ESTRO 2022 - Abstract Book

S788

Abstract book

ESTRO 2022

An ATS plan optimised from fluence was used as a gold standard against which library plans were compared. Gamma analysis ( γ dose , 3%/3mm) of library plans against the gold standard was performed. This measure was used to test for correlation with similarity to determine whether a given metric may be suitable. Finally, dose statistics were compared for the resulting library plans. Results For the two available cervix patients, a positive correlation was found between gamma pass rates and image similarity using mutual information in the PTV and surrounding 3cm where r=0.68 and r=0.71 respectively (see Figure 1). Across all 6 patients, 69% of library plans gave a closer agreement than an ATP plan (72% for cervix). In prostate patients correlation with image similarity was not seen. This is thought to be due to lower inter-fraction anatomy change in these patients. Higher similarity library plans used for the cervix patients often gave dose statistics in closer agreement with the reference plan. However, there was variation within this as shown in Figure 2.

Conclusion For patients with cervix cancer image similarity could provide a method for automatic selection from library plans accumulated on the MR-Linac. This saves a significant amount of time between scanning and delivering treatment. However, correlation of plan quality with similarity may be improved by refinement of methods. This work will be validated on a larger cohort as part of ongoing work.

PD-0896 Personalized MRI contrast for treatment guidance on an MRI-linac in patients with liver metastases

R. Navest 1 , V. van Pelt 1 , T. van de Lindt 1 , T. Janssen 1 , M. Nowee 1 , J. Sonke 1 , U. van der Heide 1 , P. van Houdt 1

1 The Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands

Purpose or Objective MRI-guided stereotactic body radiation therapy (SBRT) is an upcoming local treatment for liver metastases. The visibility of liver metastases during MRI-guided SBRT, however, is unsatisfactory in approximately 30% of patients [1]. Consequently, set-up correction and motion monitoring during treatment are challenging. We therefore propose to personalize the 4D- MRI sequence settings based on patient specific T1 and T2 values of the lesion(s) and surrounding liver tissue. Materials and Methods 23 patients with up to 3 liver metastases received 4D-MRI guided liver SBRT on a 1.5 T Unity MRI-linac (Elekta AB, Stockholm, Sweden). A 4D T2 TSE or 4D balanced GRE was used for set-up correction and motion monitoring. Additionally, before the first treatment fraction a sagittal T1 map (variable flip angle series) and transversal T2 map were acquired for all patients during free breathing. The mean and standard deviation of T1 and T2 were calculated for ROIs within liver metastases and healthy liver tissue (excluding large vessels). These T1 and T2 values were used to determine the MRI sequence settings that yield optimal contrast between the lesion and surrounding liver tissue, through Bloch simulations. Contrast was defined as the relative difference of the mean MR signal intensity of the lesion ROI with respect to the liver ROI (i.e. C = (SI lesion – SI liver ) / SI liver * 100%). Optimal values of the echo time, refocusing angle and parallel imaging (PI) factor for the 4D T2 TSE and number of startup echoes, flip-angle, PI factor and partial Fourier factor for the 4D balanced GRE were calculated. The optimal 4D-MRI