A TECHNIQUE FOR ACCURATE PLANNING OF STEREOTAXIC BRAIN IMPLANTS PRIORTO HEAD RING FIXATION

Purpose: A two-step procedure is described for accurate planning of st ereotactic brain implants prior to head-ring fixation, Methods and Mat erials: Approximately 2 weeks prior to implant a CT scan without the h ead ring is performed for treatment-planning purposes, An entry point and a reference point, both marked with barium and later tattooed, fac ilitate planning and permit correlation of the images with a later CT scan, A plan is generated using a conventional treatment-planning syst em to determine the number and activity of I-125 seeds required and th e position of each catheter, I-125 seed anisotropy is taken into accou nt by means of a modification to the treatment planning program, On th e day of the implant a second CT scan is performed with the head ring affixed to the skull and with the same points marked as in the previou s scan, The planned catheter coordinates are then mapped into the coor dinate system of the second CT scan by means of a manual translational correction and a computer-calculated rotational correction derived fr om the reference point coordinates in the two scans, Results: The rota tional correction algorithm was verified experimentally in a Rando pha ntom before it was used clinically, For analysis of the results with i ndividual patients a third CT scan is performed 1 day following the im plant and is used for calculating the final dosimetry, Conclusion: The technique that is described has two important advantages: 1) the numb er and activity of seeds required can be accurately determined in adva nce; and 2) sufficient time is allowed to derive the best possible pla n. (C) 1997 Elsevier Science Inc.