This thesis reports on the findings of an investigation into the dosimetry resulting from the abutment of en face 6 MeV electron and 6 MV photon beams as applied to the clinical challenge of radiation therapy treatments to head and neck cancer sites. Particular concern is given to the use of multi-leaf collimator (MLC) technology for photon beam definition when abutted to extended source-to-surface (SSD) electron beams. We made dosimetric comparison between MLC and Cerrobend® shielding for use in an abutment situation. The effects of extending the SSD of the electron beam were also assessed. We checked the ability of a Pinnacle3 v7.6 treatment planning system (TPS) to correctly model the dosimetry resulting from this extended electron beam SSD. Volumetric simulations of composite dosimetry resulting in water were conducted in MATLAB® for variations of surface abutment gap, and photon beam shielding type and angle. Visualization Toolkit (VTK) script was developed to visualise the resulting dosimetry. The effect of extending the SSD of the electron beam increases the beam penumbra significantly, exacerbating the challenge of matching this field to a photon beam edge. Furthermore, the TPS was shown to inaccurately model the electron beam penumbra for the extended SSD conditions. The employment of MLC shielding provides for some advantages over Cerrobend® in terms of overall composite hotspot volumes and coldspot magnitudes, though introduces detrimental dosimetric inhomogeneities in the underlying volume. Distinctly, no combination of abutment gap and shielding variables resulted in dosimetry in the range of 90% to 110% at the depth of dmax for the beam energies considered here. We provided tabulated data across these variables to outline the trade-offs present, and aid clinical decisions regarding this challenging dosimetric

This thesis reports on the findings of an investigation into the dosimetry resulting from the abutment of en face 6 MeV electron and 6 MV photon beams as applied to the clinical challenge of radiation therapy treatments to head and neck cancer sites. Particular concern is given to the use of multi-leaf collimator (MLC) technology for photon beam definition when abutted to extended source-to-surface (SSD) electron beams. We made dosimetric comparison between MLC and Cerrobend® shielding for use in an abutment situation. The effects of extending the SSD of the electron beam were also assessed. We checked the ability of a Pinnacle3 v7.6 treatment planning system (TPS) to correctly model the dosimetry resulting from this extended electron beam SSD. Volumetric simulations of composite dosimetry resulting in water were conducted in MATLAB® for variations of surface abutment gap, and photon beam shielding type and angle. Visualization Toolkit (VTK) script was developed to visualise the resulting dosimetry. The effect of extending the SSD of the electron beam increases the beam penumbra significantly, exacerbating the challenge of matching this field to a photon beam edge. Furthermore, the TPS was shown to inaccurately model the electron beam penumbra for the extended SSD conditions. The employment of MLC shielding provides for some advantages over Cerrobend® in terms of overall composite hotspot volumes and coldspot magnitudes, though introduces detrimental dosimetric inhomogeneities in the underlying volume. Distinctly, no combination of abutment gap and shielding variables resulted in dosimetry in the range of 90% to 110% at the depth of dmax for the beam energies considered here. We provided tabulated data across these variables to outline the trade-offs present, and aid clinical decisions regarding this challenging dosimetric