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Rahimi SA, Hashemi B, Mahdavi SR. Estimation of Dosimetric Parameters based on K NR and K NCSF Correction Factors for Small Field Radiation Therapy at 6 and 18 MV Linac Energies using Monte Carlo Simulation Methods. J Biomed Phys Eng 2019; 9:37-50. [PMID: 30881933 PMCID: PMC6409371] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2015] [Accepted: 12/24/2016] [Indexed: 06/09/2023]
Abstract
BACKGROUND Estimating dosimetric parameters for small fields under non-reference conditions leads to significant errors if done based on conventional protocols used for large fields in reference conditions. Hence, further correction factors have been introduced to take into account the influence of spectral quality changes when various detectors are used in non-reference conditions at different depths and field sizes. OBJECTIVE Determining correction factors (KNR and KNCSF) recommended recently for small field dosimetry formalism by American Association of Physicists in Medicine (AAPM) for different detectors at 6 and 18 MV photon beams. METHODS EGSnrc Monte Carlo code was used to calculate the doses measured with different detectors located in a slab phantom and the recommended KNR and KNCSF correction factors for various circular small field sizes ranging from 5-30 mm diameters. KNR and KNCSF correction factors were determined for different active detectors (a pinpoint chamber, EDP-20 and EDP-10 diodes) in a homogeneous phantom irradiated to 6 and 18 MV photon beams of a Varian linac (2100C/D). RESULTS KNR correction factor estimated for the highest small circular field size of 30 mm diameter for the pinpoint chamber, EDP-20 and EDP-10 diodes were 0.993, 1.020 and 1.054; and 0.992, 1.054 and 1.005 for the 6 and 18 MV beams, respectively. The KNCSF correction factor estimated for the lowest circular field size of 5 mm for the pinpoint chamber, EDP-20 and EDP-10 diodes were 0.994, 1.023, and 1.040; and 1.000, 1.014, and 1.022 for the 6 and 18 MV photon beams, respectively. CONCLUSION Comparing the results obtained for the detectors used in this study reveals that the unshielded diodes (EDP-20 and EDP-10) can confidently be recommended for small field dosimetry as their correction factors (KNR and KNCSF) was close to 1.0 for all small field sizes investigated and are mainly independent from the electron beam spot size.
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Affiliation(s)
- S A Rahimi
- PhD Candidate, Department of Medical Physics, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran
- Assistant Professor, Department of Basic Sciences, Faculty of Health Sciences, Mazandaran University of Medical Sciences, Sari, Iran
| | - B Hashemi
- Associate Professor, Department of Medical Physics, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran
| | - S R Mahdavi
- Associate Professor, Department of Medical Physics, Faculty of Medicine, Iran University of Medical Sciences, Tehran, Iran
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Wegener S, Sauer OA. Separation of scatter from small MV beams and its effect on detector response. Med Phys 2017; 44:1139-1148. [PMID: 28063164 DOI: 10.1002/mp.12091] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2016] [Revised: 12/20/2016] [Accepted: 01/01/2017] [Indexed: 11/09/2022] Open
Abstract
PURPOSE Separating the scatter from the primary component of a MV beam to study detector response separately in each case for a better understanding of the role of different effects influencing the response in nonstandard fields. METHODS Detector response in three different experimental setups was investigated for a variety of different types (diamond, shielded and unshielded diodes, ionization chamber and film): (a). Detectors positioned in water under a thin steel pole blocking the central part of the beam, yielding only the response to the scatter part of the beam. (b). Detectors positioned in air under a PMMA cap to approximate the contribution of the primary beam without scatter. (c). Detectors positioned in water in the standard open field configuration to obtain a superposition of both. RESULTS Detector differences became more clearly observable when the primary beam was blocked and detector behavior heavily depended on the construction type. It was possible to calculate the response in the open fields from the values measured in the blocked configuration with 1% accuracy for all studied field sizes between 0.8 and 10 cm and for all detectors. CONCLUSIONS The limitations of clinically used detectors in nonstandard situations were illustrated in the extreme situation of just scattered radiation reaching the detector. By experimentally separating scatter from the primary beam, the roles of different effects on the detector response were observed.
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Affiliation(s)
- Sonja Wegener
- Department of Radiation Oncology, University of Würzburg, 97080, Würzburg, Germany
| | - Otto A Sauer
- Department of Radiation Oncology, University of Würzburg, 97080, Würzburg, Germany
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Chofor N, Harder D, Selbach HJ, Poppe B. The mean photon energy ĒF at the point of measurement determines the detector-specific radiation quality correction factor kQ,M in (192)Ir brachytherapy dosimetry. Z Med Phys 2015; 26:238-50. [PMID: 26387927 DOI: 10.1016/j.zemedi.2015.08.002] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2015] [Revised: 06/19/2015] [Accepted: 08/03/2015] [Indexed: 11/17/2022]
Abstract
The application of various radiation detectors for brachytherapy dosimetry has motivated this study of the energy dependence of radiation quality correction factor kQ,M, the quotient of the detector responses under calibration conditions at a (60)Co unit and under the given non-reference conditions at the point of measurement, M, occurring in photon brachytherapy. The investigated detectors comprise TLD, radiochromic film, ESR, Si diode, plastic scintillator and diamond crystal detectors as well as ionization chambers of various sizes, whose measured response-energy relationships, taken from the literature, served as input data. Brachytherapy photon fields were Monte-Carlo simulated for an ideal isotropic (192)Ir point source, a model spherical (192)Ir source with steel encapsulation and a commercial HDR GammaMed Plus source. The radial source distance was varied within cylindrical water phantoms with outer radii ranging from 10 to 30cm and heights from 20 to 60cm. By application of this semiempirical method - originally developed for teletherapy dosimetry - it has been shown that factor kQ,M is closely correlated with a single variable, the fluence-weighted mean photon energy ĒF at the point of measurement. The radial profiles of ĒF obtained with either the commercial (192)Ir source or the two simplified source variants show little variation. The observed correlations between parameters kQ,M and ĒF are represented by fitting formulae for all investigated detectors, and further variation of the detector type is foreseen. The herewith established close correlation of radiation quality correction factor kQ,M with local mean photon energy ĒF can be regarded as a simple regularity, facilitating the practical application of correction factor kQ,M for in-phantom dosimetry around (192)Ir brachytherapy sources. ĒF values can be assessed by Monte Carlo simulation or measurement. A technique describing the local measurement of ĒF will be published separately.
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Affiliation(s)
- Ndimofor Chofor
- Medical Radiation Physics Group, University of Oldenburg and Pius-Hospital Oldenburg, Germany.
| | - Dietrich Harder
- Medical Radiation Physics Group, University of Oldenburg and Pius-Hospital Oldenburg, Germany
| | - Hans-Joachim Selbach
- Medical Radiation Physics Group, University of Oldenburg and Pius-Hospital Oldenburg, Germany
| | - Björn Poppe
- Medical Radiation Physics Group, University of Oldenburg and Pius-Hospital Oldenburg, Germany
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Determination of the energy response of closely tissue-equivalent diamond dosimeters for radiotherapy dosimetry. Appl Radiat Isot 2012; 71 Suppl:23-4. [DOI: 10.1016/j.apradiso.2012.03.016] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/02/2012] [Revised: 03/12/2012] [Accepted: 03/13/2012] [Indexed: 11/22/2022]
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Chofor N, Harder D, Poppe B. Non-reference condition correction factor kNR of typical radiation detectors applied for the dosimetry of high-energy photon fields in radiotherapy. Z Med Phys 2012; 22:181-96. [DOI: 10.1016/j.zemedi.2012.05.001] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/23/2012] [Revised: 03/29/2012] [Accepted: 05/03/2012] [Indexed: 11/25/2022]
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Cherpak A, Ding W, Hallil A, Cygler JE. Evaluation of a novel 4D in vivo dosimetry system. Med Phys 2009; 36:1672-9. [PMID: 19544784 DOI: 10.1118/1.3100264] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
A prototype of a new 4D in vivo dosimetry system capable of simultaneous real-time position monitoring and dose measurement has been developed. The radiation positioning system (RADPOS) is controlled by a computer and combines two technologies: MOSFET radiation detector coupled with an electromagnetic positioning device. Special software has been developed that allows sampling position and dose either manually or automatically in user-defined time intervals. Preliminary tests of the new device include a dosimetric evaluation of the detector in 60Co, 6 MV, and 18 MV beams and measurements of spatial position stability and accuracy. In addition, the effect of metals and other materials on the performance of the positioning system has been investigated. Results show that the RADPOS system can measure in-air dose profiles that agree, on average, within 3%-5% of diode measurements for the energies tested. The response of the detector is isotropic within 1.6% (1 SD) with a maximum deviation of +/- 4.0% over 360 degrees. The maximum variation in the calibration coefficient over field sizes from 6 x 6 to 25 x 25 cm2 was 2.3% for RADPOS probe with the high sensitivity MOSFET and 4.6% for the probe with the standard sensitivity MOSFET. Of the materials tested, only aluminum, lead, and brass caused shifts in the RADPOS read position. The magnitude of the shift varied between materials and size of the material sample. Nonmagnetic stainless steel (Grade 304) caused a distortion of less than 2 mm when placed within 10 mm of the detector; therefore, it can provide a reasonable alternative to other metals if required. The results of the preliminary tests indicate that the device can be used for in vivo dosimetry in 60Co and high-energy beams from linear accelerators.
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Affiliation(s)
- A Cherpak
- Carleton University, 1125 Colonel By Drive, Ottawa, Ontario KIS 5B6, Canada
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Abrego FC, Calcina CSG, de Almeida A, de Almeida CE, Baffa O. Relative output factor and beam profile measurements of small radiation fields with an L-alanine/K-Band EPR minidosimeter. Med Phys 2007; 34:1573-82. [PMID: 17555239 DOI: 10.1118/1.2717414] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Abstract
The performance of an L-alanine dosimeter with millimeter dimensions was evaluated for dosimetry in small radiation fields. Relative output factor (ROF) measurements were made for 0.5 x 0.5, 1 x 1, 3 x 3, 5 x 5, 10 x 10 cm(2) square fields and for 5-, 10-, 20-, 40-mm-diam circular fields. In beam profile (BP) measurements, only 1 x 1, 3 x 3, 5 x 5 cm2 square fields and 10-, 20-, 40-mm-diam circular fields were used. For square and circular field irradiations, Varian/Clinac 2100, and a Siemens/Mevatron 6 MV linear accelerators were used, respectively. For a batch of 800 L-alanine minidosimeters (miniALAs) the average mass was 4.3+/-0.5 (1 sigma) mg, the diameter was 1.22+/-0.07 (1 sigma) mm, and the length was 3.5+/-0.2 (l sigma) mm. A K-Band (24 GHz) electron paramagnetic resonance (EPR) spectrometer was used for recording the spectrum of irradiated and nonirradiated miniALAs. To evaluate the performance of the miniALAs, their ROF and BP results were compared with those of other types of detectors, such as an ionization chamber (PTW 0.125 cc), a miniTLD (LiF: Mg,Cu,P), and Kodak/X-Omat V radiographic film. Compared to other dosimeters, the ROF results for miniALA show differences of up to 3% for the smallest fields and 7% for the largest ones. These differences were within the miniALA experimental uncertainty (-5-6% at 1 sigma). For BP measurements, the maximum penumbra width difference observed between miniALA and film (10%-90% width) was less than 1 mm for square fields and within 1-2 mm for circular fields. These penumbra width results indicate that the spatial resolution of the miniALA is comparable to that of radiographic film and its dimensions are adequate for the field sizes used in this experiment. The K-Band EPR spectrometer provided adequate sensitivity for assessment of miniALAs with doses of the order of tens of Grays, making this dosimetry system (K-Band/miniALA) a potential candidate for use in radiosurgery dosimetry.
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Affiliation(s)
- Felipe Chen Abrego
- Department of Physics and Mathematics, FFCLRP-University of São Paulo, 14040-901, Ribeirão Preto-SP, Brazil
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Edwards CR, Mountford PJ, Green S, Palethorpe JE, Moloney AJ. The low energy X-ray response of the LiF:Mg:Cu:P thermoluminescent dosemeter: a comparison with LiF:Mg:Ti. Br J Radiol 2005; 78:543-7. [PMID: 15900061 DOI: 10.1259/bjr/73133162] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022] Open
Abstract
LiF:Mg:Cu:P thermoluminescent dosemeters (TLD) can be used for the same X-ray dosimetry applications as LiF:Mg:Ti, with each type having the disadvantage of a response dependent on energy, particularly at low energies. Measurements were made of the response per unit air kerma of LiF:Mg:Cu:P and LiF:Mg:Ti to nine quasi-monoenergetic X-ray beams with mean energies from 12 keV to 208 keV. Each measurement was normalized to the value produced by 6 MV X-rays. LiF:Mg:Cu:P was found to under-respond to a majority of these radiations whereas LiF:Mg:Ti over-responded to a majority. Their smallest relative measured response was produced by the lowest energy beam, and the maximum measured relative response of 1.15+/-0.07 and 1.21+/-0.07 for LiF:Mg:Cu:P and LiF:Mg:Ti, respectively, occurred at 33 keV. Energy response coefficients were derived from these measurements to estimate the error introduced by using either type of TLD to measure the dose from an X-ray spectrum different to that used for its absolute response calibration. It was calculated that if the response of either type of TLD was calibrated at 100 kVp, then an error of no more than +/-2% would be introduced into measurements of tube output at potentials of 50-130 kVp. LiF:Mg:Cu:P was found to introduce a larger error (up to 30%) into the measurement of body exit dose than LiF:Mg:Ti at tube potentials of 40-150 kVp, if its absolute response was calibrated using the corresponding body entrance beam. The method should allow this type of error to be estimated in other dosimetry applications for either type of TLD.
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Affiliation(s)
- C R Edwards
- Medical Physics Directorate, University Hospital of North Staffordshire, Princes Road, Hartshill, Stoke-on-Trent, Staffordshire ST4 7LN, UK
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Abstract
The purpose of this study was to investigate the difference between a 6 MV linear accelerator x-ray energy spectrum outside the field edge near a phantom surface, and the corresponding spectrum on the central axis. The Monte Carlo code MCNP-4A was used to calculate the spectra on the central axis and at 1, 2, 5 and 10 cm from the edge of a 4 x 4 cm2, 10 x 10 cm2 and 15 x 15 cm2 field. Compared to the spectrum on the central axis, the spectra outside the field edge showed two distinct regions: a broad peak below about 0.5 MeV, and a lower amplitude, less rapidly changing region at higher energies from 0.5 to 6 MeV. The lower energy peak was due to scattered photons, and the higher energy component was due mainly to primary photons transmitted through the jaws of the secondary collimator. The potential impact of these spectral differences on critical organ photon dosimetry was determined by calculating the ratio of the sensitivity of a Scanditronix EDD-5 diode and of a LiF:Mg:Ti thermoluminescent dosimeter (TLD) outside the field edge to their respective sensitivity at the calibration position on the central axis. The lower energy peak combined with the non-uniform energy sensitivity of each detector produced up to a two-thirds overestimate of x-ray dose outside the field by the diode, whereas the response ratio of the TLD was about unity. These results indicated that a similar evaluation was required for profile measurements of a dynamic wedged field and measurements in an intensity modulated beam with either type of detector.
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Affiliation(s)
- C R Edwards
- Radiotherapy Physics, University Hospital of North Staffordshire, Princes Road, Hartshill, Stoke-on-Trent, Staffordshire, ST4 7LN, UK.
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Yin Z, Hugtenburg RP, Beddoe AH. Response corrections for solid-state detectors in megavoltage photon dosimetry. Phys Med Biol 2005; 49:3691-702. [PMID: 15446798 DOI: 10.1088/0031-9155/49/16/015] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Solid-state detectors offer high sensitivity, stability and resolution and are frequently the dosimeter of choice for on-line dosimetry and small field therapies such as stereotactic radiosurgery. The departure from tissue equivalence of many solid-state devices, including diodes and MOSFETs, has to be carefully considered at lower energies and for Compton scattered radiation where the strongly Z-dependent photoelectric effect is significant. A modification of Burlin cavity theory is proposed that treats primary and scatter photon spectra separately and this has been applied to determine the correction factors for diode detector measurements of 6 and 15 MV linear accelerator beams. Uncorrected, an unshielded diode overestimates the dose at depth by as much as 15% for the 6 MV beam. The model predicts the effect to within 1% for both energies offering a basis for the correction of diodes for use in routine dosimetry.
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Affiliation(s)
- Z Yin
- School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK
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