A study of conformal radiotherapy methods for brain and prostate cancer
State-of-the-art radiotherapy involves a technology chain that includes 3D tumour imaging, 3D treatment planning, treatment delivery using conformal or intensity modulated techniques, and treatment verification. This thesis evaluates some of the recent imaging and planning developments to assess their role in optimisation of the technology chain for brain and prostate cancer. I focused on two major links in this technology chain. The 'imaging' link compared the use of MRI and CT in localising target volumes. An image registration protocol was developed to combine CT and MRI images in the brain. For the localisation of skull base meningiomas, MRI was found to provide contrasting information to CT. A composite target volume derived from both CT and MR information provided the most appropriate volume. For prostate radiotherapy, four MRI sequences were compared to CT. All MRI sequences provided improved localisation of relevant radiotherapy volumes-of-interest especially for the prostatic apex and rectum. The 'treatment planning' link investigated the impact of intra-fraction prostate motion for prostate planning margins, the creation of planning margins, the optimisation of beam orientations for prostate radiotherapy, and the utility of IMRT methods for brain tumours. Cine MR demonstrated a significant relationship between moderate rectal distension and increased rectal activity resulting in prostate motion. Mean prostate motion was < 5mm and lasted < 5% of a7 minute period indicating that the current 10mm prostate planning margin was adequate. The use of a 3D margin-growing method allowed the planning target margin to be accurately realised in all spatial orientations and avoided problems associated with 2D margin growing methods. A variety of co-planar arrangements using 3-, 4-, and 6-fields were evaluated for conformal prostate radiotherapy. Standard prostate plans could be optimised by proper consideration of beam orientations. A 3-field plan with gantry angles of 0", 90', 270' Provided the best rectal sparing for both prostate alone and prostate plus seminal vesicles volumes. Using this 3-field plan, dose escalation may be achieved with a smaller increase in predicted late rectal complications than with other 3-, 4-, or 6-field plans. An IMRT tomotherapy method was compared with conformal radiotherapy for convex shaped brain tumours to assess its potential for improved dose conformation. This IMRT method provided slightly improved PTV coverage but also higher OARs doses. However, these OARs doses remained within acceptable clinical limits. This IMRT tomotherapy method did not provide significant planning improvement compared to current conformal radiotherapy technique.