Dosimetric effects of residual, system-related, geometric distortion on MR-only radiotherapy planning for treatment of endometrial cancer using VMAT

Background: Traditionally, radiotherapy (RT) planning is based on CT images, however some tumours are poorly visualised on CT. Due to its superior soft-tissue contrast, MRI is increasingly being utilised as an alternative to CT in RT planning. MR-only RT planning has several advantages including: removal of the spatial uncertainty associated with registering the CT and MRI datasets; saving of time, and resources, required to perform a planning CT scan on each patient; and less dose to the patient. However, there are challenges which need to be addressed to make MR-only RT planning feasible for clinical use. MRI images are known suffer from geometric distortion. Before introducing MR-only RT planning, it is crucial that the effects of geometric distortion are quantified from a dosimetric perspective.

Objectives: The aim of this research is to determine if residual geometric distortions, from MR-only planning, could lead to significant dosimetric differences, specifically for endometrium VMAT treatments. Methods: To quantify the geometric distortion, a clinical MRI scanner will be used to acquire a MRI dataset of a distortion phantom, using a clinically relevant sequence. This distortion dataset will be used to distort retrospective CT datasets for 15 endometrium patients. Treatment plans will be produced on the distorted CTs. These plans will then be calculated on the distorted CTs and the original CTs, to enable a comparison between the two resulting dose distributions. Dose-volume-histogram statistics will be compared, using Bland-Altman analysis (p-value <0.05), to determine if the results are dosimetrically significant.

Results & Conclusions: In progress.