ESTRO 2024 - Abstract Book

S4341

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

weighted MRI recorded as part of the online adaptive workflow. Hybrid deformable image registrations (DIR) (RayStation v2023-R, RaySearch Laboratories AB, Stockholm, Sweden) were performed between the fraction MRIs and the pre-treatment MRI using the CTV, bladder, and rectum as DIR controlling contours. Hence, the clinical dose distributions – created in the Monaco treatment planning system (TPS) (v5.51.11, Elekta AB, Stockholm, Sweden) – were mapped onto the pre-treatment MRI, accumulated on a voxel-by-voxel basis, and compared to the pre treatment planned dose. In addition, we developed a pre-treatment and adaptive planning optimization strategy for the MR-linac using RayStation. As part of the treatment planning procedure we commissioned a beam model accounting for the 1.5 T external magnetic field. In Monaco, the dose was calculated on the MRIs with density overrides of the external and pelvic bones contours with CT-based average mass density values per patient. In RayStation, the simulation CT was mapped onto the MRI for dose calculations. The intensity modulated radiation therapy (IMRT) beam setup in RayStation was copied from the clinical plan accounting for the cryostat pipe and high density materials within the treatment couch. For each pre-treatment plan, the maximum number of segments was set to 70 within both TPSs. The novel pre-treatment planning strategy was compared to the clinical plans. The clinical adaptive planning approach was based on the adapt-to-shape and adapt-to-position procedure within the MR-linac system. [2] In RayStation, a warm restart of the pre-treatment plan using 4 x 40 iterations was used for adaptive planning. The estimated D Acc of the adaptive plans in both TPSs were compared. For all patients, the V95% of the CTV was >99.0% in the pre-treatment dose (D PT ) and the D Acc in both TPSs. The target conformity index (95% of prescription dose) of the clinical Monaco plan was, on average (range), 0.84 (0.81 – 0.90) for the D PT and 0.90 (0.86 – 0.94) for D Acc . Figure 1 depicts the mean dose (D mean ) of the OAR related evaluation structures of the clinical pre-treatment plan and the Monaco and RayStation D Acc . For one patient, the anal canal mean dose (D mean ) increased with 8.4 Gy in D Acc . For the rectum D mean of three patients, a difference of > 1 Gy was observed between D PT and D Acc . Target conformity was similar between the TPSs. For all plans and all analyzed OAR, the D mean was lower in the pre treatment plans optimized in RayStation (range: 0.2 – 6.8 Gy). Similarly, the accumulated adaptive doses optimized in RayStation were lower for all plans (range: 0.1 – 5.7 Gy) as well. This may be explained by the degree of treatment plan modulation, in which the Monaco pre-treatment plans contained 51 (45 – 57) segments, whereas RayStation used 69 (68 – 70) segments. Results: