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Title: SU-F-T-577: Comparison of Small Field Dosimetry Measurements in Fields Shaped with Conical Applicators On Two Different Accelerating Systems

Abstract

Purpose: To investigate small field dosimetry measurements and associated uncertainties when conical applicators are used to shape treatment fields from two different accelerating systems. Methods: Output factor measurements are made in water in beams from the CyberKnife radiosurgery system, which uses conical applicators to shape fields from a (flattening filter-free) 6 MV beam, and in a 6 MV beam from the Elekta Precise linear accelerator (with flattening filter) with BrainLab external conical applicators fitted to shape the field. The measurements use various detectors: (i) an Exradin A16 ion chamber, (ii) two Exradin W1 plastic scintillation detectors, (iii) a Sun Nuclear Edge diode, and (iv) two PTW microDiamond synthetic diamond detectors. Profiles are used for accurate detector positioning and to specify field size (FWHM). Output factor measurements are corrected with detector specific correction factors taken from the literature where available and/or from Monte Carlo simulations using the EGSnrc code system. Results: Differences in measurements of up to 1.7% are observed with a given detector type in the same beam (i.e., intra-detector variability). Corrected results from different detectors in the same beam (inter-detector differences) show deviations up to 3 %. Combining data for all detectors and comparing results from the two acceleratorsmore » results in a 5.9% maximum difference for the smallest field sizes (FWHM=5.2–5.6 mm), well outside the combined uncertainties (∼1% for the smallest beams) and/or differences among detectors. This suggests that the FWHM of a measured profile is not a good specifier to compare results from different small fields with the same nominal energy. Conclusion: Large differences in results for both intra-detector variability and inter-detector differences suggest potentially high uncertainties in detector-specific correction factors. Differences between the results measured in circular fields from different accelerating systems provide insight into sources of variability in small field dosimetric measurements reported in the literature.« less

Authors:
;  [1]; ;  [2];  [3]
  1. National Research Council, Ottawa, ON (Canada)
  2. Ottawa Hospital General Campus, Ottawa, ON (Canada)
  3. Carleton University, Ottawa, ON (Canada)
Publication Date:
OSTI Identifier:
22649152
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
07 ISOTOPES AND RADIATION SOURCES; 60 APPLIED LIFE SCIENCES; BEAMS; COMPUTERIZED SIMULATION; CORRECTIONS; DOSIMETRY; IONIZATION CHAMBERS; LINEAR ACCELERATORS; MONTE CARLO METHOD

Citation Formats

Muir, B, McEwen, M, Belec, J, Vandervoort, E, and Christiansen, E. SU-F-T-577: Comparison of Small Field Dosimetry Measurements in Fields Shaped with Conical Applicators On Two Different Accelerating Systems. United States: N. p., 2016. Web. doi:10.1118/1.4956762.
Muir, B, McEwen, M, Belec, J, Vandervoort, E, & Christiansen, E. SU-F-T-577: Comparison of Small Field Dosimetry Measurements in Fields Shaped with Conical Applicators On Two Different Accelerating Systems. United States. doi:10.1118/1.4956762.
Muir, B, McEwen, M, Belec, J, Vandervoort, E, and Christiansen, E. Wed . "SU-F-T-577: Comparison of Small Field Dosimetry Measurements in Fields Shaped with Conical Applicators On Two Different Accelerating Systems". United States. doi:10.1118/1.4956762.
@article{osti_22649152,
title = {SU-F-T-577: Comparison of Small Field Dosimetry Measurements in Fields Shaped with Conical Applicators On Two Different Accelerating Systems},
author = {Muir, B and McEwen, M and Belec, J and Vandervoort, E and Christiansen, E},
abstractNote = {Purpose: To investigate small field dosimetry measurements and associated uncertainties when conical applicators are used to shape treatment fields from two different accelerating systems. Methods: Output factor measurements are made in water in beams from the CyberKnife radiosurgery system, which uses conical applicators to shape fields from a (flattening filter-free) 6 MV beam, and in a 6 MV beam from the Elekta Precise linear accelerator (with flattening filter) with BrainLab external conical applicators fitted to shape the field. The measurements use various detectors: (i) an Exradin A16 ion chamber, (ii) two Exradin W1 plastic scintillation detectors, (iii) a Sun Nuclear Edge diode, and (iv) two PTW microDiamond synthetic diamond detectors. Profiles are used for accurate detector positioning and to specify field size (FWHM). Output factor measurements are corrected with detector specific correction factors taken from the literature where available and/or from Monte Carlo simulations using the EGSnrc code system. Results: Differences in measurements of up to 1.7% are observed with a given detector type in the same beam (i.e., intra-detector variability). Corrected results from different detectors in the same beam (inter-detector differences) show deviations up to 3 %. Combining data for all detectors and comparing results from the two accelerators results in a 5.9% maximum difference for the smallest field sizes (FWHM=5.2–5.6 mm), well outside the combined uncertainties (∼1% for the smallest beams) and/or differences among detectors. This suggests that the FWHM of a measured profile is not a good specifier to compare results from different small fields with the same nominal energy. Conclusion: Large differences in results for both intra-detector variability and inter-detector differences suggest potentially high uncertainties in detector-specific correction factors. Differences between the results measured in circular fields from different accelerating systems provide insight into sources of variability in small field dosimetric measurements reported in the literature.},
doi = {10.1118/1.4956762},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = {Wed Jun 15 00:00:00 EDT 2016},
month = {Wed Jun 15 00:00:00 EDT 2016}
}
  • Purpose: This study is aiming to identify the smallest field size for which a treatment planning system (TPS) can accurately calculate the relative dose distribution. The finding would be used as a guideline to choose the smallest proton field for clinical treatment. Methods: Mevion S250™ double scattering proton delivery system and Eclipse™ TPS (Varian) with pencil beam convolution (PBC) dose algorithm were used in this study. Square sized fields of 1 cm, 2 cm, 3 cm, 4 cm, 5 cm, and 10 cm were planned on a cubical water phantom with iso-center placed at 10 cm depth. All beams usedmore » the same proton beam option: range 15 cm and modulation 10 cm. Dose in water was calculated without any compensator. Gafchromic™ EBT3 film and diode detectors were used to measure the central axis dose distribution and lateral dose profiles at 5 cm, 10 cm, and 14 cm depth. Results: The preliminary film measurement shows good agreement between Eclipse calculated lateral dose profiles for all tested field sizes. The differences on full width half maximum were ≤ 1 mm while the differences on the penumbras were between 1 mm and 2 mm between Eclipse and film. For the depth dose, Eclipse results matched well with film measurements for field sizes down to 2 cm{sup 2}. With smaller field size of 1 cm{sup 2}, Eclipse was able to predict the decreasing of SOBP due to the lack of lateral charged particle equilibrium in depth. However, it did not match the film measurement. Diode measurement results will be available at the time of presentation. Conclusion: The PBC dose algorithm in Eclipse can accurately calculate relative dose distribution in double scattered proton system for field size down to 2 cm{sup 2}.« less
  • Purpose: To investigate the effect of plaque design and radionuclides on eye plaque dosimetry. Methods: The Monte Carlo N-particle Code version 6 (MCNP6) was used for radiation transport simulations. The 14 mm and 16 mm diameter COMS plaques and the model EP917 plaque were simulated using brachytherapy seeds containing I-125, Pd-103, and Cs-131 radionuclides. The origin was placed at the scleral inner surface. The central axis (CAX) doses of both COMS plaques at −1 mm, 0 mm, 1 mm, 2 mm, 5 mm, 10 mm, 15 mm, 20 mm, and 22.6 mm were compared to the model EP917 plaque. Dosemore » volume histograms (DVHs) were also created for both COMS plaques for the tumor and outer sclera then compared to results for the model EP917 plaque. Results: For all radionuclides, the EP917 plaque delivered higher dose (max 343%) compared to the COMS plaques, except for the 14 mm COMS plaque with Cs-131 at 1 mm and 2 mm depths from outer sclera surface. This could be due to source design. For all radionuclides, the 14 mm COMS plaque delivered higher doses compared to the 16 mm COMS plaque for the depths up to 5 mm. Dose differences were not significant beyond depths of 10 mm due to ocular lateral scatter for the different plaque designs. Tumor DVHs for the 16 mm COMS plaque with Cs-131 provided better dose homogeneity and conformity compared to other COMS plaques with I-125 and Pd-103. Using Pd-103, DVHs for the 16 mm COMS plaque delivered less dose to outer sclera compared to other plaques. Conclusion: This study identified improved tumor homogeneity upon considering radionuclides and plaque designs, and found that scleral dose with the model EP917 plaque was higher than for the 16 mm COMS plaque for all the radionuclides studied.« less
  • Purpose: For small field dosimetry, such as measurements of output factors for cones or MLC-shaped irregular small fields, ion chambers often Result in an underestimation of the dose, due to both the volume averaging effect and the lack of lateral charged particle equilibrium. This work presents a mathematical model for correction matrix for a PTW PinPoint ionization chamber for dosimetric measurements made in the newly released Incise™ Multileaf collimator fields of the CyberKnife M6™ machine. Methods: A correction matrix for a PTW 0.015cc PinPoint ionization chamber was developed by modeling its 3D dose response in twelve cone-shaped circular fields createdmore » using the 5mm, 7.5mm, 10mm, 12.5mm, 15mm, 20mm, 25mm, 30mm, 35mm, 40mm, 50mm, 60mm cones in a CyberKnife M6™ machine. For each field size, hundreds of readings were recorded for every 2mm chamber shift in the horizontal plane. The contribution of each dose pixel to a measurement point depended on the radial distance and the angle to the chamber axis. These readings were then compared with the theoretical dose as obtained with Monte Carlo calculation. A penalized least-square optimization algorithm was developed to generate the correction matrix. After the parameter fitting, the mathematical model was validated for MLC-shaped irregular fields. Results: The optimization algorithm used for parameter fitting was stable and the resulted response factors were smooth in spatial domain. After correction with the mathematical model, the chamber reading matched with the calculation for all the tested fields to within 2%. Conclusion: A novel mathematical model has been developed for PinPoint chamber for dosimetric measurements in small MLC-shaped irregular fields. The correction matrix is dependent on detector, treatment unit and the geometry of setup. The model can be applied to non-standard composite fields and provides an access to IMRT point dose validation.« less
  • Purpose: To compare two types of semiconductor detectors for in vivo dosimetry by their dependence from various parameters in different conditions. Methods: QED yellow (Sun Nuclear) and EDP (Scanditronix) Si detectors were radiated by a Varian Clinac 2300 ix with 6 and 18 MV energies. 10 cm thickness water equivalent phantom consisted of 30×30 cm{sup 2} squared plates was used for experiments. Dose dependencies for different beam angles (0 – 180°), field size (3–40 cm), dose (50 – 300 MU), and dose rates (50 – 300 MU/min) were obtained and calibrated with Standard Farmer chamber (PTW). Results: Reproducibility, linearity, dosemore » rate, angular dependence, and field size dependence were obtained for QED and EDP. They show no dose-rate dependence in available clinical dose rate range (100–600 MU/min). Both diodes have linear dependence with increasing the dose. Therefore even in case of high radiation therapy (including total body irradiation) it is not necessary to apply an additional correction during in vivo dosimetry. The diodes have different behavior for angular and field size dependencies. QED diode showed that dose value is stable for beam angles from 0 to 60°, for 60–180° correction factor has to be applied for each beam angle during in vivo measurements. For EDP diode dose value is sensitive to beam angle in whole range of angles. Conclusion: The study shows that QED diode is more suitable for in vivo dosimetry due to dose value independence from incident beam angle in the range 0–60°. There is no need in correction factors for increasing of dose and dose rate for both diodes. The next step will be to carry out measurements in non-standard conditions of total body irradiation. After this modeling of these experiments with Monte Carlo simulation for comparison calculated and obtained data is planned.« less
  • Purpose: To calculate output correction factors for Varian Clinac 2100iX beams for seven small field detectors and use the values to determine the small field output factors for the linacs at Karolinska university hospital. Methods: Phase space files (psf) for square fields between 0.25cm and 10cm were calculated using the PENELOPE-based PRIMO software. The linac MC-model was tuned by comparing PRIMO-estimated and experimentally determined depth doses and lateral dose-profiles for 40cmx40cm fields. The calculated psf were used as radiation sources to calculate the correction factors of IBA and PTW detectors with the code penEasy/PENELOPE. Results: The optimal tuning parameters ofmore » the MClinac model in PRIMO were 5.4 MeV incident electron energy and zero energy spread, focal spot size and beam divergence. Correction factors obtained for the liquid ion chamber (PTW-T31018) are within 1% down to 0.5 cm fields. For unshielded diodes (IBA-EFD, IBA-SFD, PTW-T60017 and PTW-T60018) the corrections are up to 2% at intermediate fields (>1cm side), becoming down to −11% for fields smaller than 1cm. The shielded diode (IBA-PFD and PTW-T60016) corrections vary with field size from 0 to −4%. Volume averaging effects are found for most detectors in the presence of 0.25cm fields. Conclusion: Good agreement was found between correction factors based on PRIMO-generated psf and those from other publications. The calculated factors will be implemented in output factor measurements (using several detectors) in the clinic. PRIMO is a userfriendly general code capable of generating small field psf and can be used without having to code own linac geometries. It can therefore be used to improve the clinical dosimetry, especially in the commissioning of linear accelerators. Important dosimetry data, such as dose-profiles and output factors can be determined more accurately for a specific machine, geometry and setup by using PRIMO and having a MC-model of the detector used.« less