SU‐E‐T‐207: Comparison of Integrated Tissue Air Ratio (ITAR) to Traditional TAR for Kilovoltage Pencil‐Beams

Abstract

Purpose:Clinically viable depth dose determination in kilovoltage pencil‐beams is a great challenge that resulted in a published dosimetry method called ITAR, which involves measurement of air kerma and attenuation with a detector in a low scatter environment coupled with MCNP scatter calculations. The objective of this work is to compare ITAR to traditional TAR using inherently water‐proof microchambers that have only recently become commercially available.Methods:An Exradin A26 microchamber was centered 150 mm from a 100 kVp x‐ray source with 2 mm aluminum HVL. Depth dose in water from 16 to 24 mm in 2 mm increments was determined by: (1) placing blocks of Plastic Water LR near the source to minimize scatter and using previously published conversion coefficients [ITAR method] and (2) submerging the detector in a water tank with 2 mm thick Plastic Water LR walls and jogging the tank with motor controllers while keeping the detector position fixed [traditional TAR method]. Each method was repeated four to five times. For each repetition, dose was measured free in‐air to normalize the data for exponential regression.Results:Traditional TAR indicated higher depth dose than ITAR; differences ranged from 2.1% at 24 mm depth to 2.5% at 16 mm depth. However, the results of traditional TAR did not include a correction for Pq,cham because it is unknown for this detector type in these conditions. It is estimated that the component of Pq,cham due to the effect of water displacement alone is ∼0.94, but Pq,cham is likely several percent larger than 0.94 due to the energy dependency of the microchamber in the presence of low energy scatter that is not present during in‐air calibration.Conclusion:The ITAR method remains preferable for clinical depth dose determination in kilovoltage pencil‐beams due to Pq,cham being unknown for suitable detectors in relevant conditions.All four of the authors are either current full time employees, which include stock option grants, or consultants of Oraya Therapeutics Inc.