ESTRO 2024 - Abstract Book

S3463

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

ESTRO 2024

1 HU La Paz, Medical Physics, Madrid, Spain. 2 HU La Paz, Radiation Oncology, Madrid, Spain

Purpose/Objective:

With an Elekta Unity MR-linac it is possible to perform adaptive radiotherapy treatments for each treatment fraction. Prior to radiation delivery, a magnetic resonance image (MRI) is acquired, and the dose distribution is recalculated with the Treatment Planning System (TPS) based on the dosimetric calculation performed in the initial calculation or another calculation prior to the current session. For each adapted radiotherapy session, it is possible to verify that the dosimetric criteria are met, and the TPS displays the total dose-volume histogram (DVH) for the entire treatment and this particular dose calculation, regardless of the dose distributions obtained in the previous sessions. Thus, it is not possible to obtain a DVH that takes into account the actual sum of all the sessions delivered, and therefore to know the final dose received by the PTV or OARs. The objective of this work is to obtain a total DVH that takes into account the sum of the doses delivered by all the adapted radiotherapy sessions using deformable image registration (DIR) and obtain a DVH close to what was treated. A prostate case was planned with Elekta Monaco TPS (v. 5.51.11) with a prescription of 36.25 Gy in 5 fractions. The patient was previously simulated in the Unity linac and the initial treatment plan was calculated. Each fraction was then adapted from the initial plan using the adapt-to-shape (ATS) option, which creates the adaptive plan based on the day's anatomy and optimizes the plan on the day's image by optimizing segment weights and shapes. Before each fraction delivery, dosimetric criteria for PTV and OARS coverage were verified. The total DVH was also compared to that of the reference plan to ensure that there are no significant discrepancies. At the end of the treatment, the five fractions were exported to Varian Velocity tm software. Velocity tm performs DIR, which creates a theoretical deformation matrix by deforming voxels from one image set to match the anatomy of another fixed image, and aligns the patient's anatomy by simulating real-world displacement and volume change. Velocity also includes tools to create composite dose maps of multiple fractions, which allows the total dose deposition of the different adapted fractions to be known and summed up to give a final total DVH that is closer to what was treated than the DVH shown by the TPS. The final DVH obtained with Velocity tm was compared to the DVHs shown by the TPS and the differences were evaluated. Material/Methods:

Results:

The figure shows the DVH corresponding to the original plan and the one obtained using DIR, adding the doses of each session. It can be seen that there are visible differences between the two DVHs, especially in the low medium dose range. The differences in DVH are greater for the bladder and femoral heads than for the rectum.