On the Use of a Hexapod Table to Improve Tumour Targeting in Radiation Therapy

Abstract

High precision image-guided radiotherapy demands a high degree of spatial accuracy for treatment delivery. Recent advances in imaging technology for radiotherapy have been shown to yield information with respect to the tumour position with sub-millimetre accuracy. Differences between planned and actual tumour position at treatment can be calculated in six DOF. With standard equipment, it is not possible to correct for these set-up errors with the same accuracy as they were detected. Furthermore, it is not possible to correct for all rotational errors. These limitations can be overcome with the robotic HexaPOD table. With the combination of 3D volume imaging on the linac and the HexaPOD table, an overall mean positioning accuracy of 0.3 mm and 0.3? was obtained in an experimental accuracy study. With this combination, it is possible to deliver treatment with a high degree of accuracy guided by imaging technology. This, in turn, has the potential to reduce safety margins around the tumours and hence to improve treatment outcomes. At the University of W?rzburg, 6D patient set-up correction is routinely carried out for highprecision radiotherapy. Patients who most benefit from such high accuracy are those with tumours in very close proximity to critical structures, such as lesions in the brain or paraspinal lesions. The latter are frequently wrapped around the spinal cord. Due to the elongated dimensions of paraspinal tumours surrounding a critical structure, this constellation is very sensitive to rotational set-up errors, which can be corrected adequately with the HexaPOD table. It was found that although rotational errors can be corrected it is essential to ensure that the patient is appropriately immobilized. Clinical application of 6D position correction has revealed that several non-immobilized patients who received standard treatment in the pelvic region moved after rotational errors had been corrected. This was most likely to counter act table rotations around the patient axis. Some patients ended up with set-up errors larger than before correction. This fact highlights the fine line between the benefits and detrimental effects with this new technology. A critical evaluation is indispensable to determine the exact circumstances under which a treatment can actually be improved. At the University of W?rzburg, a reasonable number of studies have been carried out to integrate image-guided radiotherapy and corrections with the HexaPOD table seamlessly into clinical routine. Today, for high-precision radiotherapy at the University of W?rzburg, the HexaPOD table is an integral part of the treatment. An active area of research is dynamic real-time compensation of tumour movement by means of WATTS. The approach described is a feasibility study. So far, its achievability has been demonstrated under non-clinical settings in a clinical environment. Although the preliminary results are very encouraging, it is still a long way from clinical implementation. Hitherto, both tracking the tumour in real-time using MV portal imaging and recording the