Medical Physics Specialist
1Northern Sydney Cancer Centre, Royal North Shore Hospital, Sydney Australia
Background and Purpose:
Total reference air kerma rate (TRAK) is a simple and unambiguous physical quantity defined as the integral of the reference air kerma rate at 1 m distance from the source over the duration of treatment. It neither takes into account dose distribution, fraction size or dose-rate but provides a useful patient-specific tool for comparison of treatment intensity (analogous to MU in EBRT) against plans of similar equieffective fractionation schedules . This work summarises the expected TRAK values for both vaginal vault and cervical HDR brachytherapy treatment schedules at Northern Sydney Cancer Centre (NSCC) where volumetric imaging has become the norm and plans are optimised to give D90 = 100% (± 1%) .
TRAK values recorded for gynaecological brachytherapy treatment plans performed at NSCC with DVH based prescriptions from years 2012 to present were retrospectively transcribed, providing n = 136 data points for vaginal vaults and n = 110 data points for cervix. Values were then normalised by the reference fractional dose dref (Gy) and HRCTV volume (cc) to provide a parameter independent of dose level and target size - with any obvious outliers removed from the analysis. A best fit to the mean value of the data using least squares polynomial regression was then used to derive the treatment specific TRAK values as a function of target size.
The TRAK per unit reference dose and volume for vaginal vaults could be fitted by the power equation y = ax-0.526, where the coefficient ranged from 0.0094 through to 0.0170 for vault diameters 20 mm and 40 mm respectively (independent of prescription depth for all practical purposes). Similarly, the TRAK per unit reference dose and volume for cervical cancer patients could be fitted by the power equation y = ax-0.568, where the coefficient was 0.0113 for intracavitary treatments and 0.0091 interstitial treatments.
Characterisation of TRAK per unit reference dose and target volume has successfully been implemented in the clinic at NSCC as a quality assurance tool of treatment intensity – one of a number of pre-treatment checks used to assess plan quality. Work remains ongoing to correlate TRAK (or a derivative) against toxicity in the certain cohorts of brachytherapy patients.
 Nkiwane, K., Anderson, E., and Champoudry, J., et al. (2017). “Total reference air kerma can accurately predict isodose surface volumes in cervix cancer brachytherapy. A multicentre study”. Brachytherapy, 16 (6):1184-1191
 Potter, R., Tanderup, K., and Kirisitis, C., et al. (2018). “The EMBRACE II study: The outcome and prospect of two decades of evolution within the GEC-ESTRO GYN working group and the EMBRACE studies”. Clinical and Translational Radiation Oncology, In-press