Evaluation of accuracy and reproducibility of a relocatable maxillary fixation system for fractionated intracranial stereotactic radiation therapy

INTRODUCTION Accurate localisation is an essential component for the delivery of intracranial stereotactic treatment. For fractionated stereotactic radiotherapy, we compared the daily localisation accuracy of a standard thermoplastic mask with a new maxillary fixation device (MFD). METHODS Daily pre-treatment kV cone-beam computed tomography (CBCT) scans of 23 patients (12 localised in the MFD and 11 in the mask) with benign skull-based lesions were reviewed retrospectively. The set up accuracy was measured in 6° of freedom, to ascertain both individual and population random and systematic errors. The appropriate clinical target volume to planning target volume margin was computed from set up error data. RESULTS A total of 682 CBCT scans were evaluated. Systematic (Σ) and random (σ) population errors were Σ = 0.8 mm, 0.2 mm and 0.2 mm and σ = 0.3 mm, 0.3 mm and 0.2 mm, respectively, for the standard mask in the left/right (LR), superior/inferior (SI), and anterior/posterior (AP) translational planes, and Σ = 0.2 mm, 0.1 mm and 0.2 mm and σ = 0.2 mm, 0.3 mm and 0.2 mm, respectively, for the MFD. There was a reduction in rotation errors in the MFD compared to the mask. Margin calculations suggested an isotropic margin could be safely reduced to 2 mm for the MFD. CONCLUSION The two devices demonstrate similar daily positional accuracy for fractionated stereotactic treatment of intracranial lesions. Combined with daily image guidance and couch correction, either of these devices is a viable frameless option for fractionated stereotactic radiation therapy.