For palliative radiotherapy treatments, two types of simulation are available at our Centre: conventional (or 2D) and virtual. Each has its advantages: conventional simulation requires less preparation time whereas virtual simulation allows accurate visualisation and identification of target volumes in 3D. We propose a new approach where treatment field parameters are determined on diagnostic CT scans, and then reproduced with reference to a patient’s bony landmarks using conventional simulation. This combines the benefits of both methods. We argue that the slight differences in set-up between diagnostic and simulation CT scans will have little impact on the determination of the target volume in palliative treatment settings.

In total, 12 patients who had diagnostic scans done within a month before their virtual simulation were randomly selected. Both scans were retrieved retrospectively for the study. An independent radiation oncologist contoured the target volumes on both scans and their relative positions were compared by fusing the digitally reconstructed radiographs generated from the respective scans. A 2D Conformity Index (2DCI) was then calculated and tabulated for 0, 0.5, 1.0, 1.5 and 2 cm margins to evaluate the accuracy of this approach and determine the margins required to account for the inherent variability of this method. In addition, the deviation or offset of the centre of field (COF) was also measured and analysed.

The median 2DCI (0 margin) was 0.85. For margins of 0.5, 1.0, 1.5 and 2.0 cm, the median 2DCIs were 0.97, 1, 1 and 1, respectively. The median displacement of the COF in the superior–inferior direction was 0.7 cm and that in the right–left direction was 0.2 cm.

Our pilot study suggests that our approach is feasible. We recommend adding an additional margin of up to 0.5 cm (to the usual treatment margins) to ensure good coverage of the target volume when using this method.