ESTRO 36 Abstract Book

S147 ESTRO 36 _______________________________________________________________________________________________

algorithms. This work investigates the feasibility of a- priori estimation and correction of OOPM. Material and Methods Data from a thoraco-abdominal numeric MRI phantom developed in-house were used 2 . A 10-phases 4DMRI, simulating the planning dataset, was registered to the exhale volume using 3D optical flow 3 , thus measuring in- plane motion (IPM 3D P ) and OOPM P along the three orthogonal slices intersecting in the GTV. In addition, IPM 2D P was obtained with 2D slice-to-slice optical flow 3 registration and the difference C = IPM 3D P − IPM 2D P represented the phase-specific a-priori correction. A 36-frames volume sequence (duration 5.4s) represented treatment data: sagittal/coronal/axial slices simulated cine-MRI sequences, whereas 3D volumes served as ground-truth. The diaphragm position measured on each sagittal slice was used to identify the corresponding breathing phase within the 4DMRI. Each axial and coronal slice of the sequence was registered to the corresponding exhale slices of the 4DMRI (IPM 2D T ) and the phase-specific correction was applied (IPM COR T = IPM 2D T + C). The average end-point distances (EPD) against ground-truth IPM (obtained through 3D registration) were measured with and without correction. OOPM was estimated for each frame as OOPM P measured in the corresponding 4DMRI phase. Finally, the planning GTV was propagated from the 4DMRI exhale phase to each treatment frame using: (1) IPM 2D T with OOPM = 0 and (2) IPM COR T combined with OOPM P . Dice indexes against ground-truth GTVs were calculated for both scenarios. The sagittal slice, showing OOPM < 1 mm, was excluded from the analysis. Results GTV motion amplitude was (4.0, 1.7, 0.2) mm (SI, AP, LR) in the 4DMRI and (5.1, 1.2, 0.6) mm in treatment data. Fig.1 reports EPDs and Dice indexes as a function of the ground-truth OOPM. On average, the a-priori correction/estimation approach resulted in EPD reduction and in Dice index increase with respect to the scenario without IPM correction and OOPM estimation (Tab.1).

Conclusion LN-based target volumes on MRI are considerably smaller than axillary levels, conventionally delineated on CT according to the ESTRO guidelines. Addition of dedicated MRI in regional RT planning leads to reduced OAR dose, and a potential reduced RT-associated toxicity for breast cancer patients . In the near future, this will be investigated for more patients, and these results will be available at ESTRO 36. Moreover, MR imaging of lymph vessels is being investigated. Introduction of MRI-guided regional RT, by direct visualization and delineation of individual LNs and OARs, and future use on the MRL, may reduce RT-induced toxicity. PV-0282 Out-of-plane motion correction in orthogonal cine-MRI registration M. Seregni 1 , C. Paganelli 1 , J. Kipritidis 2 , G. Baroni 1,3 , M. Riboldi 1 1 Politecnico di Milano University, Dipartimento di Elettronica- Informazione e Bioingegneria, Milano, Italy 2 University of Sydney, Radiation Physics Laboratory- Sydney Medical School, Sydney, Australia 3 Centro Nazionale di Adroterapia Oncologica, Bioengineering Unit, Pavia, Italy Purpose or Objective Online motion monitoring in MRI-guided treatments currently relies on the acquisition of 2D cine-MRI images that are registered to the planning anatomy 1 . However, out-of-plane motion (OOPM) cannot be measured and it could affect the accuracy of 2D-2D registration