ESTRO 37 Abstract book

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ESTRO 37

Results Validation of the functionality of the delivery system, the stability of the connection between the source guide and the shielded catheters, and the resistance to wear of the plastic needle during use has been completed. The platinum shield can reduce the dose on the shielded side by 75 % compared to the dose on the unshielded side at a radial distance of 1 cm from the source. For the same minimum dose to the hottest 90 % of the planning target volume (PTV), the IMBT plan resulted in a reduction (expressed as percentage of prescription dose) in the bladder D 2cc , rectum D 2cc , and urethral D 10 by 13%, 21%, and 10%, respectively, while improving the homogeneity within the target (PTV V 150 reduced by 25 %). Conclusion For many cancer sites the optimal dose cannot be delivered to the tumor due to normal tissue toxicity. IMBT enables dose escalation in the tumor volume while protecting OARs. Possibility to increase the dose in the target and reduce the dose spillage to OARs has the potential to improve brachytherapy treatment outcomes significantly. In addition, it will broaden the use of brachytherapy as a treatment modality on cancer sites.

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 .

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

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,

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.