Three-dimensional dose accumulation in pseudo-split-field IMRT and brachytherapy for locally advanced cervical cancer

Purpose: Dose accumulation of split-field external beam radiotherapy (EBRT) and brachytherapy (BT) is challenging because of significant EBRT and BT dose gradients in the central pelvic region. We developed a method to determine biologically effective dose parameters for combined split-field intensity-modulated radiation therapy (IMRT) and image-guided BT in locally advanced cervical cancer. Methods and Materials: Thirty-three patients treated with split-field-IMRT to 45.0-51.2Gy in 1.6-1.8Gy per fraction to the elective pelvic lymph nodes and to 20Gy to the central pelvis region were included in this study. Patients received six weekly fractions ofhigh-dose rate BT to 6.5-7.3Gy per fraction. A dose tracker software was developed to computethe equivalent dose in 2-Gy fractions (EQD₂) to gross tumor volume (GTV), organs-at-risk and point A. Total dose-volume histogram parameters were computed on the 3D combined EQD₂ dose based on rigid image registration. The dose accumulation uncertainty introduced by organ deformations between IMRT and BT was evaluated. Results: According to International Commission on Radiation Unit and Measurement and GEC European Society for Therapeutic Radiology and Oncology recommendations, D₉₈, D₉₀, D₅₀, and D_2cm3 EQD₂ dose-volume histogram parameters were computed. GTV D₉₈ was 84.0±26.5Gy and D_2cc was 99.6±13.9Gy, 67.4±12.2Gy, 75.0±10.1Gy, for bladder, rectum, and sigmoid, respectively. The uncertainties induced by organ deformation were estimated to be -1±4Gy, -3±5Gy, 2±3Gy, and -3±5Gy for bladder, rectum, sigmoid, and GTV, respectively. Conclusions: It is feasible to perform 3D EQD₂ dose accumulation to assess high and intermediate dose regions for combined split-field IMRT and BT.