Type of Document Master's Thesis Author Smith, Koren Suzette URN etd-04092007-130602 Title Investigation of Superficial Dose from a Static TomoTherapy Beam Degree Master of Science (M.S.) Department Physics & Astronomy Advisory Committee
Advisor Name Title John P. Gibbons, Jr. Committee Chair Kenneth Hogstrom Committee Co-Chair Bruce Gerbi Committee Member Greg Henkelmann Committee Member Mette Gaarde Committee Member Keywords
- superficial dose
Date of Defense 2007-03-09 Availability unrestricted AbstractAbstract
Purpose: The TomoTherapy planning system is capable of creating treatment plans that deliver a homogeneous dose to superficial targets. It is essential that the planning system accurately predicts dose to the surface and superficial depths from beams directed at every angle in the axial plane. This work concentrates on measuring and modeling the dose from a static TomoTherapy beam at normal and oblique incidence. It was hypothesized that superficial doses measured from a static TomoTherapy beam agree with doses calculated by the TomoTherapy planning system to within 5% of the maximum dose for angles of incidence from 0¢ª-83¢ª.
Methods: Doses were measured with a parallel-plate chamber and TLDs at depths ¡Â 2cm for 40x2.5cm2 and 40x5cm2 static TomoTherapy beams for multiple SSDs for incident angles of 0¢ª-83¢ª. The measurements made with TLDs were compared to those made with the parallel-plate chamber to verify the measured dose. The TomoTherapy treatment planning system was used to calculate doses from single, static beams incident on a flat phantom so that measured and calculated doses could be compared.
Results: Surface dose increased from 16.7%-18.9% as the SSD decreased from 85 to 55cm for the 40x5cm2 field and from 12.7%-14.9% for the 40x2.5cm2 field. The surface dose increased from 16.8%-44.2% as the angle of incidence increased from 0¢ª-83¢ª for the 40x5cm2 field and from 12.8%-42.6% for the 40x2.5cm2 field. For all measurement conditions, the planning system under predicted the dose at the surface by more than 5%. For the following measurement conditions and depths, the planning system also under predicted the dose by more the 5%: 85 and 70cm SCD at a depth of 0.1cm, 55cm SCD at a depth of 0.2cm, and 30¢ª and 45¢ª at a depth of 0.1cm. For 75¢ª and 83¢ª, the planning system over predicted the dose at superficial depths (0.1cm-0.3cm) by as much as 7%.
Conclusions: The results of this work indicate that the planning system under predicted the dose at the surface and superficial depths (depths ¡Â 0.3cm) from a static TomoTherapy beam at both normal and oblique incidence by more than 5%.
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