Semi-robotic 6 degree of freedom positioning for intracranial high precision radiotherapy; first phantom and clinical results

BACKGROUND To introduce a novel method of patient positioning for high precision intracranial radiotherapy. METHODS An infrared(IR)-array, reproducibly attached to the patient via a vacuum-mouthpiece(vMP) and connected to the table via a 6 degree-of-freedom(DoF) mechanical arm serves as positioning and fixation system. After IR-based manual prepositioning to rough treatment position and fixation of the mechanical arm, a cone-beam CT(CBCT) is performed. A robotic 6 DoF treatment couch (HexaPOD) then automatically corrects all remaining translations and rotations. This absolute position of infrared markers at the first fraction acts as reference for the following fractions where patients are manually prepositioned to within +/- 2 mm and +/- 2 degrees of this IR reference position prior to final HexaPOD-based correction; consequently CBCT imaging is only required once at the first treatment fraction.The preclinical feasibility and attainable repositioning accuracy of this method was evaluated on a phantom and human volunteers as was the clinical efficacy on 7 pilot study patients. RESULTS Phantom and volunteer manual IR-based prepositioning to within +/- 2 mm and +/- 2 degrees in 6 DoF was possible within a mean(+/- SD) of 90 +/- 31 and 56 +/- 22 seconds respectively. Mean phantom translational and rotational precision after 6 DoF corrections by the HexaPOD was 0.2 +/- 0.2 mm and 0.7 +/- 0.8 degrees respectively. For the actual patient collective, the mean 3D vector for inter-treatment repositioning accuracy (n = 102) was 1.6 +/- 0.8 mm while intra-fraction movement (n = 110) was 0.6 +/- 0.4 mm. CONCLUSIONS This novel semi-automatic 6DoF IR-based system has been shown to compare favourably with existing non-invasive intracranial repeat fixation systems with respect to handling, reproducibility and, more importantly, intra-fraction rigidity. Some advantages are full cranial positioning flexibility for single and fractionated IGRT treatments and possibly increased patient comfort.