Abstract Book

S91

ESTRO 37

OC-0174 Deformable image registration for dose accumulation of adaptive EBRT and BT in cervical cancer L.E. Van Heerden 1 , J. Visser 1 , C. Koedooder 1 , C.R.N. Rasch 1 , B.R. Pieters 1 , A. Bel 1 1 Academic Medical Center, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Radiotherapy for cervical cancer patients consists of external beam radiation treatment (EBRT) and a brachytherapy (BT) boost to the target. To calculate the dose to the most irradiated 2cm 3 of bladder and rectum (D 2cm3 ) from EBRT and BT, the ICRU recommends to assume that the EBRT dose is uniform near the BT boost. At the time of BT planning, it may be necessary to accumulate the delivered EBRT dose and sum it with the planned BT dose distribution using deformable image registration (DIR) to account for daily positioning variability and variation in organ filling, as well as the presence of the applicator. The purpose of this study was to investigate for bladder and rectum if the delivered EBRT dose, estimated from cone-beam computed tomography (CBCT) data, can be considered uniform near the planned BT boost, or if DIR is necessary for the calculation of D 2cm3 . Material and Methods Ten patients were studied, treated with adaptive EBRT of 46/46.2Gy EQD2 in 23/28 fractions using a plan-of-the-day approach, and an MRI-guided PDR BT boost (24 x 1.0Gy, given hourly). BT planning (Fig. 1, Left panel) was performed in Oncentra®Brachy (Elekta Brachytherapy, Veenendaal, the Netherlands). EBRT was planned with volumetric modulated arc therapy (VMAT) for nine patients, and with intensity modulated radiation therapy (IMRT) for one patient. Bladder and rectum were delineated on all CBCT scans and on the BT MRI. The delivered EBRT dose for each fraction was estimated using the daily anatomy from the CBCT scan acquired prior to irradiation. CT hounsfield units were mapped to CBCT images by registering the planning CT to CBCT images using intensity-guided DIR in VelocityAI. The daily plan was used to calculate the corresponding daily dose distribution. For dose accumulation, DIR was performed using structure-guided registration for bladder and rectum separately. For all organ-specific matches, the deformation vector fields (DVFs) obtained with CBCT-to- CBCT DIR were used to map all EBRT fraction doses to the same (CBCT of the first fraction) frame of reference (Fig. 1, Middle panel) to obtain the delivered EBRT dose. Next, the CBCT-to-MRI DVF was used to map the EBRT dose distribution to the BT frame of reference. Finally, the propagated EBRT/BT doses were summed to create the accumulated dose (Fig. 1 right panel). D 2cm3 was calculated from the accumulated dose distributions for both rectum and bladder. For the uniform dose (UD) method, the cumulative D 2cm3 was calculated by adding the EBRT prescription dose to the BT D 2cm3 .

1.0 – 3.7Gy EQD2 ) for the rectum (Fig. 2), meaning that the dose calculated with DIR was at most 104% of the dose calculated with the UD method.

Conclusion Differences between UD and DIR were small (<3.9Gy EQD2 ). The VMAT/IMRT dose delivered with adaptive EBRT to bladder and rectum near the BT boost can clinically be considered uniform. OC-0175 The BiRT project – linking radiomics to advanced approaches for treatment of prostate cancer A. Haworth 1 , Y. Sun 2 , H. Reynolds 2,3 , D. Wraith 4 , C. Mitchell 5 , D. Murphy 2,6 , B. Parameswaran 7 , M. DiFranco 8 , M. Ebert 9,10 , S. Williams 2,11 1 The University of Sydney, School of Physics, Sydney, Australia 2 University of Melbourne, Sir Peter MacCallum Dept. of Oncology, Melbourne, Australia 3 Peter MacCallum Cancer Centre, Physical Sciences, Melbourne, Australia 4 Queensland University of Technology, Institute of Health and Biomedical Innovation, Brisbane, Australia 5 Peter MacCallum Cancer Centre, Dept. of Pathology, Melbourne, Australia 6 Peter MacCallum Cancer Centre, Dept. Surgery, Melbourne, Australia 7 Peter MacCallum Cancer Centre, Cancer Imaging, Melbourne, Australia 8 Medical University of Vienna, Centre for Medical Physics and Biomedical Engineering, Vienna, Austria 9 University of Western Australia, School of Physics, Nedlands, Australia 10 Sir Charles Gairdner Hospital, Department of Radiation Oncology, Nedlands, Australia 11 Peter MacCallum Cancer Centre, Radiation Oncology, Melbourne, Australia Purpose or Objective Widespread adoption of focal approaches to treatment of prostate cancer using brachytherapy and/or external beam radiotherapy have been hampered by uncertainties in methods to define target volumes and the optimal prescribed dose. To address this, we propose a biological

Results The range in difference (DIR-UD) for D 2cm3 wa

s -0.2 –

) for the bladder and -1.9 – 3.7% (-

3.7% (-0.2 – 3.9Gy EQD2