3D CT-based volumetric dose assessment of 2D plans using GEC-ESTRO guidelines for cervical cancer brachytherapy

Purpose: To investigate two-dimensional (2D) radiograph-based plans using three-dimensional (3D) dose-volume histogram (DVH) parameters following guidelines from Gynecologic GEC-ESTRO Working Group (GEC-ESTRO). Methods and Materials: Nineteen high-dose-rate (HDR) fractions from 8 patients were studied. Prescription was 45 Gy from external beam radiation therapy plus 30 Gy in five fractions from HDR using tandem and ring/ovoids. Both radiographs and CT scan were obtained. Treatment was planned using radiographs following American Brachytherapy Society (ABS) guidelines. Retrospective evaluation of above 2D plans on a 3D volumetric basis was achieved by generating CT image-based 3D plans using same dwell times. Results: In 2D plans, International Commission on Radiation Units and Measurement (ICRU) bladder and rectal point doses were 3.8 ± 0.4 and 3.0 ± 0.5 Gy, respectively. In 3D plans, rectum D_{2 cc} is 4.0 ± 1.0 Gy and bladder D_{2 cc} is 5.4 ± 0.9 Gy. Position of actual hottest spot in 3D rectum volume was close to the position of ICRU rectal point. ICRU bladder point did not match with the actual hottest spot in 3D bladder volume. In 2D plans, H-point dose was 5.8 ± 0.2 Gy. In 3D plans, dose to CT-based cervix (D₉₀) reduced from 7.1 to 4.2 Gy as the cervical volume increased from 12 to 39 cc. Average D_{2 cc}/ICRU dose ratio was calculated to be 1.36/1.01 for bladder/rectum, respectively. Conclusions: The DVH analysis of 2D plans revealed a suboptimal coverage of CT-based cervix and a negative correlation between coverage and cervical size. Rectum dose to 2 cc weakly correlated with ICRU point dose. Currently published constraint for bladder in 3D planning is tighter than ABS guidelines in past 2D planning.