom at 20 and 10 cm depth. Wedge factors and output ratios are measured as a function of field size at 10 cm water-equivalent depth. In open beams one observes beam softening with increasing distance from the collimator axis for all energies. With an inserted wedge a beam hardening is observed at 6 MV. This beam hardening decreases at 18 MV while at 25 MV a slight beam softening is detected. Larger variations of output ratios with field sizes are observed with a wedge than without a wedge. An equivalent square formula for head-scatter factors can be used with a good accuracy for rectangular wedged fields. For irregular wedged fields a method is proposed to calculate the product of the output ratio and the wedge factor. Measurements and calculations agree within 1% for all irregular wedged fields checked.

Biophysical Phenomena↗