Small-field dosimetry with an output-stabilized tomotherapy Hi-Art machine.

摘要

The Tomotherapy Hi-Art machine is a highly advanced radiotherapy treatment device that uses a megavoltage linear accelerator (linac) mounted on a CT ring-style gantry to helically deliver intensity modulated fan-beams of radiation to patients. The aim of this work is to reduce the uncertainties in the treatment delivery process induced by variations in the Hi-Art's nominal dose rate output and limitations in the accuracy of measurements made on small radiation fields produced by narrow collimator settings.;An electronic dose rate servo circuit is designed, tested, and found to be effective in regulating the dose rates of a Varian Clinac 600 and a Siemens bench top linac. The circuit is then tested on a Hi-Art machine and found to improve the overall stability of its dose rate output.;One-dimensional detector-scanning techniques are developed and investigated as a means to accurately measure small TomoTherapy fields. Common clinical detectors such as ion chambers and radiographic film are longitudinally scanned through static rectangular fields produced by the Hi-Art. This allows detectors to make measurements under conditions of improved lateral electronic equilibrium and virtually eliminates detector volume averaging effects. A quantity termed "the integral scanned dose to slice width ratio", or (D/SW), is introduced and found to be a sensitive metric of the photon-source occlusion effect for small TomoTherapy fields. The (D/SW) quantity proved to be highly sensitive to the modeled electron source spot size in the Monte Carlo (MC) code TomoPen, making it a valuable tool for evaluating the accuracy of photon source models used in analytical/MC dose calculation algorithms.;Two-dimensional detector-scanning techniques for the measurement of stereotactic radiosurgery (SRS) fields are developed and investigated. SRS field measurements are made by scanning a standard ion chamber in a two-dimensional "raster" like pattern. The integral dose measured during each scan is divided by the area of the scanned field to obtain what is termed the "integral scanned dose to field area ratio", or (D/FA). The measured (D/FA) ratios for the SRS fields were consistent with the expected effects of source occlusion.