PRECLINICAL EVALUATION OF THE RELIABILITY OF A 50 MEV RACETRACK MICROTRON

Purpose: A 50 MeV racetrack microtron has been installed and tested at Memorial Sloan-Kettering Cancer Center. It is designed to execute mul ti-segment conformal therapy automatically under computer control usin g scanned X ray and electron beams from 10 to 50 MeV. Prior to accepta nce of the machine from the manufacturer, formal reliability testing w as carried out. Only in this way could confidence be gained in its use fulness for routine 3D computer-controlled conformal therapy. Material s and Methods: To assess reliability, a set of 25 multi-segment test c ases, each consisting of 10 to 17 fixed segments, was developed. The f ield arrangements and modalities for some of the test cases were ident ical to 3D conformal treatments that were being delivered with multipl e static fields on conventional linear accelerators at our institution . Other cases were designed to explore reliability under more complex sets of conditions. These cases were ''treated'' repeatedly during a t otal period of 45 hours, over 5 days. During the treatments, ion chamb ers attached to the head of the machine provided dosimetric data for e ach field. Data from sensors connected to every set-up parameter (for example, couch positions, gantry angle, collimator leaf positions, etc .) were recorded and verified by an external computer. Results: While preliminary tests indicated an interlock rate of 5%, final reliability test results demonstrated an interlock fault rate of approximately 0. 5%. The reproducibility of dosimetric data and geometric setup paramet ers was within specifications. As an example, leaf position reproducib ility in the patient plane was within 0.5 mm for 97% of the setups. Th e times required to carry out treatments were recorded and compared wi th the times to carry out identical treatments on a conventional linea r accelerator with cerrobend blocks. Areas where additional time savin gs can be achieved were identified. Conclusion: As an integral part of acceptance testing, the Scanditronix MM50 was rigorously tested for r eliability. The machine successfully passed these tests, providing inc reased confidence in its usefulness for routine 3D conformal therapy.