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The 'cutting away' of potential secondary electron tracks explains the effects of beam size and detector wall density in small-field photon dosimetry
| Content Provider | Scilit |
|---|---|
| Author | Looe, Hui Khee Delfs, Björn Poppinga, Daniela Jiang, Ping Harder, Dietrich Poppe, Björn |
| Copyright Year | 2017 |
| Description | Journal: Physics in Medicine & Biology The well-known field-size dependent overresponse in small-field photon-beam dosimetry of solid-state detectors equipped with very thin sensitive volumes, such as the PTW microDiamond, cannot be caused by the photon and electron interactions within these sensitive layers because they are only a few micrometers thick. The alternative explanation is that their overresponse is caused by the combination of two effects, the modification of the secondary electron fluence profile (i) by a field size too small to warrant lateral secondary electron equilibrium and (ii) by the density-dependent electron ranges in the structural detector materials placed in front of or backing the sensitive layer. The present study aims at the numerical demonstration and visualization of this combined mechanism. The lateral fluence profiles of the secondary electrons hitting a 1 µm thick scoring layer were Monte-Carlo simulated by modelling their generation and transport in the upstream or downstream adjacent layers of thickness 0.6 mm and densities from 0.0012 to 3 g $cm^{−3}$, whose atomic composition was constantly kept water-like. The scoring layer/adjacent layer sandwich was placed in an infinite water phantom irradiated by circular$ ^{60}$Co, 6 MV and 15 MV photon beams with diameters from 3 to 40 mm. The interpretation starts from the ideal case of lateral secondary electron equilibrium, where the Fano theorem excludes any density effect. If the field size is then reduced, electron tracks potentially originating from source points outside the field border will then be numerically 'cut away'. This geometrical effect reduces the secondary electron fluence at the field center, but the magnitude of this reduction also varies with the density-dependent electron ranges in the adjacent layers. This combined mechanism, which strongly depends on the photon spectrum, explains the field size and material density effect on the response of detectors with very thin sensitive layers used in small-field photon-beam dosimetry. |
| Related Links | http://iopscience.iop.org/article/10.1088/1361-6560/aa9b46/pdf |
| ISSN | 00319155 |
| e-ISSN | 13616560 |
| DOI | 10.1088/1361-6560/aa9b46 |
| Journal | Physics in Medicine & Biology |
| Issue Number | 1 |
| Volume Number | 63 |
| Language | English |
| Publisher | IOP Publishing |
| Publisher Date | 2017-12-14 |
| Access Restriction | Open |
| Subject Keyword | Journal: Physics in Medicine & Biology Radiology, Nuclear Medicine and Imaging Fano Theorem Densities of Detector Structure Materials Photon-beam Dosimetry Secondary Electron Transport Small Fields |
| Content Type | Text |
| Resource Type | Article |
| Subject | Radiology, Nuclear Medicine and Imaging Radiological and Ultrasound Technology |
