Modifying a plan delivery without re-optimisation to account for patient offset in tomotherapy

Patient positioning in tomotherapy can be achieved either by using the computed tomographic (CT) capabilities or by using registration of sinograms in projection space. In both cases a reference image associated to the patient anatomy obtained in a previous fraction, is registered with the new one. From this registration processes, the six offset parameters (three translational, and three rotational) can be obtained. In principle there are two possible ways to accommodate the patient accordingly to the geometry of the optimized treatment plan: (a) Shift the patient manually in order to account for the patient offset. This may be, however, a prone error methodology. (b) Use a couch with three-dimensional movement capabilities to accounting for the patient's shift. However, accounting for rotational patient's offset such as pitch, jaw and roll is extremely difficult. Another possibility will be to maintain the patient in the same position on the couch and modify the treatment delivery. The method must be fast enough to be performed with the patient on the couch. The authors are developing such delivery modification (DM) methodology. Specifically, delivery modification is the set of transformations applied to the planned energy fluence sinogram to account for the patient offset. A DM test is presented for a tomotherapy prostate treatment. Different offsets were applied to the CT and the delivery sinograms were modified to accommodate these offsets. Dose volume histograms and dose distributions are presented to analyze capabilities and limitations of this method.