Total reference air kerma: To what extent can it predict intracavitary volume enclosed by isodose surfaces during multiple high-dose rate brachytherapy?

Background: The International Commission on Radiation Units and Measurements (ICRU) report 38 recommends reporting of total reference air kerma (TRAK) and reference ICRU isodose volumes during intracavitary brachytherapy (ICBT) in cancer of the cervix. The present study attempts to estimate the volumes enclosed by isodose surfaces from TRAK and evaluate its utility to represent doses to organs of interest. Material and Methods: Volumes encompassed by isodose surfaces of 3 Gy, 6 Gy, 9 Gy, and 12 Gy were obtained for 90 high-dose rate (HDR) ICBT procedures. These were used to derive a relation between isodose volumes, TRAK/dose (K/D), and rectal and bladder doses. Results: Actual volumes (V) encompassed by isodose surfaces were reflected as a quadratic function of K/D (r2 = 0.998) and the expression, V = −23.09 + 1295.99(K/D) + 5661.65(K/D)2 gave the best estimates for various volumes. No correlation was observed between TRAK and bladder (r2 = 0.086) or rectal doses (r2 = 0.082). Conclusions: Estimates of volumes encompassed by different dose levels from TRAK could be derived with reasonable certainty. However, TRAK fails to correlate with rectal and bladder doses.