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- Beaulieu, Luc, et al.
(author)
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Report of the Task Group 186 on model-based dose calculation methods in brachytherapy beyond the TG-43 formalism : Current status and recommendations for clinical implementation
- 2012
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In: Medical physics (Lancaster). - : American Association of Physicists in Medicine. - 0094-2405. ; 39:10, s. 6208-6236
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Journal article (peer-reviewed)abstract
- The charge of Task Group 186 (TG-186) is to provide guidance for early adopters of model-based dose calculation algorithms (MBDCAs) for brachytherapy (BT) dose calculations to ensure practice uniformity. Contrary to external beam radiotherapy, heterogeneity correction algorithms have only recently been made available to the BT community. Yet, BT dose calculation accuracy is highly dependent on scatter conditions and photoelectric effect cross-sections relative to water. In specific situations, differences between the current water-based BT dose calculation formalism (TG-43) and MBDCAs can lead to differences in calculated doses exceeding a factor of 10. MBDCAs raise three major issues that are not addressed by current guidance documents: (1) MBDCA calculated doses are sensitive to the dose specification medium, resulting in energy-dependent differences between dose calculated to water in a homogeneous water geometry (TG-43), dose calculated to the local medium in the heterogeneous medium, and the intermediate scenario of dose calculated to a small volume of water in the heterogeneous medium. (2) MBDCA doses are sensitive to voxel-by-voxel interaction cross sections. Neither conventional single-energy CT nor ICRU∕ICRP tissue composition compilations provide useful guidance for the task of assigning interaction cross sections to each voxel. (3) Since each patient-source-applicator combination is unique, having reference data for each possible combination to benchmark MBDCAs is an impractical strategy. Hence, a new commissioning process is required. TG-186 addresses in detail the above issues through the literature review and provides explicit recommendations based on the current state of knowledge. TG-43-based dose prescription and dose calculation remain in effect, with MBDCA dose reporting performed in parallel when available. In using MBDCAs, it is recommended that the radiation transport should be performed in the heterogeneous medium and, at minimum, the dose to the local medium be reported along with the TG-43 calculated doses. Assignments of voxel-by-voxel cross sections represent a particular challenge. Electron density information is readily extracted from CT imaging, but cannot be used to distinguish between different materials having the same density. Therefore, a recommendation is made to use a number of standardized materials to maintain uniformity across institutions. Sensitivity analysis shows that this recommendation offers increased accuracy over TG-43. MBDCA commissioning will share commonalities with current TG-43-based systems, but in addition there will be algorithm-specific tasks. Two levels of commissioning are recommended: reproducing TG-43 dose parameters and testing the advanced capabilities of MBDCAs. For validation of heterogeneity and scatter conditions, MBDCAs should mimic the 3D dose distributions from reference virtual geometries. Potential changes in BT dose prescriptions and MBDCA limitations are discussed. When data required for full MBDCA implementation are insufficient, interim recommendations are made and potential areas of research are identified. Application of TG-186 guidance should retain practice uniformity in transitioning from the TG-43 to the MBDCA approach.
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| 2. |
- Gastineau, R., et al.
(author)
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Haslea ostrearia-like Diatoms: Biodiversity out of the Blue
- 2014
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In: Advances in Botanical Research. - London : Academic Press Ltd-Elsevier Science Ltd. - 0065-2296. ; 71, s. 441-465
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Journal article (peer-reviewed)abstract
- Diatoms are usually referred to as golden-brown microalgae, due to the colour of their plastids and to their pigment composition, mainly carotenoids (fucoxanthin, diadinoxanthin, diatoxanthin), which mask chlorophylls a and c. The species Haslea ostrearia Gaillon/Bory (Simonsen) appears unique because of its extraplastidial bluish colour, a consequence of the presence of a water-soluble blue pigment at cell apices, marennine. When released in seawater, marenbine can be fixed on gills of oysters and other bivalves, which turn green. This greening phenomenon is economically exploited in Southwestern France, as it gives an added value to oysters. For decades, this singularity ascribed a worldwide distribution to H. ostrearia, first as Vibrio ostrearius, then Navicula ostrearia, last as H. ostrearia, when the genus Haslea was proposed by R. Simonsen (1974). Indeed, this 'birthmark' (presence of blue apices) made H. ostrearia easily recognisable without further scrutiny and identification of the microalga as well as its presence easily deduced from the greening of bivalves. Consequently, the widely admitted cosmopolitan character of H. ostrearia has only been questioned recently, following the discovery in 2008, of a new species of blue diatom in the Black Sea, Haslea karadagensis. The biodiversity of blue diatoms suddenly increased with the finding of other blue species in the Mediterranean Sea, the Canary Islands, etc., the taxonomic characterization of which is in progress. This review thus focuses on the unsuspected biodiversity of blue diatoms within the genus Haslea. Methods for species determination (morphometrics, chemotaxonomy, genomics), as well as a new species, are presented and discussed.
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