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Output correction factors for nine small field detectors in 6 MV radiation therapy photon beams: A PENELOPE Monte Carlo study
Stockholm University, Faculty of Science, Department of Physics. Karolinska Institutet, Sweden.
Stockholm University, Faculty of Science, Department of Physics. Karolinska Institutet, Sweden.
2014 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 41, no 4, 041711- p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2014. Vol. 41, no 4, 041711- p.
Keyword [en]
small field dosimetry, Monte Carlo, PENELOPE, output correction factors
National Category
Radiology, Nuclear Medicine and Medical Imaging
Research subject
Medical Radiation Physics
Identifiers
URN: urn:nbn:se:su:diva-103969DOI: 10.1118/1.4868695ISI: 000334287000014OAI: oai:DiVA.org:su-103969DiVA: diva2:721824
Note

AuthorCount:3;

Available from: 2014-06-05 Created: 2014-05-27 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Key Data for the Reference and Relative Dosimetry of Radiotherapy and Diagnostic and Interventional Radiology Beams
Open this publication in new window or tab >>Key Data for the Reference and Relative Dosimetry of Radiotherapy and Diagnostic and Interventional Radiology Beams
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2015. 68 p.
Keyword
Backscatter factors, Diagnostic radiology dosimetry, Mass energy-absorption coefficients, Monte Carlo, Output correction factors, Radiotherapy dosimetry, Reference dosimetry, Relative dosimetry, Small photon fields
National Category
Physical Sciences
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-114413 (URN)978-91-7649-111-9 (ISBN)
Public defence
2015-04-22, Föreläsningssalen, (P1:01), Radiumhemmet, Karolinska Universitetssjukhuset, Solna, 14:30 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 6: Manuscript.

Available from: 2015-03-31 Created: 2015-03-03 Last updated: 2015-05-12Bibliographically approved

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