USE OF A 1 MM COLLIMATOR TO TEST THE ACCURACY OF STEREOTAXIC RADIOTHERAPY

Purpose: To develop a method of measuring locations of the center of d ose in stereotactic radiotherapy relative to the center of the target, and thereby obtain a test of the accuracy of stereotactic radiotherap y (SRT). Methods and Materials: An insert was mounted in an SRT collim ator on a 6 MV linear accelerator to provide a photon beam similar to 1 mm in diameter at isocenter, and a method of measuring radiation cen ter coordinates of arced SRT beams, To simulate a small intracranial t arget, two halves of a Barium paste column were embedded in two adjace nt slabs of a humanoid phantom, A film was placed between the slabs to image the radiation relative to the target center, A surgical head ri ng and computerized tomography (CT) localizer were attached to the pha ntom and CT scans were obtained, The scans were entered in a three-dim ensional computerized treatment-planning system and radiation isocente r coordinates determined by iteratively moving the 90% isodose surface center of arced beam dose distributions to coincide with the target c enter, The phantom was bolted to an SRT floorstand with isocenter coor dinates obtained from the treatment plan, and then irradiated in two s ets of experiments, The first set applied five 1 mm noncoplanar arced beams with and without offsets of the planned coordinates in the trans verse plane, The second set applied one large transverse are coplanar to the film with and without offsets in the craniocaudal direction, Ir radiations with coordinate offsets tested the sensitivity of the metho d, Films were developed and digitized with a high resolution film scan ner to measure the location of the radiation relative to the target ce nter. Results and Conclusion: The radiation center was found from 0.0 to 0.3 mm of the target center, within requirements of our clinical qu ality assurance program, The measurement and evaluation of coincidence of radiation and target centers are, thus, proposed as elements of ra diosurgery facility acceptance and annual quality assurance.