In modern radiotherapy, 3D conformal dose distributions are achieved using several beam ports each having pre-calculated planar distributions of photon beam intensity. The intensity matrix for a given beam port is generated by independent motion of the leaves of a multileaf collimator (MLC). In this thesis, we have used the step-and-shoot approach to intensity-modulated beam delivery, the safest and most popular approach at the moment. The first component of this thesis was to write a leaf sequence algorithm to control the MLC fitted to our Clinac 2300 C/D linear accelerator. Our algorithm is more efficient than other published step-and-shoot type algorithms, and takes into account the MLC transmission, MLC penumbra, and change in scatter conditions with field size. / Although sophisticated means to calculate and deliver these spatially-modulated beams have been developed by our group as well as by other medical physics research centres, means to verify their actual delivery are definitely the most problematic at the moment, making equipment and treatment quality assurance difficult to enforce. The second (and major) component of this thesis has been to investigate the use of a new portal imaging device for dosimetric verification purposes. We show that an electronic portal imaging device of the scanning liquid ionization chamber type yields images which, once calibrated from a previously-determined calibration curve, provide highly-precise planar maps of the incident dose rate distribution. For verification of an intensity-modulated beam delivered in the segmented approach with an MLC, a portal image is acquired for each subfield of the leaf sequence. Subsequent to their calibration, the images are multiplied by their respective associated monitor unit settings, and summed to produce a planar dose distribution at the measurement depth in phantom. The excellent agreement of our portal imager measurements with calculations of our treatment planning system and measurements with a one-dimensional beam profiler attests to the usefulness and relative simplicity of this method for the planar verification of intensity-modulated fields, which are produced in the segmented approach on a computerized linear accelerator equipped with an MLC.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.35687 |
| Date | January 1998 |
| Creators | Curtin-Savard, Arthur. |
| 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 | Doctor of Philosophy (Department of Physics.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001656726, proquestno: NQ50138, Theses scanned by UMI/ProQuest. |
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