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Monte Carlo calculated and experimentally determined output correction factors for small field detectors in Leksell Gamma Knife Perfexion beams
Stockholm University, Faculty of Science, Department of Physics. Karolinska University Hospital, Sweden.
Stockholm University, Faculty of Science, Department of Physics. Karolinska University Hospital, Sweden.
(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: urn:nbn:se:su:diva-114411OAI: oai:DiVA.org:su-114411DiVA: diva2:792300
Available from: 2015-03-03 Created: 2015-03-03 Last updated: 2016-01-29Bibliographically 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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