Evaluation of deformable image registration algorithm for determination of accumulated dose for brachytherapy of cervical cancer patients

Purpose: This study was designed to assess the dose accumulation (DA) of bladder and rectum between brachytherapy fractions using hybrid-based deformable image registration (DIR) and compare it with the simple summation (SS) approach of GEC-ESTRO in cervical cancer patients. Material and methods: Patients (n = 137) with cervical cancer treated with 3D conformal radiotherapy and three fractions of high-dose-rate brachytherapy were selected. CT images were acquired to delineate organs at risk and targets according to GEC-ESTRO recommendations. In order to determine the DA for the bladder and rectum, hybrid-based DIR was done for three different fractions of brachytherapy and the results were compared with the standard GEC-ESTRO method. Also, we performed a phantom study to calculate the uncertainty of the hybrid-based DIR algorithm for contour matching and dose mapping. Results: The mean +/- standard deviation (SD) of the Dice similarity coefficient (DICE), Jaccard, Hausdorff distance (HD) and mean distance to agreement (MDA) in the DIR process were 0.94 +/- 0.02, 0.89 +/- 0.03, 8.44 +/- 3.56 and 0.72 +/- 0.22 for bladder and 0.89 +/- 0.05, 0.80 +/- 0.07, 15.46 +/- 10.14 and 1.19 +/- 0.59 for rectum, respectively. The median (Q1, Q3; maximum) Gy EQD2 differences of total D 2cc between DIR-based and SS methods for the bladder and rectum were reduced by -1.53 (-0.86, -2.98; -9.17) and -1.38 (-0.80, -2.14; -7.11), respectively. The mean +/- SD of DICE, Jaccard, HD, and MDA for contour matching were 0.98 +/- 0.008, 0.97 +/- 0.01, 2.00 +/- 0.70 and 0.20 +/- 0.04, respectively for large deformation. Maximum uncertainty of dose mapping was about 3.58%. Conclusions: The hybrid-based DIR algorithm demonstrated low registration uncertainty for both contour matching and dose mapping. The DA difference between DIR-based and SS approaches was statistically significant for both bladder and rectum and hybrid-based DIR showed potential to assess DA between brachytherapy fractions.