Premium
TU‐C‐T‐6E‐04: Clinical Reference Dosimetry of a “Hi‐Art II” Helical Tomotherapy Machine
Author(s) -
Followill D,
Molineu A,
McGary J,
Ibbott G
Publication year - 2005
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1998372
Subject(s) - ionization chamber , tomotherapy , dosimetry , calibration , thermoluminescent dosimeter , nuclear medicine , physics , imaging phantom , laser beam quality , beam (structure) , materials science , ionization , optics , ion , dosimeter , radiation therapy , medicine , laser , quantum mechanics , laser beams
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 (8.9 2 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, k Q , was determined, P ion was measured and P pol was assumed to be unity. The ion chamber center electrode was placed at d max , 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 d max 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 cm 2 field instead of 10 × 10 cm 2 and calibration at d max 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.