Clinical reference dosimetry of a "Hi-Art II" helical tomotherapy machine

Purpose: To develop a procedure for calibrating “Hi‐Art II” helical tomotherapy machines using TG‐51 and analyze the uncertainties associated with minor departures from the TG‐51 protocol. Method and Materials: Physical limitations of the tomotherapy unit (85cm SAD) restrict the user from performing a rigorous TG‐51 calibration. The Radiological Physics Center determined the ionization ratio (IR) using a water phantom and Exradin A12 ion chamber for a 40cm × 5cm field (8.92 eq.sq.). The IR was converted to %dd(10)x using a published relationship between %dd(10)x and IR. The beam quality conversion factor, kQ, was determined, Pion was measured and Ppol was assumed to be unity. The ion chamber center electrode was placed at dmax, instead of the recommended 10cm depth. The output/min was calculated using TG‐51 and verified using a newly designed TLD jig. Results: The measured IR indicates a beam energy slightly greater than that for a 4 MV x‐ray beam. The reference calibration resulted in a dose rate of 882.7 cGy/min at dmax for a 40cm × 5cm field at 85cm SAD that was within 1% of the dose rate (890.6 cGy/min) set by the factory where 0.4% of the difference was due to ADCL calibration differences and an un‐flattened beam. The TLD/ion‐chamber dose ratio was 0.993 (std.dev.= 0.007). An analysis of uncertainties associated with measuring IR and conversion to %dd(10)x, use of a 40 × 5 cm2 field instead of 10 × 10 cm2 and calibration at dmax instead of 10 cm results in a possible increased uncertainty of 0.5% in the final reference calibration. We believe this to be negligible. Conclusion: A procedure was developed to calibrate “Hi‐Art II” tomotherapy machines using TG‐51 with a minimal increase in the uncertainty of the final calculated dose rate. Work supported by PHS grant CA10953 awarded by NCI, DHHS. © 2005, American Association of Physicists in Medicine. All rights reserved.