Clinical implementation of MRI-only radiotherapy treatment workflow for prostate cancer with a standard linac
Tesi di Dottorato
Data di Pubblicazione:
2020
Citazione:
Clinical implementation of MRI-only radiotherapy treatment workflow for prostate cancer with a standard linac / M.M.J. Felisi. - (2020 Nov 12).
Abstract:
BACKGROUND AND AIM
The purpose of this work is the clinical implementation of radiotherapy treatment workflow for prostate cancer based only on MRI images (MRI-only) with a standard linac, using the technological tools available in a big general hospital. In the recent years the growing interest in using solely MRI images, due to the excellent soft tissue contrast that makes it unique, both for identification and precise delineation of target volumes and organs at risk (OAR), emerged strongly. Especially in those regions characterized by soft tissues, as the pelvis site, the method has several benefits, including improvement of inter-observer robustness in target delineation, a reduction in contoured structures which potentially leads to smaller irradiated volumes and a major efficiency of the workflow by both reducing the time and costs of patients imaging and removing residual registration error compared to a CT/MR workflow. However, there are several challenges in the implementation and application of MRI-only workflow; the most important one is the Electron Densities (ED) estimation. In fact, unlike CT images, where the voxels intensities are directly linked to the physical properties of the tissues ED, required by the Treatment Planning System (TPS) for dose calculation, MRI signals do not depend on ED but correlate with tissue proton density and thus cannot be directly used. Therefore, it is mandatory to convert MRI data to ED maps to allow the dose distribution even in this case. These generated maps are known as synthetic CT (sCT). Several aspects of the implementation have been examined in this thesis in order to validate the overall workflow.
MATERIAL AND METHODS
Imaging acquisition was performed using a 3D T1 VIBE Dixon gradient echo sequence that is a good compromise between image quality and acquisition time; it takes 2 minutes and 30 seconds and provides 4 contrasts (Water, Fat, In-phase and Out-of-phase) useful for the visualization of target and different OARs. The acquisition on the MR scanner was possible thanks to the use of a home-made table-top, commercial MR safe markers and properly controlled internal laser system; furthermore, the geometric distortions in obtained images were evaluated. A hybrid method was adopted to generate sCT from MRI; it consists in a combination of bulk ED and multi atlas-based approach. For this purpose, the MR images of 20 volunteers and patients undergoing diagnostic acquisitions were collected, manually contoured in bones and main OARs, and then inserted, after verification by a radiation oncologist, as reference in the database of ADMIRE software (research version 1.13.5, Elekta AB, Stockholm, Sweden), utilized for automatic contouring process. The software approximates the anatomy contours by comparing several individual atlases, applying elements of maximum likelihood forms to a new patient image-set, and creates a structure set to fit the actual patient’s anatomy. Therefore, sCTs were finally generated in the TPS by assigning to each structure bulk ED values, calculated averaging over 20 patients who previously underwent prostate radiotherapy. Once the sCT was generated, different evaluations were made using quantitative methods such as dose-volume histogram (DVH) and 2D local gamma analysis, in order to test the dose differences between the sCT and the gold standard CT, both in terms of homogeneous EDs assignment and contours delineated on the MR images. At first, 20 VMAT prostate treatment plans, calculated on standard CT images with 10 MV X-rays, were recalculated using the same field, segments and monitor unit (MU) arrangements on the sCT, where only fixed EDs were assigned instead of the heterogenous ones. Afterwards, the combination between the assignment of
Tipologia IRIS:
Tesi di specializzazione
Keywords:
Prostate cancer radiotherapy; Magnetic Resonance Imaging; MRI-only
Elenco autori:
M.M.J. Felisi
Link alla scheda completa: