| Summary: | The measurement of stereotactic radiosurgical dose distributions requires an
integrating, high-resolution dosimeter capable of providing a spatial map of absorbed
dose. Although radiographic film is an accessible detector fulfilling these criteria, its
application to the dosimetry of larger photon fields in radiotherapy has been limited by
dependencies of emulsion sensitivity on depth in phantom, field size and orientation
relative to the beam axis. The first part of this thesis examines the applicability of
radiographic film specifically for the dosimetry of 6 MV radiosurgical beams. We show
that while, for large (e.g. 20 cm x 20 cm) photon beams, the error in measured dose due
to a depth-dependence of emulsion reaches 15%, the corresponding maximum error for a
2.5 cm diameter radiosurgical beam is reduced to 1.5%. For radiosurgical beams this
error is comparable to the measured achievable reproducibility of film dosimetry (1.1%)
and the potential error incurred due to orientation dependence (1.5%). We also
demonstrate that the dependence of film sensitivity on field size is negligible for beams
ranging from 1.0 cm to 4.0 cm in diameter at isocentre. The marked difference between
radiosurgical and larger 6 MV photon beams in the context of film dosimetry is explained
using EGS4 Monte Carlo simulation. For larger fields, significant increases in the
Compton-scattered photon population, particularly below 400 keV, result in
dependencies on depth and field size. In contrast, the relative increase of this low-energy
component is negligible for radiosurgical photon fields. Finally, the problem of volume
averaging in radiosurgical film dosimetry is addressed by evaluating a new, highresolution
CCD-based transparency digitizer in terms of spatial linearity, dynamic range,
signal-to-noise ratio and uniformity.
The second part of this thesis presents the design considerations and clinical
evaluation of a novel phantom system facilitating the measurement of conformal
radiosurgical dose distributions using one or multiple arrays of up to 20 radiographic
films separated by 3.2 mm-thick tissue-equivalent spacers. Using EGS4 Monte Carlo
simulation and experimental measurement, we show that this geometry preserves tissue-equivalence
to within 1%. The phantom provides 0.25 mm in-plane spatial resolution,
and bicubic-interpolated isodose surfaces may be interpolated with an estimated spatial
accuracy of 1.0 mm throughout the dose volume. Dedicated software has been developed
to automate the process film digitization, ordering and orienting of film images,
conversion of scanned pixel value to dose, interpolation within the measured volume and
export of images in DICOM format for co-registration of planned and measured three-dimensional
dose distributions. Benchmark tests and example conformal dose
verification studies demonstrate that this technique provides a practical method of
quantifying even minor errors in radiosurgical treatment delivery. === Science, Faculty of === Physics and Astronomy, Department of === Graduate
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