Role of deformable image registration for delivered dose accumulation of adaptive external beam radiation therapy and brachytherapy in cervical cancer

Purpose
Deformable image registration (DIR) can be used to accumulate the absorbed dose distribution of daily image-guided adaptive external beam radiation treatment (EBRT) and brachytherapy (BT). Since dose-volume parameter addition assumes a uniform delivered EBRT dose around the planned BT boost, the added value of DIR over direct addition was investigated for dose accumulation in bladder and rectum.

Material and methods
For 10 patients (EBRT 46/46.2 GyEQD2, EBRT + BT: D90 85-90 GyEQD2, in equivalent dose in 2 Gy fractions), the actually delivered dose from adaptive volumetric-modulated arc therapy (VMAT)/intensity-modulated radiotherapy (IMRT) EBRT was calculated using the daily anatomy from the cone-beam computed tomography (CBCT) scans acquired prior to irradiation. The CBCT of the first EBRT fraction and the BT planning MRI were registered using DIR. The cumulative dose to the 2 cm3 with the highest dose (D2cm3) from EBRT and BT to the bladder and rectum was calculated and compared to direct addition assuming a uniform EBRT dose (UD).

Results
Differences (DIR-UD) in the total EBRT + BT dose ranged between –0.2-3.9 GyEQD2 (bladder) and –1.0-3.7 GyEQD2 (rectum). The total EBRT + BT dose calculated with DIR was at most 104% of the dose calculated with the UD method.

Conclusions
Differences between UD and DIR were small (< 3.9 GyEQD2). The dose delivered with adaptive VMAT/IMRT EBRT to bladder and rectum near the planned BT boost can be considered uniform for the evaluation of bladder/rectum D2cm3.