A DUAL COMPUTED-TOMOGRAPHY LINEAR-ACCELERATOR UNIT FOR STEREOTAXIC RADIATION-THERAPY - A NEW APPROACH WITHOUT CRANIALLY FIXATED STEREOTAXICFRAMES

Purpose: To perform stereotactic radiation therapy (SRT) without crani ally fixated stereotactic frames, we developed a dual computed tomogra phy (CT) linear accelerator (linac) treatment unit. Methods and Materi als: This unit is composed of a linac, CT, and motorized table. The li nac and CT are set up at opposite ends of the table, which is suitable for both machines. The gantry axis of the linac is coaxial with that of the CT scanner. Thus, the center of the target detected with the CT can be matched easily with the gantry axis of the linac by rotating t he table. Positioning is confirmed with the CT for each treatment sess ion. Positioning and treatment errors with this unit were examined by phantom studies. Between August and December 1994, 8 patients with 11 lesions of primary or metastatic brain tumors received SRT with this u nit. All lesions were treated with 24 Gy in three fractions to 30 Gy i n 10 fractions to the 80% isodose line, with or without conventional e xternal beam radiation therapy. Results: Phantom studies revealed that treatment errors with this unit were within 1 mm after careful positi oning. The position was easily maintained using two tiny metallic ball s as vertical and horizontal marks. Motion of patients was negligible using a conventional heat-flexible head mold and dental impression. Th e overall time for a multiple noncoplanar arcs treatment for a single isocenter was less than 1 h on the initial tretment day and usually le ss than 20 min on subsequent days. Treatment was outpatient-based and well tolerated with no acute toxicities. Satisfactory responses have b een documented. Conclusion: Using this treatment unit, multiple fracti onated SRT is performed easily and precisely without cranially fixated stereotactic frames.