Data acquisition for linac based stereotactic radiosurgery

In September 1991 a Philips Stereotactic Radiosurgery System (SRS 200) was installed in the Radiotherapy Department at Sir Charles Gairdner Hospital for use with an existing 6MV linear accelerator. The treatment planning software requires tables of Output Factors, Tissue Maximum Ratios (TMRs) and Off-Axis Ratios (OARs) for each specific linac/collimator set combination. Conventional techniques of acquiring radiation dose data for patient treatment planning were not suitable for the very small field sizes (5-30mm diameter) to be measured. Output Factors relative to a 100 x 100 mm field were measured using a large water tank and solid state diode detectors. The SRS 200 base frame which usually supports the special collimators could not be used with the tank in place and so a method for aligning the central axis of the beam, the SRS 200 collimators and diode was developed to improve the accuracy of measurement. TMRs were measured using two independent methods, similarly for the OARs. The first method of measuring TMRs made use of the large water tank and diode detectors. The second method employed the use of solid water phantom slabs seated on the base frame of the SRS 200. OARs were first measured with the solid water phantom slabs and diode; the second method made use of the technique of film dosimetry. Finally, a spherical water phantom was designed and built to check the total system accuracy. Agreement between sets of results measured by the independent methods was within 0.3% on average for TMRs and 0.25mm at the 50% dose level in most cases for the OARs.(ABSTRACT TRUNCATED AT 250 WORDS)