Year

2014

Degree Name

Masters of Science (Research)

Department

Centre for Medical Radiation Physics

Abstract

Stereotactic radiation therapy or radiosurgery involves the delivery of a radiation dose, using small radiation beams to treat tumours or lesions, typically within the brain. To derive accurate beam models for stereotactic treatment planning, high spatial and dosimetric accuracy is required. The stereotactic beams are required to be characterised with regards to dosimetry. The MOSkin detector developed by the Centre of Medical Physics at the University of Wollongong was investigated to determine if it could be used for dose characterisation of stereotactic small field radiation beams.

The MOSkin was compared to the IBA Stereotactic field diode (SFD), a PTW 31014 PinPoint chamber and Gafchromic EBT3 film. The dose characteristics examined were beam profiles and output factors over stereotactic cone diameters of 5 mm to 45 mm.

The beam profiles for the smallest field diameters measured with the MOSkin were conducted in two orientations ‘edge on’ and ‘face on’. The change in orientation from ‘face on to ‘edge on’ reduced the sensitive volume visible to the beam central axis. The MOSkin in the edge on position had improved spatial resolution, demonstrated by the smaller penumbra width (80% - 20%) for all nominal field sizes measured.

When comparing all investigated dosimeters, the stereotactic field diode demonstrated the smallest penumbra width for all beam profile measurements. A broader penumbra was observed with the PinPoint chamber over all field sizes. The MOSkin in the face on position was consistently within ±0.03 mm for all field sizes with respect to the SFD. EBT3 film results showed a broader penumbra for the smaller field sizes, which was unexpected and requires further investigation. The penumbral width was consistently larger for EBT3 film with respect to the MOSkin for nominal field sizes of 5 mm – 10 mm.

In analysis of the output factors, the values were compared to in-house calculated Monte Carlo data. EBT3 film was shown to correlate well with the Monte Carlo data in the smaller field sizes, possibly due to its near water equivalence. The MOSkin, when compared to EBT3 film was shown to exhibit an over response of 2.6% at the smallest nominal field size of 5 mm, which could be due to the over response from the silicon components within the MOSkin. The MOSkin however, was shown to under respond at larger fields, in comparison to the Monte Carlo values. The SFD over responded for all output factor values in comparison to the Monte Carlo. The PinPoint chamber was observed to under respond at field sizes less than 10 mm, with an under response of 10.7% with respect to the Monte Carlo value at the 5 mm cone diameter.

The results of this investigation showed that the PinPoint should not be used for beam profile characterisation for stereotactic small fields unless a correction factor is applied. Overall, the MOSkin could possibly be used as a dosimeter for beam profile measurements, especially in the ‘edge on’ orientation, however data may warrant comparison to SFD measurements to minimise error.

Overall, no dosimeter can be used singularly to determine output factors in small field sizes, and all except the EBT3 film require correction factors due to non-water equivalence. The PinPoint in addition, requires a correction factor due to its significant volume averaging, causing significant dose perturbation at the smaller fields. The possibility of using the average output factor over all dosimeter used in this investigation to determine an output factor for each respective nominal field size should be investigated further.

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