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Source modeling for Monte Carlo dose calculation of CT examinations with a radiotherapy treatment planning system
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science. Clin Res Ctr, SE-79182 Falun, Sweden..
Raysearch Labs AB, Box 3297, SE-10365 Stockholm, Sweden..
Umea Univ, Dept Radiat Sci Radiat Phys, SE-90185 Umea, Sweden..
Sunderby Hosp, Norrbotten Cty Council, Dept Med Radiat Phys, SE-97180 Lulea, Sweden..
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2016 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 43, no 11, p. 6118-6128Article in journal (Refereed) Published
Abstract [en]

Purpose: Radiation dose to patients undergoing examinations with Multislice Computed Tomography (MSCT) as well as Cone Beam Computed Tomography (CBCT) is a matter of concern. Risk management could benefit from efficient replace rational dose calculation tools. The paper aims to verify MSCT dose calculations using a Treatment Planning System (TPS) for radiotherapy and to evaluate four different variations of bow-tie filter characterizations for the beam model used in the dose calculations. Methods: A TPS (RayStation (TM), RaySearch Laboratories, Stockholm, Sweden) was configured to calculate dose from a MSCT (GE Healthcare, Wauwatosa, WI, USA). The x-ray beam was characterized in a stationary position the by measurements of the Half-Value Layer (HVL) in aluminum and kerma along the principal axes of the isocenter plane perpendicular to the beam. A Monte Carlo source model for the dose calculation was applied with four different variations on the beam-shaping bow-tie filter, taking into account the different degrees of HVL information but reconstructing the measured kerma distribution after the bow-tie filter by adjusting the photon sampling function. The resulting dose calculations were verified by comparison with measurements in solid water as well as in an anthropomorphic phantom. Results: The calculated depth dose in solid water as well as the relative dose profiles was in agreement with the corresponding measured values. Doses calculated in the anthropomorphic phantom in the range 26-55 mGy agreed with the corresponding thermo luminescence dosimeter (TLD) measurements. Deviations between measurements and calculations were of the order of the measurement uncertainties. There was no significant difference between the different variations on the bow-tie filter modeling. Conclusions: Under the assumption that the calculated kerma after the bow-tie filter replicates the measured kerma, the central specification of the HVL of the x-ray beam together with the kerma distribution can be used to characterize the beam. Thus, within the limits of the study, a flat bow-tie filter with an HVL specified by the vendor suffices to calculate the dose distribution. The TPS could be successfully configured to replicate the beam movement and intensity modulation of a spiral scan with dose modulation, on the basis of the specifications available in the metadata of the digital images and the log file of the CT.

Place, publisher, year, edition, pages
2016. Vol. 43, no 11, p. 6118-6128
Keywords [en]
radiation exposure, radiation oncology, tomography, x-ray computed, Monte Carlo method, risk management
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
URN: urn:nbn:se:uu:diva-310771DOI: 10.1118/1.4965043ISI: 000387007500033PubMedID: 27806588OAI: oai:DiVA.org:uu-310771DiVA, id: diva2:1057902
Available from: 2016-12-19 Created: 2016-12-19 Last updated: 2017-12-08Bibliographically approved
In thesis
1. Dose Management in Diagnostic Radiology - application of the DICOM imaging standard and a Monte Carlo dose engine for exposure surveillance
Open this publication in new window or tab >>Dose Management in Diagnostic Radiology - application of the DICOM imaging standard and a Monte Carlo dose engine for exposure surveillance
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Ionizing radiation is used in diagnostic radiology with a large contribution to the health of the patients. The regulations to limit the detrimental effects, e.g. cancer induction, are based on recommendations from the International Commission on Radiological Protection (ICRP). Epidemiological evidence for radiation induced cancer is expressed as a function of absorbed dose in the irradiated organs. The committee for Biological Effects of Ionizing Radiation has favored the use of Lifetime Attributable Risk, a risk estimator applicable to individuals exposed in medical applications. The imaging in radiology complies with a technical standard that potentiates the retrieval of exposure information that can be used in optimization of patient exposure. The information can also be used as input in organ dose calculations.

The aims were to apply the benefits of the technical image standard to radiation safety management by automated collection and analysis of exposure data and to adapt a Therapy Planning System (TPS) for radiotherapy to calculate dose for a Computed Tomography (CT) machine.

An automated workflow for extraction, communication and analysis of exposure data from the image files in the central image archive was defined and implemented at the institution (papers I-II). A source model for Monte Carlo simulation of the CT was developed taking into consideration the energy spectrum of the photons, the spiral movement of the X-ray beam, the beam shaping filter and the tube current modulation (paper III). The source model was used exploring the possibilities to utilize the tissue characterization methods and segmentation tools available in the TPS to devise a strategy to automate organ dose calculations for patients undergoing thorax examinations in a CT (paper IV).

The exposure data workflow was finalized showing that the technical standard for images could supply a framework for automated assembly and analysis of the data, supporting the local implementation of optimization. The CT was modeled with regard to its irradiation characteristics with uncertainties in the dose calculations below 4%. Dose calculations with the tissue characterization methods available in the TPS deviated by less than 2% from measurements and a strategy for automation of organ dose calculations was devised that could facilitate individual risk estimates in CT.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. p. 44
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine, ISSN 1651-6206 ; 1407
Keywords
Radiology, metadata, DICOM, radiation safety, Monte Carlo, source model, patient model, optimization, justification
National Category
Other Medical Sciences not elsewhere specified
Research subject
Medical Radiophysics
Identifiers
urn:nbn:se:uu:diva-335575 (URN)978-91-513-0179-2 (ISBN)
Public defence
2018-02-02, Lecture hall, Falu lasarett, Lasarettsvägen 10, Falun, 13:00 (English)
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Supervisors
Available from: 2018-01-08 Created: 2017-12-07 Last updated: 2018-03-07

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Källman, Hans-ErikAhnesjö, Anders

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