Year

2018

Degree Name

Master of Science (Medical Radiation Physics)

Department

School of Physics

Abstract

Dose targeting is critical in lung tumours that are treated with stereotactic ablative body radiotherapy (SABR). However, the small fields associated with these techniques in lung can cause increased lateral electron disequilibrium (LED), creating a reduction in the absorbed dose delivered to the tumour when compared with the treatment planning dose calculation.

The accuracy of the Philips Pinnacle ® collapsed cone convolution calculation in lung at densities 0.1g/cm3 to 1g/cm3 was investigated. A simple lung slab phantom was simulated for 10MV and 6MV photon beams, and for field sizes of 2cm x 2cm, 3cm x 3cm, 5cm x 5cm and 10cm x 10cm. Data from the calculation for lung depth dose and penumbral width (80% - 20%) were both investigated. Off axis dose profiles collected from these simulations were then benchmarked at 0.3g/cm3 lung density using Gaf EBT3 ® film in a CIRS® lung phantom. Depth dose measurements were also undertaken in the lung phantom with an IBA CC04 ion chamber and an Advanced Markus parallel plate ionization chamber. Results were then finally benchmarked against EGSnrc Monte Carlo calculations.

The depth dose trends in lung were as follows. The ionization chambers showed an over-response when compared with convolution, which in turn showed a slight overresponse compared to Monte Carlo simulations for small fields sizes down to 2cm x 2cm. However, dose for these three methods converged as the field size increased and the amount of LED was reduced, with a close match for a 10cm x 10cm field size. The impact of lung density change on the central axis dose were estimated using the convolution calculation at three different densities. For a 3cm x 3cm field at 10MV, the percentage depth dose values in the mid lung were 55.4%, 62.6%, 66.9% for 0.2g/cm3, 0.3g/cm3 and 0.4g/cm3 densities respectively. The penumbral 80% - 20% widths were 1.12cm, 0.90cm, 0.80cm and 0.54cm for densities 0.2g/cm3, 0.3g/cm3, 0.4g/cm3, and 1.0g/cm3 respectively. The central axis deficits and penumbral flaring were also quantified at 6MV and were of lesser magnitude than at 10MV.

Lateral electron disequilibrium has a significant impact on dose coverage and needs to be considered for SABR applications. The convolution method showed good agreement with Monte Carlo and film simulations. There were only significant differences between the convolution and Monte Carlo compared to ionization chamber measurements at very small field sizes, with the latter over-responding compared with Monte Carlo and convolution in lung.

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