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Uppsala, Sweden

Enger S.A.,Laval University | Ahnesjo A.,Uppsala University | Ahnesjo A.,Elekta Instrument AB | Verhaegen F.,Maastricht University | And 2 more authors.
Physics in Medicine and Biology | Year: 2012

It has been suggested that modern dose calculation algorithms should be able to report absorbed dose both as dose to the local medium, D m,m,and as dose to a water cavity embedded in the medium, D w,m, using conversion factors from cavity theory. Assuming that the cell nucleus with its DNA content is the most important target for biological response, the aim of this study is to investigate, by means of Monte Carlo (MC) simulations, the relationship of the dose to a cell nucleus in a medium, D n,m,to D m,mand D w,m, for different combinations of cell nucleus compositions and tissue media for different photon energies used in brachytherapy. As D n,mis very impractical to calculate directly for routine treatment planning, while D m,mand D w,mare much easier to obtain, the questions arise which one of these quantities is the best surrogate for D n,mand which cavity theory assumptions should one use for its estimate. The Geant4.9.4 MC code was used to calculate D m,m,D w,mand D n,mfor photon energies from 20 (representing the lower energy end of brachytherapy for 103Pd or 125I) to 300keV (close to the mean energy of 192Ir) and for the tissue media adipose, breast, prostate and muscle. To simulate the cell and its nucleus, concentric spherical cavities were placed inside a cubic phantom (10×10×10mm 3). The diameter of the simulated nuclei was set to 14m. For each tissue medium, three different setups were simulated; (a) D n,mwas calculated with nuclei embedded in tissues (MC-D n,m). Four different published elemental compositions of cell nuclei were used. (b) D w,mwas calculated with MC (MC-D w,m) and compared with large cavity theory calculated D w,m(LCT-D w,m), and small cavity theory calculated D w,m(SCT-D w,m). (c) D m,mwas calculated with MC (MC-D m,m). MC-D w,mis a good substitute for MC-D n,mfor all photon energies and for all simulated nucleus compositions and tissue types. SCT-D w,mcan be used for most energies in brachytherapy, while LCT-D w,mshould only be considered for source spectra well below 50keV, since contributions to the absorbed dose inside the nucleus to a large degree stem from electrons released in the surrounding medium. MC-D m,mis not an appropriate substitute for MC-D n,mfor the lowest photon energies for adipose and breast tissues. The ratio of MC-D m,mto MC-D n,mfor adipose and breast tissue deviates from unity by 34% and 15% respectively for the lowest photon energy (20keV), whereas the ratio is close to unity for higher energies. For prostate and muscle tissue MC-D m,mis a good substitute for MC-D n,m. However, for all photon energies and tissue types the nucleus composition with the highest hydrogen content behaves differently than other compositions. Elemental compositions of the tissue and nuclei affect considerably the absorbed dose to the cell nuclei for brachytherapy sources, in particular those at the low-energy end of the spectrum. Thus, there is a need for more accurate data for the elemental compositions of tumours and healthy cells. For the nucleus compositions and tissue types investigated, MC-D w,mis a good substitute to MC-D n,mfor all simulated photon energies. Whether other studied surrogates are good approximations to MC-D n,mdepends on the target size, target composition, composition of the surrounding tissue and photon energy. © 2012 Institute of Physics and Engineering in Medicine.

Sjoberg C.,Uppsala University | Sjoberg C.,Elekta Instrument AB | Lundmark M.,Uppsala University Hospital | Granberg C.,Umea University | And 4 more authors.
Radiation Oncology | Year: 2013

Background: Semi-automated segmentation using deformable registration of selected atlas cases consisting of expert segmented patient images has been proposed to facilitate the delineation of lymph node regions for three-dimensional conformal and intensity-modulated radiotherapy planning of head and neck and prostate tumours. Our aim is to investigate if fusion of multiple atlases will lead to clinical workload reductions and more accurate segmentation proposals compared to the use of a single atlas segmentation, due to a more complete representation of the anatomical variations.Methods: Atlases for lymph node regions were constructed using 11 head and neck patients and 15 prostate patients based on published recommendations for segmentations. A commercial registration software (Velocity AI) was used to create individual segmentations through deformable registration. Ten head and neck patients, and ten prostate patients, all different from the atlas patients, were randomly chosen for the study from retrospective data. Each patient was first delineated three times, (a) manually by a radiation oncologist, (b) automatically using a single atlas segmentation proposal from a chosen atlas and (c) automatically by fusing the atlas proposals from all cases in the database using the probabilistic weighting fusion algorithm. In a subsequent step a radiation oncologist corrected the segmentation proposals achieved from step (b) and (c) without using the result from method (a) as reference. The time spent for editing the segmentations was recorded separately for each method and for each individual structure. Finally, the Dice Similarity Coefficient and the volume of the structures were used to evaluate the similarity between the structures delineated with the different methods.Results: For the single atlas method, the time reduction compared to manual segmentation was 29% and 23% for head and neck and pelvis lymph nodes, respectively, while editing the fused atlas proposal resulted in time reductions of 49% and 34%. The average volume of the fused atlas proposals was only 74% of the manual segmentation for the head and neck cases and 82% for the prostate cases due to a blurring effect from the fusion process. After editing of the proposals the resulting volume differences were no longer statistically significant, although a slight influence by the proposals could be noticed since the average edited volume was still slightly smaller than the manual segmentation, 9% and 5%, respectively.Conclusions: Segmentation based on fusion of multiple atlases reduces the time needed for delineation of lymph node regions compared to the use of a single atlas segmentation. Even though the time saving is large, the quality of the segmentation is maintained compared to manual segmentation. © 2013 Sjöberg et al.; licensee BioMed Central Ltd.

Backstrom G.,Uppsala University | Galassi M.E.,CONICET | Tilly N.,Uppsala University | Tilly N.,Elekta Instrument AB | And 2 more authors.
Medical Physics | Year: 2013

Purpose: The LIonTrack (Light Ion Track) Monte Carlo (MC) code for the simulation of H+, He2+, and other light ions in liquid water is presented together with the results of a novel investigation of energy-deposition site properties from single ion tracks. Methods: The continuum distorted-wave formalism with the eikonal initial state approximation (CDW-EIS) is employed to generate the initial energy and angle of the electrons emitted in ionizing collisions of the ions with H2O molecules. The model of Dingfelder ["Electron inelastic-scattering cross sections in liquid water," Radiat. Phys. Chem. 53, 1-18 (1998)10.1016/S0969-806X(97)00317-4; Dingfelder "Comparisons of calculations with PARTRAC and NOREC: Transport of electrons in liquid water," Radiat. Res. 169, 584-594 (2008)10.1667/RR1099.1] is linked to the general-purpose MC code PENELOPE/penEasy to simulate the inelastic interactions of the secondary electrons in liquid water. In this way, the extended PENELOPE/penEasy code may provide an improved description of the 3D distribution of energy deposits (EDs), making it suitable for applications at the micrometer and nanometer scales. Results: Single-ionization cross sections calculated with the ab initio CDW-EIS formalism are compared to available experimental values, some of them reported very recently, and the theoretical electronic stopping powers are benchmarked against those recommended by the ICRU. The authors also analyze distinct aspects of the spatial patterns of EDs, such as the frequency of nearest-neighbor distances for various radiation qualities, and the variation of the mean specific energy imparted in nanoscopic targets located around the track. For 1 MeV/u particles, the C6+ ions generate about 15 times more clusters of six EDs within an ED distance of 3 nm than H+. Conclusions: On average clusters of two to three EDs for 1 MeV/u H+ and clusters of four to five EDs for 1 MeV/u C6+ could be expected for a modeling double strand break distance of 3.4 nm. © 2013 American Association of Physicists in Medicine.

Sjberg C.,Uppsala University | Sjberg C.,Elekta Instrument AB | Johansson S.,Uppsala University | Ahnesj A.,Uppsala University
Radiation Oncology | Year: 2014

Background and purpose: Multi-atlas segmentation can yield better results than single atlas segmentation, but practical applications are limited by long calculation times for deformable registration. To shorten the calculation time pre-calculated registrations of atlases could be linked via a single atlas registered in runtime to the current patient. The primary purpose of this work is to investigate and quantify segmentation quality changes introduced by such linked registrations. We also determine the optimal parameters for fusing linked multi-atlas labels using probabilistic weighted fusion. Material and methods: Computed tomography images of 10 head and neck cancer patients were used as atlases, with parotid glands, submandibular glands, the mandible and lymph node levels II-IV segmented by an experienced radiation oncologist following published consensus guidelines. The change in segmentation quality scored by Dice similarity coefficient (DSC) for linking free-form deformable registrations, modeled by B-splines, was investigated for both single- and multi-atlas label fusion by using a leave-one-out approach. Results: The median decrease of the DSC was in the range 2.8% to 8.4% compared to direct registrations for all structures while reducing the computer calculation time to that of a single deformable registration. Linking several registrations showed a DSC decrease almost linear to the number of links, suggesting that extrapolation to zero links provides an observer independent measure of the inherent precision with which the segmentation guidelines can be applied. Conclusions: Linking pre-made registrations of multiple atlases via a runtime registration of a single atlas provides a feasible method for reducing computation time in multi-atlas registration. © Sjöberg et al.; licensee BioMed Central.

Benmakhlouf H.,Karolinska University Hospital | Benmakhlouf H.,University of Stockholm | Johansson J.,Elekta Instrument AB | Paddick I.,Cromwell Gamma Knife Center | And 2 more authors.
Physics in Medicine and Biology | Year: 2015

The measurement of output factors (OF) for the small photon beams generated by Leksell Gamma Knife« (LGK) radiotherapy units is a challenge for the physicist due to the under or over estimation of these factors by a vast majority of the detectors commercially available. Output correction factors, introduced in the international formalism published by Alfonso (2008 Med. Phys. 35 5179-86), standardize the determination of OFs for small photon beams by correcting detector-reading ratios to yield OFs in terms of absorbed-dose ratios. In this work output correction factors for a number of detectors have been determined for LGK Perfexion™ 60Co γ-ray beams by Monte Carlo (MC) calculations and measurements. The calculations were made with the MC system PENELOPE, scoring the energy deposited in the active volume of the detectors and in a small volume of water; the detectors simulated were two silicon diodes, one liquid ionization chamber (LIC), alanine and TLD. The calculated LIC output correction factors were within ± 0.4%, and this was selected as the reference detector for experimental determinations where output correction factors for twelve detectors were measured, normalizing their readings to those of the LIC. The MC-calculated and measured output correction factors for silicon diodes yielded corrections of up to 5% for the smallest LGK collimator size of 4 mm diameter. The air ionization chamber measurements led to extremely large output correction factors, caused by the well-known effect of partial volume averaging. The corrections were up to 7% for the natural diamond detector in the 4 mm collimator, also due to partial volume averaging, and decreased to within about ± 0.6% for the smaller synthetic diamond detector. The LIC, showing the smallest corrections, was used to investigate machine-to-machine output factor differences by performing measurements in four LGK units with different dose rates. These resulted in OFs within ± 0.6% and ± 0.2% for the 4 mm and 8 mm collimators, respectively, providing evidence for the use of generic OFs for these LGK beams. Using the experimentally derived output correction factors, OFs can be measured using a wide range of commercially available detectors. © 2015 Institute of Physics and Engineering in Medicine.

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