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Monte Carlo investigation of backscatter factors for skin dose determination in interventional neuroradiology procedures
Stockholm University, Faculty of Science, Department of Physics. Karolinska University Hospital, Sweden.
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2014 (English)In: Medical Imaging 2014: Physics of Medical Imaging. Proceedings vol. 9033 / [ed] Bruce R. Whiting; Christoph Hoeschen, SPIE - International Society for Optical Engineering, 2014, , 8 p.Conference paper, Published paper (Refereed)
Abstract [en]

Complex interventional and diagnostic x-ray angiographic (XA) procedures may yield patient skin doses exceeding thethreshold for radiation induced skin injuries. Skin dose is conventionally determined by converting the incident air kermafree-in-air into entrance surface air kerma, a process that requires the use of backscatter factors. Subsequently, theentrance 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 howthe 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 anda cubic water phantom that includes a cranial bone layer. The simulations were performed for different clinical x-rayspectra, 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 withconventional backscatter factors calculated for a homogeneous water phantom. The reduction increases for thicker bonelayers, softer incident beam qualities, and larger field sizes, indicating that, due to the increased photoelectric crosssectionof 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 phantomcontaining a cranial bone layer increase the accuracy of the skin dose determination.

Place, publisher, year, edition, pages
SPIE - International Society for Optical Engineering, 2014. , 8 p.
National Category
Physical Sciences
Research subject
Medical Radiation Physics
Identifiers
URN: urn:nbn:se:su:diva-114134OAI: oai:DiVA.org:su-114134DiVA: diva2:789935
Conference
SPIE Medical Imaging conference, San Diego, California, USA, February 15-20, 2014
Available from: 2015-02-20 Created: 2015-02-20 Last updated: 2015-03-04
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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