Stereotactic Target point Verification in Actual Treatment Position of Radiosurgery

Yun, Hyong Geun; Lee, Hyun Koo

Abstract

PURPOSE: Authors tried to enhance the safety and accuracy of radiosurgery by verifying stereotctic target point in actual treatment position prior to irradiation.
MATERIALS AND METHODS
Before the actual treatment, several sections of anthropomorphic head phantom were used to create a condition of unknown coordinated of the target point. A film was sandwitched between the phantom sections and punctured by sharp needle tip. The tip of the needle represented the target point. The head phantom was fixed to the stereotactic ring and CT scan was done with CT localizer attached to the ring. After the CT scanning, the stereotactic coordinates of the target point were determined. The head phantom was secured to accelerator's treatment couch and the movement of laser isocenter to the stereotactic coordinates determined by CT scanning was performed using target positioner. Accelerator's anteroposterior and lateral portal films were taken using angiographic localizers. The stereotactic coordinates determined by analysis of portal films were compared with the stereotactic coordinates previously determined by CT scanning. Following the correction of discrepancy, the head phantom was irradiated using a stereotactic thechnique of several arcs. After the irradiation, the film which was sandwitched between the phantom sections was developed and the degree of coincidence between the center of the radiation distribution with the target point represented by the hole in the film was measured. In the treatment of actual patients, the way of determining the stereotactic coordinates with CT localizers and angiographic localizers between two sets of coordinates, we proceeded to the irradiation of the actual patient.
RESULTS
In the phantom study, the agreement between the center of the radiation distribution and the localized target point was very good. By measuring optical density profiles of the sandwitched film along axes that intersected the target point, authors could confirm the discrepancy was 0.3mm. In the treatment of an actual patient, the discrepancy between the stereotactic coordinates with CT localizers and angiographic localizers was 0.6mm.
CONCLUSION
By verifying stereotactic target point in actual treatment position prio to irradiation, the accuracy and safety of streotactic radiosurgery procedure were established.