COMPUTER-PLANNING OF STEREOTAXIC I-125 SEED BRACHYTHERAPY FOR RECURRENT MALIGNANT GLIOMAS

At St Thomas' Hospital, we have developed a computer program on a Tita n graphics supercomputer to plan the stereotactic implantation of iodi ne-125 seeds for the palliative treatment of recurrent malignant gliom as. Use of the Gill-Thomas-Cosman relocatable frame allows planning an d surgery to be carried out at different hospitals on different days. Stereotactic computed tomography (CT) and positron emission tomography (PET) scans are performed and the images transferred to the planning computer. The head, tumour and frame fiducials are outlined on the rel evant images, and a three-dimensional model generated. Structures whic h could interfere with the surgery or radiotherapy, such as major vess els, shunt tubing etc., can also be outlined and included in the displ ay. Catheter target and entry points are set using a three-dimensional cursor controlled by a set of dials attached to the computer. The pro gram calculates and displays the radiation dose distribution within th e target volume for various catheter and seed arrangements. The CT co- ordinates of the fiducial rods are used to convert catheter co-ordinat es from CT space to frame space and to calculate the catheter insertio n angles and depths. The surgically implanted catheters are after-load ed the next day and the seeds left in place for between 4 and 6 days, giving a nominal dose of 50 Gy to the edge of the target volume. 25 pa tients have been treated so far.