student profile: Miss Madelaine Tyler


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Thesis work

Thesis title: Radiation dosimetry methods and analysis

Supervisors: Natalka SUCHOWERSKA , David MCKENZIE

Thesis abstract:

Technological advancements in ancillary equipment for linear accelerators, in particular the replacement of fixed beam-limiting devices with multi-leaf collimators (MLCs) has seen an increase in complexity of treatment techniques used for radiotherapy with widespread implementation of Intensity-Modulated Radiation Therapy, Volumetric-Modulated Arc Therapy (VMAT) and more recently Stereotactic Ablative Radiation Therapy (SABR). Modern MLC designs have leaf widths (projected at the treatment distance) of 5 mm (Elekta Agility, Varian 120 leaf MLC) and as small as  mm (Brainlab, Novalis, Varian Edge). These MLC designs provide the ability to deliver complex, inhomogeneous dose distributions to target volumes by subdividing each treatment beam into many small beamlets. The beamlet size is only limited by the physical size of the MLCs and by constraints placed on the calculation engine that optimises dose in the treatment planning system. Accurate measurement and commissioning of small photon fields has become vital in modern radiotherapy physics. Small-field dosimetry is complex due to various effects that are not present in large fields including a lack of lateral charge particle equilibrium (CPE), partial source occlusion and perturbations introduced by the detecotrs themselves. CPE and partial source occlusion are effects directly related to the settings and design of the linear accelerator and collimator and will be constant for a particular field size. With the implementation of complex treatment techniques there is an increased need for data collection of small radiation fields in almost all clinics. A fundamental analysis and understanding of detector perturbations is required to allow the clinical medical physicist to conduct measurements with confidence and a high degree of accuracy. Perturbation effects of dosimeters and their effect on measurement accuracy in small radiation fields are not yet well understood. There is currently a lock of consensus in the literature on the origin and magnitude of perturbations for detectors of the same type and the corrections required to offset them. The absence of a fundamental understanding of the underlying physics has lead to different correction factors being proposed by different groups. A component of the proposed PhD research topic will be to provide a fundamental understanding of the physics contributing to the extreme conditions created with small radiation fields increasingly encountered in the clinic. The results from this research will enable clinical medical physicists to conduct the small field dosimetry that underpins modern cancer treatment accurately and with confidence.

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