Identifier

etd-11132009-010511

Degree

Master of Science (MS)

Department

Physics and Astronomy

Document Type

Thesis

Abstract

Purpose: The purpose of this work was to investigate the potential of a prototype electron multi-leaf collimator (eMLC) to deliver segmented-field electron conformal therapy (ECT) and to improve dose homogeneity to the planning target volume (PTV) by feathering the abutting edge of the higher energy electron fields. Methods: Software was developed to define the eMLC leaf positions that most closely fit a general field shape. Electron beams (6-20 MeV) using a prototype eMLC were commissioned for the pencil beam dose algorithm in the Pinnacle treatment planning system. A discrete (5-step) Gaussian edge spread function was used to match electron dose penumbras of differing energies at a specified depth in a water phantom. The effect of 1D edge feathering on dose homogeneity was computed and measured for segmented-field ECT treatment plans for three 2D PTVs in a water phantom (depths varied along axis parallel to leaf motion) and one 3D PTV (depth varied along both axes normal to beam). Additionally, the effect of 2D edge feathering was computed for the 3D PTV. Results: 1D discrete Gaussian edge feathering reduced the standard deviation of dose in the 2D PTVs by 34, 34, and 39%. In the 3D PTV, 1D discrete Gaussian edge feathering reduced the standard deviation of dose by 19%. The physical constraints (1-cm leaf width) of the eMLC hindered the 2D application of the feathering solution to the 3D PTV, and the standard deviation of dose increased by 10%. However, 2D discrete Gaussian edge feathering with a smooth-aperture (infinitesimal leaf width) reduced the standard deviation of dose in the 3D PTV by 33%. Conclusions: A 5-step discrete Gaussian edge spread function applied in 2D improves the abutment dosimetry but requires an eMLC leaf resolution better than 1 cm.

Date

2009

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Hogstrom, Kenneth R.

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