Dissertation Abstract

Analysis of dose characteristics from beveled applicators in intraoperative radiation therapy (IORT) using Monte Carlo simulation, water scan measurement and film dosimetry

A Study of the intraoperative radiation therapy (TORT) has been done by using Welhofer scanning system, film Dosmetry measurements as well as Monte Carlo simulations. The IORT system consists of a Varian clinac 1800 and a set of applicators of 0°, 15° and 30° beveled angles of different diameters from 1 inch to 4 inch with ¼ inch increment. The depth dose and isodose distributions obtained by the three different modalities are in good agreement. Results showed that as the beveled angle increases, the depth dose shifts upstream along the beam. As the beveled angle, diameter and electron beam energy increase, the symmetry of the isodose line gets worse. Hot spots will be generated near toe and heel position asymmetrically under such circumstances, suggesting that large diameter and/or large beveled angle should be avoided in clinical practice as much as possible for better control of the dose distribution, especially for higher electron energies.

Also it is found that the formula for estimating the percent depth dose along the clinical axis and along the geometrical axis using the simple cosine function is not suitable especially for the small size applicators, large beveled angle and higher energy. A hypothetical explanation for this has been discussed. Since it is difficult to measure the percent depth dose curve along the clinical axis in the beveled case, it is recommended that the isodose distribution and the percent dose along the geometrical axis be used to get the full information about the dose distribution for a given circumstance.

[Dr. Li is an ARRT registrant.]

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