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UBC Theses and Dissertations

Film dosimetry and three-dimensional verification of conformal dose distributions in stereotactic radiosurgery Robar, James L.

Abstract

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.

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