Identifier

etd-06162014-153414

Degree

Master of Science (MS)

Department

Physics and Astronomy

Document Type

Thesis

Abstract

Purpose: To evaluate dosimetric differences of copper inserts compared to lead-alloy inserts for electron beam therapy. Methods: Copper inserts were manufactured by .decimal, Inc. and matching lead-alloy, Cerrobend®, inserts were constructed in-house for 32 square field sizes (2x2 to 20x20 cm&178;) for five applicator sizes (6x6 to 25x25 cm&178;). Percent depth-dose and off-axis relative dose profiles were measured using an electron diode in water for the copper and Cerrobend® inserts for a subset of insert sizes (6x6, 10x10, 25x25 cm&178;) and energies (6, 12, 20 MeV) at 100 and 110 cm source-to-surface distances (SSD). Dose outputs were measured for all field size-insert combinations and available energies (6-20 MeV) at 100 cm SSD and for a smaller subset at 110 cm SSD. Using these data, 2D planar absolute dose distributions were generated and compared. Criteria for agreement were ±2% of maximum dose or 1 mm distance-to-agreement for 99% of points. Results: A gamma analysis of the beam dosimetry showed 94 of 96 combinations of insert size, applicator, energy, and SSD were within the 2%/1 mm criteria. Failures were found for combinations of small field sizes in large applicators at 20 MeV and 100-cm SSD. Copper inserts showed less bremsstrahlung production due to copper’s lower atomic number compared to Cerrobend® (greatest difference was 2.5% at 20 MeV). This effect was most prominent at the highest energies for combinations of large applicators with small field sizes. Also, more electrons scattered from the collimator edge of copper compared to Cerrobend®, resulting in an increased dose at the field edge for copper at shallow depths (greatest increase was 1% at 20 MeV). Conclusions: Inserts for field sizes ≥6x6 cm&178; at any energy, or for small fields (≤4x4 cm&178;) at energies <20 MeV, showed dosimetric differences less than 2%/1 mm for more than 99% of points. All areas of comparison criteria failures were from lower out-of-field dose from copper inserts due to a reduction in bremsstrahlung production, a dosimetric difference which is clinically beneficial in reducing dose to healthy tissue outside of the planned treatment volume. All field size-applicator size-energy combinations passed 3%/1 mm criteria for 100% of points.

Date

2014

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Carver, Robert

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