ESTRO 2022 - Abstract Book

S693

Abstract book

ESTRO 2022

Examples of CBCTs, sCTs and a planning CT are shown in figure 1. Calculated metrics are summarized in table 1. The sCTs outperformed their corresponding CBCTs on all metrics, except FWHM ( P<.001 for all metrics). This shows that the NN improves correspondence with the reference CT, but loses some sharpness with respect to the CBCT. The CBCTs with ASG outperformed the CBCTs without, except in FWHM ( P<.05 for all metrics). The sCTs with ASG outperformed the sCTs without, except in FWHM ( P<.05 for all metrics except MAE).

Conclusion The NN in Admire reliably enhanced the CBCT images. We observed some loss of sharpness, which may be explained by the downsampling of the training data. The NN benefitted from the higher quality in CBCTs with ASG, even though it was trained exclusively on data without ASG. We expect, however, that the differences between sCTs with/without ASG may not be dosimetrically relevant.

Proffered Papers: Novel approaches towards IGRT

OC-0775 Dosimetric consistency between MR-only and deformed CT for MR-guided online adaptive radiotherapy

G. Valdes Santurio 1 , J. Edmund 1 , K.I. Nousiainen 2

1 Herlev Hospital, Radiotherapy, Herlev, Denmark; 2 Helsinki University, Helsinki University Hospital, Helsinki, Finland

Purpose or Objective MR-guided online adaptive radiotherapy (MRgOART) has become more frequent in radiotherapy clinics with the inclusion of MR-linacs (MRL). In the workflows for adaptive fractions, an MR scan is first acquired using a dedicated MRL sequence, the structure set re-contoured, and the plan re-optimized and re-calculated. For plan re-calculation, an electron densities (ED) map is assigned from either the deformed or rigid CT simulated before treatment. The ED map extracted from the deformed CT (dCT) is not necessarily consistent with the MR image and thus subject to QA prior the treatment delivery. To assess the dosimetric impact of incorrect ED alignment with the MRI, bulk EDs were manually assigned to the MR images and the plan was re-calculated using those 2 conditions. The bulk ED assignment is typically called MR-only Materials and Methods The 0.35T MRL from ViewRay was used for calculating the plans and acquiring MR images. In this system, a Monte Carlo based treatment planning system uses the ED relative to water (RED) for material assignment and calculating dose-to- medium. 2 bulk ED were manually assigned: “water-like” for all the structures inside the patient (RED=1), and “lung-like” for the lungs (RED=0.12). 15 patients were included with lesions mainly located in the upper-abdominal/thoracic regions such as pancreas and liver. 1 online-adaptive fraction per patient was selected as a representation of the full treatment.

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