Browsing by Author "Chofor, Ndimofor"

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 x 10 cm(2), 4 x 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).

In this work we present a new parameter for characterizing the emitted photon spectra of Co-60 radiotherapy units. It is intended to propose this parameter for the revised DIN standard 6809-1. In the previous DIN regulation, it had been sufficient to state the nature of the radioactive material within the source. However, scatter processes within the radioactive material as well as the source housing and the collimator system influence the shape of the photon spectrum, with a noticeable contribution in the low-energy portion. The fraction of the air kerma for a given distance from the source, position and beam size in air comprising all contributions by scattered photons up to an upper energy limit for the emitted spectrum from Co-60 decay, will be proposed as a typical parameter. The new quantity, which is termed the 'fraction of air kerma attributable to scattered photons', PE Scatter, has been calculated for E=1.17 MeV and compared for four different Monte Carlo- simulated spectra of used Co-60 devices. Not included in this new formalism is the air kerma contribution by scattered photons in between the two lines of the Co-60 spectrum. A simple measurement procedure based on the signal ratio of two Farmer chamber detectors with different wall materials is discussed and its feasibility shown.

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. (C) 2010 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 95 (2010) 158-165

The unshielded Si diode PTW 60012, used for accurate measurements of the transversal close profiles of narrow photon beams, has been investigated with regard to its linearity, photon energy dependence and spatial resolution. The diode shows a slight supralinearity, i.e. increase of the response with pulse dose, by 3% over the pulse dose range 0.1 to 0.8 mGy. In p-type silicon, supralinearity results from the increased chance for radiation-induced electrons to escape recombination when the pulse dose increases. Over the energy range from 6 to 15 MV, the response decreases by about 4%. This small variation of the response results front partial compensation between the influences of the secondary electron energy oil the mass stopping power ratio silicon/water and on electron backscattering from the silicon chip. The lateral response function of the examined diode has a full half width of 1.3 mm. Dose profiles of 5 tent half-width can still he recorded with negligible error.

Purpose: The dosimetric properties of the OCTAVIUS Detector 1500 (OD1500) ionization chamber array (PTW-Freiburg, Freiburg, Germany) have been investigated. A comparative study was carried out with the OCTAVIUS Detector 729 and OCTAVIUS Detector 1000 SRS arrays. Methods: The OD1500 array is an air vented ionization chamber array with 1405 detectors in a 27x 27 cm(2) measurement area arranged in a checkerboard pattern with a chamber-to-chamber distance of 10 mm in each row. A sampling step width of 5 mm can be achieved by merging two measurements shifted by 5 mm, thus fulfilling the Nyquist theorem for intensity modulated dose distributions. The stability, linearity, and dose per pulse dependence were investigated using a Semiflex 31013 chamber (PTW-Freiburg, Freiburg, Germany) as a reference detector. The effective depth of measurement was determined by measuring TPR curves with the array and a Roos chamber type 31004 (PTW-Freiburg, Freiburg, Germany). Comparative output factor measurements were performed with the array, the Semiflex 31010 ionization chamber and the Diode 60012 (both PTW-Freiburg, Freiburg, Germany). The energy dependence of the OD1500 was measured by comparing the array's readings to those of a Semiflex 31010 ionization chamber for varying mean photon energies at the depth of measurement, applying to the Semiflex chamber readings the correction factor k(NR) for nonreference conditions. The Gaussian lateral dose response function of a single array detector was determined by searching the convolution kernel suitable to convert the slit beam profiles measured with a Diode 60012 into those measured with the array's central chamber. An intensity modulated dose distribution measured with the array was verified by comparing a OD1500 measurement to TPS calculations and film measurements. Results: The stability and interchamber sensitivity variation of the OD1500 array were within +/- 0.2% and +/- 0.58%, respectively. Dose linearity was within 1% over the range from 5 to 1000 MU. The effective point of measurement of the OD1500 for dose measurements in RW3 phantoms was determined to be (8.7 +/- 0.2) mm below its front surface. Output factors showed deviations below 1% for field sizes exceeding 4x4 cm(2). The dose per pulse dependence was smaller than 0.4% for doses per pulse from 0.2 to 1 mGy. The energy dependence of the array did not exceed +/- 0.9%. The parameter sigma of the Gaussian lateral dose response function was determined as sigma(6MV) = (2.07 +/- 0.02) mm for 6 MV and sigma(15MV) = (2.09 +/- 0.02) mm for 15 MV. An IMRT verification showed passing rates well above 90% for a local 3 mm/3% criterion. Conclusions: The OD1500 array's dosimetric properties showed the applicability of the array for clinical dosimetry with the possibility to increase the spatial sampling frequency and the coverage of a dose distribution with the sensitive areas of ionization chambers by merging two measurements. (C) 2015 American Association of Physicists in Medicine.

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.

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 relativemagnitude 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 energyshifted 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 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)(200keV) 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)(200keV) is bellshaped, with a wide-ranging exponential tail of half value 6 to 7 cm.. The P(D)(200keV) value obtained on the central axis of a photonfield shows an approximately proportional increase with,field size. Out-of-field P(D)(200keV) 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)(200keV) 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)(200keV) 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.

In photon-beam radiotherapy, the absorbed close in at? 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 efftectiveness (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 M V 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 close fraction attributable to low-energy Compton scattered photons can be presented as an innovative instrument of describing radiation quality in radiotherapy.

According to accepted dosimetry protocols, the "radiation quality correction factor" k(Q) accounts for the energy-dependent changes of detector responses under the conditions of clinical dosimetry for high-energy photon radiations. More precisely, a factor k(QR) is valid under reference conditions, i.e. at a point on the beam axis at depth 10 cm in a large water phantom, for 10 x 10 cm(2) field size, SSD 100 cm and the given radiation quality with quality index Q. Therefore, a further correction factor k(NR) has been introduced to correct far the influences of spectral quality changes when detectors are used under non-reference conditions such as other depths, field sizes and off-axis distances, while under reference conditions k(NR) is normalized to unity. In this paper, values of k(NR) are calculated for 6 and 15 MV photon beams, using published data of the energy-dependent responses of various radiation detectors to monoenergetic photon radiations, and weighting these responses with validated photon spectra of clinical high-energy photon beams from own Monte-Carlo-calculations for a wide variation of the non-reference conditions within a large water phantom. Our results confirm the observation by Scarboro et al. [26] that k(NR) can be represented by a unique function of the mean energy Em, weighted by the spectral photon fluence. Accordingly, the numerical variations of Em with depth, field size and off-axis distance have been provided. Throughout all considered conditions, the deviations of the k(NR) values from unity are at most 2% for a Farmer type ion chamber, and they remain below 15% for the thermoluminescent detectors LiF:Mg Ti. and LiF:Mg,Cu,P. For the shielded diode EDP-10, k(NR) varies from unity up to 20%, while the unshielded diode EDD-5 shows deviations up to 60% in the peripheral region. Thereby, the restricted application field of unshielded diodes has been clarified. For small field dosimetry purposes k(NR) can be converted into k(NCSF), the non-calibration condition correction factor normalized to unity for a 4 x 4 cm(2) calibration field. For the unshielded Si diodes needed in small-field dosimetry, the values of k(NCSF) are closer to unity than the associated k(NR) values.