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) GyEQD2 differences of total D2cc 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.