Structure-based deformable image registration: Added value for dose accumulation of external beam radiotherapy and brachytherapy in cervical cancer

Structure-based deformable image registration (DIR) can be used to calculate accumulated brachytherapy (BT) and external-beam radiation therapy (EBRT) dose-volume histogram (DVH) parameters in cervical cancer. Since direct parameter addition does not take dose non-uniformity into account, the added value of DIR over addition methods was investigated for bladder and rectum. For twelve patients (EBRT: 46Gy, EBRT+BT: D90 85-90GyEQD2 in equivalent dose in 2Gy fractions) the EBRT planning CT and BT planning MRI were registered using DIR. Affected lymph nodes, located far from the BT boost region, received an EBRT boost (9.2Gy) not contributing to the BT boost dose. Cumulative bladder/rectum D2cm3/D1cm3 were calculated and compared to direct addition methods, assuming uniform EBRT doses (UD), or overlapping high dose volumes (OHD). Between the three methods, the maximum differences in the cumulative DVH parameters were 3.2GyEQD2 (bladder) and 3.3GyEQD2 (rectum). The difference between DIR and UD was <1.8GyEQD2 for both organs. The UD method provides a better estimate of D2cm3/D1cm3 than the OHD method. There is no added value of DIR since differences with direct addition methods are clinically insignificant. EBRT dose distributions can be considered uniform in bladder and rectum for the evaluated dose parameters