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 Gy(EQD2), EBRT + BT: D(90) 85-90 Gy(EQD2), 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 cm(3) with the highest dose (D(2cm3)) 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 Gy(EQD2) (bladder) and –1.0-3.7 Gy(EQD2) (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 Gy(EQD2)). 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 D(2cm3).