PURPOSE: To demonstrate that V60 Gy, V75 Gy, and V85 Gy isodose surface volumes can be accurately estimated from total reference air kerma (TRAK) in cervix cancer MRI-guided brachytherapy (BT).

METHODS AND MATERIALS: 60 Gy, 75 Gy, and 85 Gy isodose surface volumes levels were obtained from treatment planning systems (VTPS) for 239 EMBRACE study patients from five institutions treated with various dose rates, fractionation schedules and applicators. An equation for estimating VTPS from TRAK was derived. Furthermore, a surrogate Point A dose (Point A*) was proposed and tested for correlation with V75 Gy.

RESULTS: Predicted volumes Vpred = 4965 (TRAK/dref) 3/2 + 170 (TRAK/dref) - 1.5 gave the best fit to VTPS. The difference between VTPS and predicted volumes was 0.0% ± 2.3%. All volumes were predicted within 10%. The prediction was valid for (1) high-dose rate and pulsed dose rate, (2) intracavitary vs. intracavitary/interstitial applicators, and (3) tandem-ring, tandem-ovoid, and mold. Point A* = 14 TRAK was converted to total EQD2 and showed high correlation with V75 Gy.

CONCLUSIONS: TRAK derived Isodose surface volumes may become a tool for assessment of treatment intensity. Furthermore, surrogate Point A(∗) doses can be applied for both intracavitary and intracavitary/interstitial BT and can be used to compare treatments across fractionation schedules.