The treatment head of the Clinac-18 medical linear accelerator was modelled using-the EGS4 Monte Carlo simulation package. Photon-energy spectra for fields ranging from 2 x 2 cm$ sp2$ to 20 x 20 cm$ sp2$ in size were generated and the primary and scatter spectra were analyzed separately. The generated x-ray spectra were used in the calculation of the percent depth dose (PDD) distributions for flattened and unflattened 10 MV x-ray beams in a water phantom at a source-surface distance of 100 cm for the various field sizes. The agreement between calculated and measured depth doses is excellent. / Measurements of the dose in the build-up region show that the depth of dose maximum (d$ sb{max}$) increases with increasing field size for fields up to 5 x 5 cm$ sp2$ for both the flattened and unflattened beams. As the field size is increased beyond 5 x 5 cm$ sp2,$ d$ sb{max}$ decreases with increasing field size for the flattened x-ray beam while remaining nearly constant for the unflattened beam. Additionally, the surface dose of the flattened beam is found to approach that of the unflattened beam for large field sizes. Calculations show that the decrease in d$ sb{max}$ as the field size is increased above 5 x 5 cm$ sp2,$ and the rapid increase in the surface dose for the flattened x-ray beam with increasing field size, are due to the degradation of the flattened-beam parameters caused by low-energy photons produced in the flattening filter.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.55405 |
| Date | January 1994 |
| Creators | Zankowski, Corey E. |
| Contributors | Podgorsak, E. B. (advisor) |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Master of Science (Department of Medical Radiation Physics.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001426166, proquestno: AAIMM00069, Theses scanned by UMI/ProQuest. |
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