Experimental verification of buildup region dose calculation for a commercial treatment planning system
Date
2011-06-01
Authors
Journal Title
Journal ISSN
Volume Title
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ORCID
Type
Degree Level
Masters
Abstract
The purpose of this research was to verify experimentally the buildup region dose calculation for Pinnacle3 (version 9.0), a commercial treatment planning system, commissioned and in use at the Saskatoon Cancer Center. To achieve this, buildup dose measurements using Attix parallel-plate ionization chamber and calculations by Pinnacle3, for a variety of clinical setups, were compared. The clinical setups involved 6 MV and 15 MV photon beams, open fields, enhanced dynamic wedges, physical wedges, block tray, 85, 100 and 120 cm source-to-surface distances (SSDs), and field sizes 3 x 3, 4 x 4, 5 x 5, 8 x 8, 10 x 10, 12 x 12, 15 x 15, 20 x 20, 30 x 30 cm2. The dose difference (DD) and distance-to-agreement (DTA) were used to evaluate the discrepancy between measured and calculated dose values. Significant discrepancies between measured and calculated buildup dose values were observed because the modeling in Pinnacle3 is based on measurements made using a cylindrical ionization chamber. Based on the criteria of DD less than 2% or DTA less than 2 mm, 93.7% of 1,710 dose points for the 6 MV photon beam passed while for the 15 MV photon beam, 96.1% of the 2,244 dose points passed. The dose points that did not pass these criteria were mostly for open fields, block tray fields, fields with physical wedges of 15 degrees and 30 degrees and for fields with shorter source-to-surface distances. This is attributed to the high electron contamination associated with these fields. The low levels of discrepancies between measured and calculated dose values for the 15 MV beam as compared to those of the 6 MV beam need further investigations. The good agreement between measured and calculated dose values after remodeling the Electron Contamination in Pinnacle3 based on Attix chamber measurements is an indication that the Electron Contamination equation in Pinnacle3 may be adequate for modeling of electron contamination in the buildup dose region. The disagreement between Attix chamber and EBT 2 film measured buildup dose values was less than 3% for 89.9% of the buildup dose measurements compared. It is recommended to use a good parallel plate ionization chamber, such as the Attix chamber, for measurements in the buildup region.
Description
Keywords
Buildup region dose, Pinnacle 3, Attix parallel plate ionization chamber
Citation
Degree
Master of Science (M.Sc.)
Department
Physics and Engineering Physics
Program
Physics and Engineering Physics