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Key Data for the Reference and Relative Dosimetry of Radiotherapy and Diagnostic and Interventional Radiology Beams
Stockholm University, Faculty of Science, Department of Physics.
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 [en]
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: urn:nbn:se:su:diva-114413ISBN: 978-91-7649-111-9 (print)OAI: oai:DiVA.org:su-114413DiVA: diva2:792325
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
List of papers
1. Ten years after: Impact of recent research in photon and electron beam dosimetry on the IAEA TRS-398 Code of Practice
Open this publication in new window or tab >>Ten years after: Impact of recent research in photon and electron beam dosimetry on the IAEA TRS-398 Code of Practice
2011 (English)In: Standards, Applications and Quality Assurance in Medical Radiation Dosimetry (IDOS): Proceedings of an International Symposium. V. 1 / [ed] Benmakhlouf, H.; Andreo, P., Vienna: International Atomic Energy Agency, 2011, , 14 p.139-152 p.Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
Vienna: International Atomic Energy Agency, 2011. 14 p.
Series
Proceedings series / International Atomic Energy Agency, ISSN 0074-1884
National Category
Other Physics Topics
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-114132 (URN)978-92-0-116210-6 (ISBN)
Conference
International Symposium on Standards, Applications and Quality Assurance in Medical Radiation Dosimetry, Vienna, Austria, 9-12 November 2010
Available from: 2015-02-20 Created: 2015-02-20 Last updated: 2015-03-05Bibliographically approved
2. Backscatter factors and mass energy-absorption coefficient ratios for diagnostic radiology dosimetry
Open this publication in new window or tab >>Backscatter factors and mass energy-absorption coefficient ratios for diagnostic radiology dosimetry
2011 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 56, no 22, 7179-7204 p.Article in journal (Refereed) Published
Abstract [en]

Backscatter factors, B, and mass energy-absorption coefficient ratios, (mu(en)/rho)(omega,) (air), for the determination of the surface dose in diagnostic radiology were calculated using Monte Carlo simulations. The main purpose was to extend the range of available data to qualities used in modern x-ray techniques, particularly for interventional radiology. A comprehensive database for mono-energetic photons between 4 and 150 keV and different field sizes was created for a 15 cm thick water phantom. Backscattered spectra were calculated with the PENELOPE Monte Carlo system, scoring track-length fluence differential in energy with negligible statistical uncertainty; using the Monte Carlo computed spectra, B factors and (mu(en)/rho)(omega), air were then calculated numerically for each energy. Weighted averaging procedures were subsequently used to convolve incident clinical spectra with mono-energetic data. The method was benchmarked against full Monte Carlo calculations of incident clinical spectra obtaining differences within 0.3-0.6%. The technique used enables the calculation of B and (mu(en)/rho)(w), air for any incident spectrum without further time-consuming Monte Carlo simulations. The adequacy of the extended dosimetry data to a broader range of clinical qualities than those currently available, while keeping consistency with existing data, was confirmed through detailed comparisons. Mono-energetic and spectra-averaged values were compared with published data, including those in ICRU Report 74 and IAEA TRS-457, finding average differences of 0.6%. Results are provided in comprehensive tables appropriated for clinical use. Additional qualities can easily be calculated using a designed GUI interface in conjunction with software to generate incident photon spectra.

National Category
Physical Sciences
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-70659 (URN)10.1088/0031-9155/56/22/012 (DOI)000296768700016 ()
Note

authorCount :4

Available from: 2012-01-24 Created: 2012-01-23 Last updated: 2017-12-08Bibliographically approved
3. Influence of phantom thickness and material on the backscatter factors for diagnostic x-ray beam dosimetry
Open this publication in new window or tab >>Influence of phantom thickness and material on the backscatter factors for diagnostic x-ray beam dosimetry
2013 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 58, no 2, 247-260 p.Article in journal (Refereed) Published
Abstract [en]

Most of the existing backscatter factors for the dosimetry of clinical diagnostic x-ray beams have been calculated for 15 cm thick phantoms; these data are used for skin dose determinations which in general ignore the influence of phantom material and thickness. The former should strictly be required whenever dosimetry measurements are made on phantom materials different from those used for the backscatter factor calculations. The phantom or patient thickness is of special importance when skin dose determinations are made for infants or paediatric patients. In this work, the recently published formalism for reference dosimetry and comprehensive database of backscatter factors for clinical beams and water phantoms have been extended using two correction factors which account for phantom material and thickness. These were determined with simulations using the PENELOPE Monte Carlo system, for PMMA to analyse the influence of the phantom material relative to water, and for a broad range of thicknesses of water and PMMA to investigate the role of this parameter in patient dose estimates. The material correction factor was found to be in the range 3-10%, depending on the field size and the HVL. The thickness correction factor was in the range 2-12% for a 5 cm thick phantom and square field sizes between 5 and 35 cm, reaching a plateau of about ±1% for thicknesses beyond 13 cm. Expressions in the form of surface fits over the calculated data are provided which streamline the determination of backscatter factors for arbitrary thicknesses and phantom materials, as well as field sizes. Results demonstrate the inadequacy of using conventional backscatter factors (calculated for 15 cm thick phantoms) without correction factors that take into account the phantom material and its thickness.

Keyword
small field dosimetry, Monte Carlo, PENELOPE, output correction factors
National Category
Physical Sciences
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-114133 (URN)10.1088/0031-9155/58/2/247 (DOI)000312674400006 ()23257169 (PubMedID)
Available from: 2015-02-20 Created: 2015-02-20 Last updated: 2017-12-04Bibliographically approved
4. Output correction factors for nine small field detectors in 6 MV radiation therapy photon beams: A PENELOPE Monte Carlo study
Open this publication in new window or tab >>Output correction factors for nine small field detectors in 6 MV radiation therapy photon beams: A PENELOPE Monte Carlo study
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.

Keyword
small field dosimetry, Monte Carlo, PENELOPE, output correction factors
National Category
Radiology, Nuclear Medicine and Medical Imaging
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-103969 (URN)10.1118/1.4868695 (DOI)000334287000014 ()
Note

AuthorCount:3;

Available from: 2014-06-05 Created: 2014-05-27 Last updated: 2017-12-05Bibliographically approved
5. Monte Carlo investigation of backscatter factors for skin dose determination in interventional neuroradiology procedures
Open this publication in new window or tab >>Monte Carlo investigation of backscatter factors for skin dose determination in interventional neuroradiology procedures
Show others...
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:nbn:se:su:diva-114134 (URN)
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
6. Monte Carlo calculated and experimentally determined output correction factors for small field detectors in Leksell Gamma Knife Perfexion beams
Open this publication in new window or tab >>Monte Carlo calculated and experimentally determined output correction factors for small field detectors in Leksell Gamma Knife Perfexion beams
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The measurement of output factors (OF) for the small beams of the Leksell Gamma Knife® (LGK) is a challenge for the physicist due to the over- or underestimation of these factors by a vast majority of commercially available detectors. Output correction factors, introduced in the new international formalism published by Alfonso et al. (2008) standardizes the determination of OFs for small photon beams by correcting the detector reading ratios with output correction factors in order to yield the correct OF. Output correction factors have, in this work, been determined for LGK Perfexion™ 60Co γ-ray beams by Monte Carlo (MC) calculations and measurements. The MC calculations were performed using the MC system PENELOPE scoring the doses to the active volumes of the detectors and to a small volume of water. Two silicon diodes, one liquid ionization chamber (LIC), one alanine and one TLD detector were included in the MC derivation of the output correction factors. The LIC resulted in correction factors within ±0.4% and was therefore selected as the reference detector for the measurements. Twelve detectors were used in the experimental determination of the output correction factors by normalizing their detector readings to those of the LIC. The MC-calculated and experimentally determined output correction factors for the silicon diodes resulted in up to a -4% correction for the smallest collimator size. The air ionization chamber measurements resulted in extremely large output correction factors, due to the well-known effect of partial volume averaging (PVA). The natural diamond detector resulted in 6% correction for the 4 mm collimator, also due to PVA, whereas the smaller synthetic diamond detector resulted in a correction within ±1%. The LIC, requiring the smallest correction, was used to explore machine-to-machine differences in the OFs by performing measurements in four LGK units with different dose rates. This resulted in OFs within ±0.6% and ±0.3% for the 4 mm and 8 mm collimators, respectively, favouring the use of generic OFs. Using these experimentally derived correction factors, OFs can now be measured using a wide range of commercially available detectors.

National Category
Physical Sciences
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-114411 (URN)
Available from: 2015-03-03 Created: 2015-03-03 Last updated: 2016-01-29Bibliographically approved

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