| 1. |
- Benmakhlouf, Hamza, et al.
(author)
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Backscatter factors and mass energy-absorption coefficient ratios for diagnostic radiology dosimetry
- 2011
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In: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 56:22, s. 7179-7204
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Journal article (peer-reviewed)abstract
- Backscatter factors, B, and mass energy-absorption coefficient ratios, (mu(en)/rho)(omega,) (air), for the determination of the surface dose in diagnostic radiology were calculated using Monte Carlo simulations. The main purpose was to extend the range of available data to qualities used in modern x-ray techniques, particularly for interventional radiology. A comprehensive database for mono-energetic photons between 4 and 150 keV and different field sizes was created for a 15 cm thick water phantom. Backscattered spectra were calculated with the PENELOPE Monte Carlo system, scoring track-length fluence differential in energy with negligible statistical uncertainty; using the Monte Carlo computed spectra, B factors and (mu(en)/rho)(omega), air were then calculated numerically for each energy. Weighted averaging procedures were subsequently used to convolve incident clinical spectra with mono-energetic data. The method was benchmarked against full Monte Carlo calculations of incident clinical spectra obtaining differences within 0.3-0.6%. The technique used enables the calculation of B and (mu(en)/rho)(w), air for any incident spectrum without further time-consuming Monte Carlo simulations. The adequacy of the extended dosimetry data to a broader range of clinical qualities than those currently available, while keeping consistency with existing data, was confirmed through detailed comparisons. Mono-energetic and spectra-averaged values were compared with published data, including those in ICRU Report 74 and IAEA TRS-457, finding average differences of 0.6%. Results are provided in comprehensive tables appropriated for clinical use. Additional qualities can easily be calculated using a designed GUI interface in conjunction with software to generate incident photon spectra.
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| 2. |
- Benmakhlouf, Hamza, et al.
(author)
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Influence of phantom thickness and material on the backscatter factors for diagnostic x-ray beam dosimetry
- 2013
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In: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 58:2, s. 247-260
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Journal article (peer-reviewed)abstract
- Most of the existing backscatter factors for the dosimetry of clinical diagnostic x-ray beams have been calculated for 15 cm thick phantoms; these data are used for skin dose determinations which in general ignore the influence of phantom material and thickness. The former should strictly be required whenever dosimetry measurements are made on phantom materials different from those used for the backscatter factor calculations. The phantom or patient thickness is of special importance when skin dose determinations are made for infants or paediatric patients. In this work, the recently published formalism for reference dosimetry and comprehensive database of backscatter factors for clinical beams and water phantoms have been extended using two correction factors which account for phantom material and thickness. These were determined with simulations using the PENELOPE Monte Carlo system, for PMMA to analyse the influence of the phantom material relative to water, and for a broad range of thicknesses of water and PMMA to investigate the role of this parameter in patient dose estimates. The material correction factor was found to be in the range 3-10%, depending on the field size and the HVL. The thickness correction factor was in the range 2-12% for a 5 cm thick phantom and square field sizes between 5 and 35 cm, reaching a plateau of about ±1% for thicknesses beyond 13 cm. Expressions in the form of surface fits over the calculated data are provided which streamline the determination of backscatter factors for arbitrary thicknesses and phantom materials, as well as field sizes. Results demonstrate the inadequacy of using conventional backscatter factors (calculated for 15 cm thick phantoms) without correction factors that take into account the phantom material and its thickness.
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| 3. |
- Benmakhlouf, Hamza, 1985-
(author)
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Key Data for the Reference and Relative Dosimetry of Radiotherapy and Diagnostic and Interventional Radiology Beams
- 2015
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Doctoral thesis (other academic/artistic)abstract
- Accurate dosimetry is a fundamental requirement for the safe and efficient use of radiation in medical applications. International Codes of Practice, such as IAEA TRS-398 (2000) for radiotherapy beams and IAEA TRS-457 (2007) for diagnostic radiology beams, provide the necessary formulation for reference and relative dosimetry and the data required for their implementation. Research in recent years has highlighted the shortage of such data for radiotherapy small photon beams and for surface dose estimations in diagnostic and interventional radiology, leading to significant dosimetric errors that in some instances have jeopardized patient’s safety and treatment efficiency.The aim of this thesis is to investigate and determine key data for the reference and relative dosimetry of radiotherapy and radiodiagnostics beams. For that purpose the Monte Carlo system PENELOPE has been used to simulate the transport of radiation in different media and a number of experimental determinations have also been made. A review of the key data for radiotherapy beams published after the release of IAEA TRS-398 was conducted, and in some cases the considerable differences found were questioned under the criterion of data consistency throughout the dosimetry chain (from standards laboratories to the user). A modified concept of output factor, defined in a new international formalism for the dosimetry of small photon beams, requires corrections to dosimeter readings for the dose determination in small beams used clinically. In this work, output correction factors were determined, for Varian Clinac 6 MV photon beams and Leksell Gamma Knife Perfexion 60Co gamma-ray beams, for a large number of small field detectors, including air and liquid ionization chambers, shielded and unshielded silicon diodes and diamond detectors, all of which were simulated by Monte Carlo with great detail.Backscatter factors and ratios of mass energy-absorption coefficients required for surface (skin) determinations in diagnostic and interventional radiology applications were also determined, as well as their extension to account for non-standard phantom thicknesses and materials. A database of these quantities was created for a broad range of monoenergetic photon beams and computer codes developed to convolve the data with clinical spectra, thus enabling the determination of key data for arbitrary beam qualities.Data presented in this thesis has been contributed to the IAEA international dosimetry recommendations for small radiotherapy beams and for diagnostic radiology in paediatric patients.
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| 4. |
- Benmakhlouf, Hamza, et al.
(author)
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Monte Carlo calculated and experimentally determined output correction factors for small field detectors in Leksell Gamma Knife Perfexion beams
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Other publication (other academic/artistic)abstract
- The measurement of output factors (OF) for the small beams of the Leksell Gamma Knife® (LGK) is a challenge for the physicist due to the over- or underestimation of these factors by a vast majority of commercially available detectors. Output correction factors, introduced in the new international formalism published by Alfonso et al. (2008) standardizes the determination of OFs for small photon beams by correcting the detector reading ratios with output correction factors in order to yield the correct OF. Output correction factors have, in this work, been determined for LGK Perfexion™ 60Co γ-ray beams by Monte Carlo (MC) calculations and measurements. The MC calculations were performed using the MC system PENELOPE scoring the doses to the active volumes of the detectors and to a small volume of water. Two silicon diodes, one liquid ionization chamber (LIC), one alanine and one TLD detector were included in the MC derivation of the output correction factors. The LIC resulted in correction factors within ±0.4% and was therefore selected as the reference detector for the measurements. Twelve detectors were used in the experimental determination of the output correction factors by normalizing their detector readings to those of the LIC. The MC-calculated and experimentally determined output correction factors for the silicon diodes resulted in up to a -4% correction for the smallest collimator size. The air ionization chamber measurements resulted in extremely large output correction factors, due to the well-known effect of partial volume averaging (PVA). The natural diamond detector resulted in 6% correction for the 4 mm collimator, also due to PVA, whereas the smaller synthetic diamond detector resulted in a correction within ±1%. The LIC, requiring the smallest correction, was used to explore machine-to-machine differences in the OFs by performing measurements in four LGK units with different dose rates. This resulted in OFs within ±0.6% and ±0.3% for the 4 mm and 8 mm collimators, respectively, favouring the use of generic OFs. Using these experimentally derived correction factors, OFs can now be measured using a wide range of commercially available detectors.
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| 5. |
- Benmakhlouf, Hamza, et al.
(author)
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Monte Carlo calculated and experimentally determined output correction factors for small field detectors in Leksell Gamma Knife Perfexion beams
- 2015
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In: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 60:10, s. 3959-3973
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Journal article (peer-reviewed)abstract
- The measurement of output factors (OF) for the small photon beams generated by Leksell Gamma Knife (R) (LGK) radiotherapy units is a challenge for the physicist due to the under or over estimation of these factors by a vast majority of the detectors commercially available. Output correction factors, introduced in the international formalism published by Alfonso (2008 Med. Phys. 35 5179-86), standardize the determination of OFs for small photon beams by correcting detector-reading ratios to yield OFs in terms of absorbed-dose ratios. In this work output correction factors for a number of detectors have been determined for LGK Perfexion (TM) Co-60 gamma-ray beams by Monte Carlo (MC) calculations and measurements. The calculations were made with the MC system PENELOPE, scoring the energy deposited in the active volume of the detectors and in a small volume of water; the detectors simulated were two silicon diodes, one liquid ionization chamber (LIC), alanine and TLD. The calculated LIC output correction factors were within +/- 0.4%, and this was selected as the reference detector for experimental determinations where output correction factors for twelve detectors were measured, normalizing their readings to those of the LIC. The MC-calculated and measured output correction factors for silicon diodes yielded corrections of up to 5% for the smallest LGK collimator size of 4 mm diameter. The air ionization chamber measurements led to extremely large output correction factors, caused by the well-known effect of partial volume averaging. The corrections were up to 7% for the natural diamond detector in the 4 mm collimator, also due to partial volume averaging, and decreased to within about +/- 0.6% for the smaller synthetic diamond detector. The LIC, showing the smallest corrections, was used to investigate machine-to-machine output factor differences by performing measurements in four LGK units with different dose rates. These resulted in OFs within +/- 0.6% and +/- 0.2% for the 4 mm and 8 mm collimators, respectively, providing evidence for the use of generic OFs for these LGK beams. Using the experimentally derived output correction factors, OFs can be measured using a wide range of commercially available detectors.
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| 6. |
- Benmakhlouf, Hamza, et al.
(author)
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Output correction factors for nine small field detectors in 6 MV radiation therapy photon beams : A PENELOPE Monte Carlo study
- 2014
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In: Medical physics (Lancaster). - : Wiley. - 0094-2405 .- 2473-4209. ; 41:4, s. 041711-
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Journal article (peer-reviewed)abstract
- Purpose: To determine detector-specific output correction factors, k(Qclin,Qmsr)(fclin,fmsr) in 6 MV small photon beams for air and liquid ionization chambers, silicon diodes, and diamond detectors from two manufacturers. Methods: Field output factors, defined according to the international formalism published by Alfonso et al. [Med. Phys. 35, 5179-5186 (2008)], relate the dosimetry of small photon beams to that of the machine-specific reference field; they include a correction to measured ratios of detector readings, conventionally used as output factors in broad beams. Output correction factors were calculated with the PENELOPE Monte Carlo (MC) system with a statistical uncertainty (type-A) of 0.15% or lower. The geometries of the detectors were coded using blueprints provided by the manufacturers, and phase-space files for field sizes between 0.5 x 0.5 cm(2) and 10 x 10 cm(2) from a Varian Clinac iX 6 MV linac used as sources. The output correction factors were determined scoring the absorbed dose within a detector and to a small water volume in the absence of the detector, both at a depth of 10 cm, for each small field and for the reference beam of 10 x 10 cm(2). Results: The Monte Carlo calculated output correction factors for the liquid ionization chamber and the diamond detector were within about +/- 1% of unity even for the smallest field sizes. Corrections were found to be significant for small air ionization chambers due to their cavity dimensions, as expected. The correction factors for silicon diodes varied with the detector type (shielded or un-shielded), confirming the findings by other authors; different corrections for the detectors from the two manufacturers were obtained. The differences in the calculated factors for the various detectors were analyzed thoroughly and whenever possible the results were compared to published data, often calculated for different accelerators and using the EGSnrc MC system. The differences were used to estimate a type-B uncertainty for the correction factors. Together with the type-A uncertainty from the Monte Carlo calculations, an estimation of the combined standard uncertainty was made, assigned to the mean correction factors from various estimates. Conclusions: The present work provides a consistent and specific set of data for the output correction factors of a broad set of detectors in a Varian Clinac iX 6 MV accelerator and contributes to improving the understanding of the physics of small photon beams. The correction factors cannot in general be neglected for any detector and, as expected, their magnitude increases with decreasing field size. Due to the reduced number of clinical accelerator types currently available, it is suggested that detector output correction factors be given specifically for linac models and field sizes, rather than for a beam quality specifier that necessarily varies with the accelerator type and field size due to the different electron spot dimensions and photon collimation systems used by each accelerator model. (C) 2014 American Association of Physicists in Medicine.
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| 7. |
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| 8. |
- dos Santos Matias, Lucilio, et al.
(author)
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Characterization of the radiation field surrounding the Leksell Gamma Knife® and shielding applications
- 2023
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In: Applied Radiation and Isotopes. - 0969-8043 .- 1872-9800. ; 198
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Journal article (peer-reviewed)abstract
- The aim of this study is to improve the characterization and modeling of the radiation field surrounding the Leksell Gamma Knife®-PerfexionTM. The improved characterization of the radiation field enables more accurate shielding calculations to be performed for the areas adjacent to the treatment room. With the aid of a high-purity germanium detector and a satellite dose rate meter, ?-ray spectra and ambient dose equivalent H*(10) data were acquired at various locations in the field of a Leksell Gamma Knife unit in a treatment room at Karolinska University Hospital, Sweden. These measurements were used to validate the results of the PEGASOS Monte Carlo simulation system with a PENELOPE kernel. The levels of the radiation that passes through the shielding of the machine (leakage radiation) are shown to be much lower than what is suggested by various bodies, e.g. the National Council on Radiation Protection and Measurements, to be used when calculating radiation shielding barriers. The results clearly indicate that Monte Carlo simulations may be used in structural shielding design calculations for γ? rays from the Leksell Gamma Knife.
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| 9. |
- Ghazal, Mohammed, et al.
(author)
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6-MV small field output factors: intra-/intermachine comparison and implementation of TRS-483 using various detectors and several linear accelerators
- 2019
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In: Medical physics (Lancaster). - : WILEY. - 0094-2405 .- 2473-4209. ; 46:11, s. 5350-5359
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Journal article (peer-reviewed)abstract
- Purpose To investigate the applicability of output correction factors reported in TRS-483 on 6-MV small-field detector-reading ratios using four solid-state detectors. Also, to investigate variations in 6-MV small-field output factors (OF) among nominally matched linear accelerators (linacs). Methods The TRS-483 Code of Practice (CoP) introduced and provided output correction factors to be applied to measured detector-reading ratios to obtain OFs for several small-field detectors. Detector readings for 0.5 cm x 0.5 cm to 8 cm x 8 cm fields were measured and normalized to that of 10 cm x 10 cm field giving the detector-reading ratios. Three silicon diodes, IBA PFD, IBA EFD (IBA, Schwarzenbruck, Germany), PTW T60017, and one microdiamond, PTW T60019 (PTW, Freiburg, Germany), were used. Output correction factors from the CoP were applied to measured detector-reading ratios. Measurements were performed on six Clinac and six TrueBeam linacs (Varian Medical Systems, Palo Alto, USA). An investigation of the relationship between the size of small fields and corresponding detector-reading ratio among the linacs was performed by measuring lateral dose profiles for 0.5 cm x 0.5 cm fields to determine the full width half maximum (FWHM). The relationship between the linacs focal spot size and the small-field detector-reading ratio was investigated by measuring 10 cm x 10 cm lateral dose profiles and determining the penumbra width reflecting the focal spot size. Measurement geometry was as follows: gantry angle = 0 degrees, collimator angle = 0 degrees, source-to surface distance (SSD) = 90 cm, and depth in water = 10 cm. Results For a given linac and 0.5 cm x 0.5 cm field, the deviations in detector-reading ratios among the detectors were 9%-15% for the Clinacs and 4%-5% for the TrueBeams. Use of output correction factors reduced these deviations to 6%-12% and 3%-4%, respectively. For field sizes equal to or larger than 0.8 cm x 0.8 cm, the deviations were corrected to 1% using output correction factors for both Clinacs and TrueBeams. For a given detector and 0.5 cm x 0.5 cm field, the deviations in detector-reading ratios among the linacs were 11%-17% for the Clinacs and 5-6% for the TrueBeams. For 1 cm x 1 cm the deviations were 1%-2% for Clinacs and 1% for TrueBeams. For field sizes larger than 1 cm x 1 cm the deviations were within 1% for both Clinacs and TrueBeams. No relationship between FWHMs and detector-reading ratios for 0.5 cm x 0.5 cm was observed. For Clinacs, larger 10 cm x 10 cm penumbra width yielded lower 0.5 cm x 0.5 cm detector-reading ratio indicating an effect of the focal spot size. For TrueBeams, the spread of penumbra widths was lower compared to Clinacs and no similar relationship was observed. Conclusions Output correction factors from the TRS-483 CoP are not sufficient for accurate determination of OF for 0.5 cm x 0.5 cm fields but are applicable for 0.8 cm x 0.8 cm to 8 cm x 8 cm fields. Nominally matched Clinacs and TrueBeams show large differences in detector-reading ratios for fields smaller than 1 cm x 1 cm.
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| 10. |
- Häger, Wille, et al.
(author)
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Experimental investigation of TRS-483 reference dosimetry correction factors for Leksell Gamma Knife (R) Icon (TM) beams
- 2021
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In: Medical physics (Lancaster). - : Wiley. - 0094-2405 .- 2473-4209. ; 48:1, s. 434-444
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Journal article (peer-reviewed)abstract
- Purpose: Radiosurgery using the Leksell Gamma Knife (R) (LGK) Icon (TM) is an established technique used for treating intracranial lesions. The largest beam field size the LGK Icon can produce is a 16 mm diameter sphere. Despite this, reference dosimetry on the LGK Icon is typically performed using ionization chambers calibrated in 10 x 10 cm(2) fields. Furthermore, plastic phantoms are widely used instead of liquid water phantoms. In an effort to resolve these issues, the International Atomic Energy Agency (IAEA) in collaboration with American Association of Physicists in medicine (AAPM) recently published Technical Report Series No. 483 (TRS-483) as a Code of Practice for small-field dosimetry. TRS-483 includes small-field correction factors, k(Qmsr,Q0)(fmsr,fref), intended to account for the differences between setups when using small-field modalities such as the LGK Icon, and conventional setups. Since the publication of TRS-483, at least three new sets of values of k(Qmsr,Q0)(fmsr,fref) for the LGK Icon have been published. The purpose of this study was to experimentally investigate the published values of k(Qmsr,Q0)(fmsr,fref) for commonly used phantom and ionization chamber (IC) models for the LGK Icon.Methods: Dose-rates from two LGK units were determined using acrylonitrile butadiene styrene (ABS) and Certified Medical Grade Solid Water (R) (SW) phantoms, and PTW 31010 and PTW 31016 ICs. Correction factors were applied, and the resulting dose-rates compared. Relative validity of the correction factors was investigated by taking the ratios of dose-rate correction factor products. Additionally, dose-rates from the individual sectors were determined in order to calculate the beam attenuation caused by the ABS phantom adapter.Results and Conclusions: It was seen that the dose-rate is underestimated by at least 1% when using the ABS phantom, which was attributed to fluence perturbation caused by the IC and phantom adapter. Published correction factors k(Qmsr,Q0)(fmsr,fref) account for these effects to varying degree and should be used. The SW phantom is unlikely to underestimate the dose-rate by more than 1%, and applying k(Qmsr,Q0)(fmsr,fref) could not be shown to be necessary. Out of the two phantom models, the ABS phantom is not recommended for use in LGK reference dosimetry. The use of newly published values of k(Qmsr,Q0)(fmsr,fref) should be considered.
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| 11. |
- Kaveckyte, Vaiva, 1991-
(author)
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Development of Experimental Brachytherapy Dosimetry Using Monte Carlo Simulations for Detector Characterization
- 2021
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Doctoral thesis (other academic/artistic)abstract
- Brachytherapy (BT) is a type of interventional radiotherapy that is advantageous due to high absorbed dose conformity and possibility to deliver high dose in few fractions. It is often used for prostate and gynecological tumors as monotherapy or a boost alongside external beam radiotherapy (EBRT). However, there is a number of things that can compromise treatment delivery, starting from incorrect source data in a treatment planning system to malfunctioning of a treatment delivery unit. None of the established quality assurance (QA) procedures emulate treatment delivery where the planned dose could be compared with the experimentally determined value. While such practices are employed in EBRT, BT suffers from the lack of detectors that would be water-equivalent and convenient to use for regular measurements. First-choice thermoluminescence dosimeters are water-equivalent but have passive readout. Sporadic attempts to use other detectors have not led to any established practices at clinical sites. Stepping ahead, the safety of treatment delivery could be further evaluated using real-time in vivo dosimetry. If detectors were characterized with high-accuracy, a reliable error detection level could be set to terminate treatments if needed. Contrary to in-phantom QA, there are detectors suitable for such applications but their characterization is incomplete. In this thesis we address both problems.Focusing on high-dose-rate 192Ir remote afterloading treatments, which are among the most common in BT, we investigate and propose a direct readout synthetic diamond detector for in-phantom QA of treatment units. The detector was designed for small-field high-energy EBRT dosimetry but our findings demonstrate its suitability for BT dosimetry. Additionally, due to detector calibration with traceability to absorbed dose to water primary standards of high-energy photon beams and combined experimental and Monte Carlo (MC) characterization, the uncertainties in absorbed dose to water were comparable to passive readout detectors and lower than for other direct readout detectors. We complemented detector investigation with a theoretical study on diamond material properties and which values (mass density, mean excitation energy, number of conduction electrons per atom) shall be used for the most faithful description of ionizing radiation interactions in diamond for MC simulations and calculations of mass electronic stopping power. The findings improve diamond dosimetry accuracy, and subsequently, experimental dosimetry of not only BT but all radiotherapy beam qualities where the detectors are used.Aiming to further contribute to experimental BT dosimetry, we focused on high atomic number inorganic scintillators used for in vivo dosimetry: ZnSe, CsI, and Al2O3. These are already existing dosimeters exhibiting promising luminescence properties, but until now, their investigation has been solely experimental. MC simulations are not subject to detector positioning uncertainties which are high due to steep dose gradients and other detector response artifacts, thus we used the method to investigate the absorbed-dose energy response of detectors, its dependence on radial distance and polar angles, scatter conditions, as well as detector design. We clarified how error-prone high atomic number detector characterization might be if experimental and MC methods are not combined. Both have certain limitations and have to complement each other.Though the thesis addresses two different types of detectors for two different applications, the underlying theme is to understand the detector at hand. The use of MC simulations allowed introducing a new synthetic diamond detector into BT field and improving accuracy of in vivo dosimetry systems using inorganic scintillators. We also raised awareness to the lack of unified detector calibration and characterization practice in BT dosimetry.
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| 12. |
- Kaveckyte, Vaiva, et al.
(author)
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Investigation of a synthetic diamond detector response in kilovoltage photon beams
- 2020
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In: Medical physics (Lancaster). - : Wiley-Blackwell Publishing Inc.. - 0094-2405 .- 2473-4209. ; 47:3, s. 1268-1279
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Journal article (peer-reviewed)abstract
- Purpose An important characteristic of radiation dosimetry detectors is their energy response which consists of absorbed-dose and intrinsic energy responses. The former can be characterized using Monte Carlo (MC) simulations, whereas the latter (i.e., detector signal per absorbed dose to detector) is extracted from experimental data. Such a characterization is especially relevant when detectors are used in nonrelative measurements at a beam quality that differs from the calibration beam quality. Having in mind the possible application of synthetic diamond detectors (microDiamond PTW 60019, Freiburg, Germany) for nonrelative dosimetry of low-energy brachytherapy (BT) beams, we determined their intrinsic and absorbed-dose energy responses in 25-250 kV beams relative to a Co-60 beam, which is usually the reference beam quality for detector calibration in radiotherapy. Material and Methods Three microDiamond detectors and, for comparison, two silicon diodes (PTW 60017) were calibrated in terms of air-kerma free in air in six x-ray beam qualities (from 25 to 250 kV) and in terms of absorbed dose to water in a Co-60 beam at the national metrology laboratory in Sweden. The PENELOPE/penEasy MC radiation transport code was used to calculate the absorbed-dose energy response of the detectors (modeled based on blueprints) relative to air and water depending on calibration conditions. The MC results were used to extract the relative intrinsic energy response of the detectors from the overall energy response. Measurements using an independent setup with a single ophthalmic BEBIG I25.S16 I-125 BT seed (effective photon energy of 28 keV) were used as a qualitative check of the extracted intrinsic energy response correction factors. Additionally, the impact of the thickness of the active volume as well as the presence of extra-cameral components on the absorbed-dose energy response of a microDiamond detector was studied using MC simulations. Results The relative intrinsic energy response of the microDiamond detectors was higher by a factor of 2 in 25 and 50 kV beams compared to the Co-60 beam. The variation in the relative intrinsic energy response of silicon diodes was within 10% over the investigated photon energy range. The use of relative intrinsic energy response correction factors improved the agreement among the absorbed dose to water values determined using microDiamond detectors and silicon diodes, as well as with the TG-43 formalism-based calculations for the I-125 seed. MC study of microDiamond detector design features provided a possible explanation for inter-detector response variation at low-energy photon beams by differences in the effective thickness of the active volume. Conclusions MicroDiamond detectors had a non-negligible variation in the relative intrinsic energy response (factor of 2) which was comparable to that in the absorbed-dose energy response relative to water at low-energy photon beams. Silicon diodes, in contrast, had an absorbed-dose energy dependence on photon energy that varied by a factor of 6, whereas the intrinsic energy dependence on beam quality was within 10%. It is important to decouple these two responses for a full characterization of detector energy response especially when the user and reference beam qualities differ significantly, and MC alone is not enough.
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| 13. |
- Kaveckyte, Vaiva, et al.
(author)
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Suitability of microDiamond detectors for the determination of absorbed dose to water around high-dose-rate Ir-192 brachytherapy sources
- 2018
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In: Medical physics (Lancaster). - : WILEY. - 0094-2405 .- 2473-4209. ; 45:1, s. 429-437
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Journal article (peer-reviewed)abstract
- Purpose: Experimental dosimetry of high-dose-rate (HDR) Ir-192 brachytherapy (BT) sources is complicated due to high dose and dose-rate gradients, and softening of photon energy spectrum with depth. A single crystal synthetic diamond detector microDiamond (PTW 60019, Freiburg, Germany) has a small active volume, high sensitivity, direct readout, and nearly water-equivalent active volume. The purpose of this study was to evaluate the suitability of microDiamond detectors for the determination of absorbed dose to water around HDR Ir-192 BT sources. Three microDiamond detectors were used, allowing for the comparison of their properties. Methods: In-phantom measurements were performed using microSelectron and VariSource iX HDR Ir-192 BT treatment units. Their treatment planning systems (TPSs), Oncentra (v. 4.3) and BrachyVision (v. 13.6), respectively, were used to create irradiation plans for a cubic PMMA phantom with the microDiamond positioned at one of three source-to-detector distances (SDDs) (1.5, 2.5, and 5.5 cm) at a time. The source was stepped in increments of 0.5 cm over a total length of 6 cm to yield absorbed dose of 2 Gy at the nominal reference-point of the detector. Detectors were calibrated in Co-60 beam in terms of absorbed dose to water, and Monte Carlo (MC) calculated beam quality correction factors were applied to account for absorbed-dose energy dependence. Phantom correction factors were applied to account for differences in dimensions between the measurement phantom and a water phantom used for absorbed dose calculations made with a TPS. The same measurements were made with all three of the detectors. Additionally, dose-rate dependence and stability of the detectors were evaluated in Co-60 beam. Results: The percentage differences between experimentally determined and TPS-calculated absorbed doses to water were from -1.3% to +2.9%. The values agreed to within experimental uncertainties, which were from 1.9% to 4.3% (k = 2) depending on the detector, SDD and treatment delivery unit. No dose-rate or intrinsic energy dependence corrections were applied. All microDiamonds were comparable in terms of preirradiation dose, stability of the readings and energy response, and showed a good agreement. Conclusions: The results indicate that the microDiamond is potentially suitable for the determination of absorbed dose to water around HDR Ir-192 BT sources and may be used for independent verification of TPSs calculations, as well as for QA measurements of HDR Ir-192 BT treatment delivery units at clinical sites. (C) 2017 American Association of Physicists in Medicine
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| 14. |
- Omar, Artur, et al.
(author)
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Monte Carlo investigation of backscatter factors for skin dose determination in interventional neuroradiology procedures
- 2014
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In: Medical Imaging 2014. - : SPIE - International Society for Optical Engineering. - 9780819498267
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Conference paper (peer-reviewed)abstract
- Complex interventional and diagnostic x-ray angiographic (XA) procedures may yield patient skin doses exceeding the threshold for radiation induced skin injuries. Skin dose is conventionally determined by converting the incident air kerma free-in-air into entrance surface air kerma, a process that requires the use of backscatter factors. Subsequently, the entrance surface air kerma is converted into skin kerma using mass energy-absorption coefficient ratios tissue-to-air, which for the photon energies used in XA is identical to the skin dose. The purpose of this work was to investigate how the cranial bone affects backscatter factors for the dosimetry of interventional neuroradiology procedures. The PENELOPE Monte Carlo system was used to calculate backscatter factors at the entrance surface of a spherical and a cubic water phantom that includes a cranial bone layer. The simulations were performed for different clinical x-ray spectra, field sizes, and thicknesses of the bone layer. The results show a reduction of up to 15% when a cranial bone layer is included in the simulations, compared with conventional backscatter factors calculated for a homogeneous water phantom. The reduction increases for thicker bone layers, softer incident beam qualities, and larger field sizes, indicating that, due to the increased photoelectric crosssection of cranial bone compared to water, the bone layer acts primarily as an absorber of low-energy photons. For neurointerventional radiology procedures, backscatter factors calculated at the entrance surface of a water phantom containing a cranial bone layer increase the accuracy of the skin dose determination.
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| 15. |
- Stenman, Maria, et al.
(author)
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Albumin, corticosteroid use and Karnofsky performance status predict outcome of single-fraction gamma knife radiosurgery in clear cell renal cell carcinoma patients with brain metastases
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Other publication (other academic/artistic)abstract
- Background and Purpose: To evaluate the effects of single fraction gamma knife radiosurgery (sf-GKRS) on patients with renal cell carcinoma (RCC) brain metastases (BM) in the era of targeted agents (TA).Material and Methods: Clear cell metastatic RCC patients treated with sf-GKRS due to BM in 2005-2014 at three European centres were retrospectively analysed (n=43). Median follow-up was 56 months. Ninety-five percent had prior nephrectomy, 53% synchronous metastasis and 86% extracranial disease at first sf-GKRS. Karnofsky performance status (KPS) ranged from 60-100%. Outcome measures were overall survival (OS), local control (LC) and adverse radiation effects (ARE).Results: One hundred and ninety-four targets were irradiated. Median number of targets at first sf-GKRS were two. The median prescription dose was 22 Gy. Thirty-seven percent had repeated sf-GKRS. Eighty-eight percent received TA. LC at 12 and 18 months were 97% and 90%. Median OS from first sf-GKRS was 15.7 months. Serum albumin (HR 5.3), corticosteroids pre sf-GKRS (HR 5.8) and KPS (HR 9.1) were independent prognostic factors. MSKCC risk group, synchronous metastasis, age, number of BM or extracranial metastases provided no additional prognostic information. Other prognostic scores for BM radiosurgery, including DS-GPA, Renal-GPA, LLV-SIR and CITV-SIR, did not add prognostic value. ARE were seldom symptomatic and were associated with tumour volume, 10-Gy volume and pre-treatment perifocal oedema. ARE were less common among patients treated with TA within one month of sf-GKRS.Conclusions: We identified three independent prognostic factors with potential impact on clinical decision making in patients with clear cell RCC BM.
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