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SU‐E‐T‐426: Assessment of the Limitations of Intensity Modulated Radiation Therapy Quality Control Procedure
Author(s) -
Girard F,
Lacroix F
Publication year - 2011
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3612380
Subject(s) - multileaf collimator , imaging phantom , dosimeter , offset (computer science) , ionization chamber , optics , materials science , dosimetry , nuclear medicine , detector , dose profile , planar , collimator , physics , linear particle accelerator , radiation , beam (structure) , medicine , computer science , ion , computer graphics (images) , ionization , quantum mechanics , programming language
Purpose: To determine the impact of dose delivery errors due to multileaf collimator bank offset and gantry angle offset on the resulting gamma value of an IMRT Quality Control (QC) test as determined by a planar 2D dosimeter. Also to determine the impact of the planar 2D dosimeter array resolution on the gamma. Methods: Systematic errors such as MLC leaf bank offset and gantry angle offset were numerically introduced in IM verification plans. The dose received by a phantom was calculated using anisotropic analytical algorithm with a grid size of 1 mm. A corona plane (30×30 cm2) was selected for each modified plan and compared t original using the geometric gamma approach in absolute or relative using a 3%/3 mm criteria. The plans were then irradiated in a phan composed of 20×20 cm2 slabs of plastic water. The phantom was im using a CT scan and imported into the treatment planning system Ec (Varian inc.). A radiochromic EBT2 (ISP Corp. inc.) film was placed depth of 4 cm. Results: Absolute dose measurements are much m sensitive to MLC bank offsets than relative measurements. In con modifying the gantry angle by 1 degree has little effect on the relativ absolute gamma value. There is a clear increase in the variance of gamma value as the detector spacing is increased beyond 2–3 Conclusions: Absolute dose measurements should be performed maximize QC sensitivity. Ideally, the detector spacing in a 2D planar a should be less than 2–3 mm in order to minimize the uncertainty on gamma. This work has been supported by the Ministère de la Santé et des Ser Sociaux and Natural Sciences and Engineering Research Council Disco Grant No. 357402.