ESTRO 2024 - Abstract Book

S3475

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

the translation and rotation values of the registration matrix of each registration between the pre- and post treatment MRI. Finally, the dose to the CTV and GTV of each fraction was estimated by projecting the planned dose distribution to the post-treatment MRI and compared with the planned dose on the pre-treatment MRI.

Results:

115 treatment fractions were analyzed. The mean time interval between the pre- and post-treatment MRI was 17 min (14 – 27 min). Mean CTV translations were 0.0 mm (± 0.9 mm) in the left-right (LR) direction, -1.1 mm (± 1.4 mm) in the anterior-posterior (AP) direction and -0.9 mm (± 1.2 mm) in the cranial-caudal (CC) direction. The CTV showed mean rotations of -0.1 ° (± 1.9 °), 0.0 ° (± 0.4 °) and 0.0 ° (± 0.5 °) in the LR, AP and CC direction, respectively. The mean planned D98% to the CTV and GTV was 30.7 Gy (30.1 – 31.5 Gy) and 44.1 Gy (41.1 – 47.4 Gy) compared to a mean delivered D98% of 30.1 Gy (22.7 – 31.6 Gy) and 43.0 Gy (30.8 – 47.6 Gy) based on the post-treatment MRI, respectively. Sub-analysis of fractions with translations that did not exceed 2 mm in any direction (n = 90), showed a mean delivered D98% of 30.6 Gy (29.0 – 31.6 Gy) to the CTV and 44.0 Gy(40.0 – 47.6 Gy) to the GTV (Figure 2).

Conclusion:

In case of using a 0 mm PTV margin the delivered dose to the CTV and GTV will be reduced compared to the planned dose by intra-fraction motion. However, even with a PTV margin of 0 mm, the delivered dose to the CTV was >29 Gy in the majority of fractions. Although further analysis based on accumulated dose of all fractions is