Correlation of total reference air-kerma (TRAK) to prescription isodose surface volume in vaginal cylinder high-dose-rate brachytherapy

Introduction
The purpose of this technical note was to investigate correlation of total reference air-kerma (TRAK) with volume enclosed by the prescription isodose surface in vaginal cylinder high-dose-rate (HDR) brachytherapy.

Material and methods
Treatment plans of 175 gynecological cancer patients treated at our institution with iridium-192 (192Ir) HDR brachytherapy using a single-channel vaginal cylinder applicator were retrospectively reviewed. Applicator size in diameter ranged from 20 mm to 40 mm. Treatment length ranged from 30 mm to 90 mm (median, 50 mm). Brachytherapy fractional dose was 5 Gy (DoseRef) prescribed to 5 mm distance from cylinder surface. Parameters TRAK (cGy), source activity during treatment (Ci), total treatment time (s), and prescription isodose surface volume ISVRef (cm3) were recorded from individual treatment plans. In each case, vaginal tissue volume (VVT) enclosed by prescription isodose was calculated by subtracting cylinder volume enclosed by the prescription isodose from ISVRef.

Results
Total reference air-kerma correlated with the total volume enclosed by the prescription isodose via ISVRef = 4768 × (TRAK/DoseRef)1.47. TRAK related linearly to the volume of vaginal tissue enclosed by the prescription dose via VVT = ((138.3 × TRAK) – 8.2). Secondarily, TRAK related to the treatment time through time (s) = 882 (s/cGy) × TRAK (cGy), where 882 is (1/air-kerma strength) for 10 Ci apparent activity of 192Ir source.

Conclusions
The correlation of TRAK to the vaginal tissue volume encompassed by the prescription dose surface yields a useful predictive equation. The TRAK treatment time relationship enables quick verification of planned treatment time by knowing TRAK in any HDR brachytherapy application.