Proceedings of the 38th Annual ASTRO Meeting 257

1026 MRI LOCALIZATION 0~ IRRI~~I,ARLY SHAPED INTRACRANIAL TARGETS FOR FRAMELESS STEREOTACTIC RADIOTHERAPY

Douglas Jones*, Donald Christopherson*, John W. Rieke, M.D.“, Mark D. Hafermann, M.D.**, and John Travaglini, M.D.** *Northwest Medical Physics Center, Lynnwood, Washington **Virginia Mason Medical Center, Seattle, Washington

PURPOSE - To devise a method to precisely locate gold filled titanium screws, fixed to the skull, in an MRI study and define a process for conformal stereotactic irradiation of irregularly shaped targets

METHODS & MATERIALS - A computer program, MRTOFLM, has been written which performs a least squares fit to two 3-D data sets, calculates an offset for each point transformed from one coordinate system to the other, and automatically whittles the data set excluding points with large offsets. The 3-D data sets consist of points, defined by markers, in isocentric X-Ray film studies and axial, coronal and sagittal MRI series. The gold fiducial markers, used for stereotactic localization of the target, are located in the film studies and transformed to the MRJ coordinate system, at which point treatment planning can proceed as for CT scanning which has been reported (1). The selection of stereotactic multiple arc. radiotherapy (SMART) or stereotactic radiotherapy with irregular ports (STRIP) is determined by calculating an irregularity factor for the target, which is the ratio of the surface area of the target to the surface area of a sphere with the same volume as the target.

RESULTS - In our experience when the irregularity factor exc$eds 1.25, substantial sparing of adjacent normal tissue is obtained by using six to ten stationary fields with BEV shaping of the apertures and about one-third of our stereotactically localized patients are treated in this manner. On average the ratio of the volume of the prescription isodose shell to the target volume (RATIO PITV) is reduced by a factor of about two using STRIP over SMART. The vector offset in the set up of the fiducial points for stereotactic radiotherapy was less than 0.5 mm in two patients, when a direct measure of the fiducials in a CT study was compared to indirect calculation using MRTOFLM on MR data.

CONCLUSION - We have used conformal stationary fields for stereotactic radiotherapy since June 1993, at this time (March 1996) forty-two patients have been treated by the method. The technique of using simulator films with markers has evolved from a method of correlating anatomical points in CT & MRJ studies and is considerably less expensive than the earlier method in that it doe not require a CT scan and the image sets do not require the carehI scrutiny of a neuroradiologist.

(1) Jones, D.; Christopherson, D.A.; Washington, J.T., Hafermann, M. D.; Rieke, J. W.; Travaglini, J. J.; and Vermuelen, S.S. A frameless method for stereotactic radiotherapy. Br. J. Radiol., 66, 1142-I 150, 1993.

1027 MRI-EVALUATION OF LATE TISSUE RESPONSE AFTER RADIOSURGERY IN THE RAT BRAIN

Christian P. Karger, Ph.D.; Giinther H. Hartmann, Ph.D.; Peter Peschke, Ph.D.; Jiirgen Debus, Ph.D., M.D.; Ulf Hoffmann, PhD.; Gunnar Brix, Ph.D.; Eric Hahn, Ph.D.; Walter J. Lorenz, Ph.D.

German Cancer Research Center Department of Biophysics and Medical Radiation Physics, FS05

D-69120 Heidelberg, Germany E-mail: C.Karger@dkfz-heidelberg.de

Purpose: only few quantitative data are available on late effects in the healthy brain after radiosurgery. An animal model would contribute to systematically investigate such late effects. For this reason a rat model applying radiosurgery at the rat brain was established. This investigation comprised several steps: (I) design for a special fixation- and localization device to perform linac based stereotactic radiosurgery at the rat; (2) feasibility and accuracy study of irradiation and MRJ-evaluation in the rat brain; (3) a long term (I % year) follow up study with a group of animals.

Materials & Methods: A localization technique was developed to irradiate a small target volume within the rat brain. A mean spatial uncertainty of I mm was verified by phantom measurements. At 60 animals, a small area of the brain was irradiated stereotactically (1 S MV linac). Different doses doses of 20, 30,40, 50, and 100 Gy with two field sizes using the 2 and 3 mm collimator were administered. These dose levels were selected to be equally distributed between 0 and 100 % effect propability according to the Flickinger model assuming comparable radiosensitivity between rat and human brain. The diameter of the spherical dose distribution (80%-isodose) was 3.9 and 5.8 mm, respectively. The alteration of the permeability of the blood brain barrier was investigated, using magnetic resonance imaging and Gd-DTPA contrast agent. An intracranial contrast enhancement was interpreted as a first indication for brain necrosis. Half of the animals were killed for histology after 9 and I8 months, respectively.

Results: A first intracranial signal enhancement was observed 160 days after irradiation. Within one year, all animal in the two 100 Gy groups showed contrast enhancement, but none of the other groups. The incident rates were 6/6 for the 2 mm collimator and 5/5 for the 3 mm collimator. Contrast enhancement volume and signal intensity were significantly different between these two groups. After 18 months, however, other animals also showed contrast enhancements. The incidence rate was 2/3 for the SO Gy/3 mm group and the 40 Gy/3 mm group, and 113 for the 30 Gy/3 mm group and the 50 GyC! mm group. All other animals did not show any contrast enhancement within I8 months.

Conclusions: Linac based stereotactic radiosurgery can be succesfully applied at the rat brain. The animal model is appropriate to study late normal brain tissue response. Contrast enhancement as an indication of late radionecrosis was found I % year after even moderate doses of radiosurgery. The behaviour of radioresponse appeared to followed the prediction of the Flickinger model for human brain. The techniques used can also be applied to study modifications in the irradiation modality, i.e. fractionation, irregular volumes, or radiation quality.