Improving treatment dose accuracy in radiation therapy

Wong, T 2006, Improving treatment dose accuracy in radiation therapy, Doctor of Philosophy (PhD), Applied Sciences, RMIT University.


Document type: Thesis
Collection: Theses

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Title Improving treatment dose accuracy in radiation therapy
Author(s) Wong, T
Year 2006
Abstract The thesis aims to improve treatment dose accuracy in brachytherapy using a high dose rate (HDR) Ir-192 stepping source and in external beam therapy using intensity modulated radiation therapy (IMRT). For HDR brachytherapy, this has been achieved by investigating dose errors in the near field and the transit dose of the HDR brachytherapy stepping source. For IMRT, this study investigates the volume effect of detectors in the dosimetry of small fields, and the clinical implementation and dosimetric verification of a 6MV photon beam for IMRT.

For the study of dose errors in the near field of an HDR brachytherapy stepping source, the dose rate at point P at 0.25 cm in water from the transverse bisector of a straight catheter was calculated with Monte Carlo code MCNP 4.A. The Monte Carlo (MC) results were used to compare with the results calculated with the Nucletron Brachytherapy Planning System (BPS) formalism. Using the MC calculated radial dose function and anisotropy function with the BPS formalism, 1% dose calculation accuracy can be achieved even in the near field with negligible extra demand on computation time.

A video method was used to analyse the entrance, exit and the inter-dwell transit speed of the HDR stepping source for different path lengths and step sizes ranging from 2.5 mm to 995 mm. The transit speeds were found to be ranging from 54 to 467 mm/s. The results also show that the manufacturer has attempted to compensate for the effects of inter-dwell transit dose by reducing the actual dwell time of the source. A well-type chamber was used to determine the transit doses. Most of the measured dose differences between stationary and stationary plus inter-dwell source movement were within 2%.

The small-field dosimetry study investigates the effect of detector size in the dosimetry of small fields and steep dose gradients with a particular emphasis on IMRT measurements. Due to the finite size of the detector, local discrepancies of more than 10 % are found between calculated cross profiles of intensity modulated beams and intensity modulated profiles measured with film. A method to correct for the spatial response of finite sized detectors and to obtain the "real" penumbra width of cross profiles from measurements is introduced. Output factor measurements are performed with different detectors and are presented as a function of detector size for a 1 x 1 cm2 field.

The study on the clinical implementation and dosimetric verification of a 6 MV photon beam for IMRT provides a systematic guideline for effective clinical implementation of IMRT in radiotherapy centres. Despite the beam model not taking in account for the effects of transmission through round leaf ends, adjusting the leaf position to account for the effective widening of the leaf opening shows good dose agreement within the acceptable criterion of ±3% or 2 mm in our in-house dose verification process. The Radiological Physics Centre (RPC) IMRT phantom provides a valuable independent check by the RPC on IMRT commissioning, validation and QA process.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Applied Sciences
Keyword(s) Radioisotope brachytherapy
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