ESTRO 2020 Abstract Book

S924 ESTRO 2020

Material and Methods Currently published literature on DW-MRI of irradiated parotid glands in HNC patients have been included in this overview. The literature search was conducted in PubMed and Web of Science databases and resulted in 12 studies written in English and published from 2001 to 2018. The studies are sorted by their population size, from 8 to 34 patients. Comparisons between the studies were performed using descriptive statistics. Results Published literature shows a wide range of low and high b- values used for calculation of ADC (Figure 1a). Figure 1b shows that most preferred b-values are b-value=0 and b- value=1000. Only one study uses low b-values, while the rest of the studies uses combination of both low and high b-values (2 studies) or only high b-values (9 studies). ROI selection methods were whole-gland ROI (4 studies), selected slice ROI (6 studies) and reader-based circular ROI in selected slices (2 studies). The time point of post treatment DW-MRI examinations ranges from at the same day after end of RT to 270 days after RT (Figure 2). Figure 2 also shows a changes in ADC before (ADC pre) and after (ADC post) RT treatment. Compared with the ADC pre values, the ADC post values increases in almost all studies. In only one study, the decrease in ADC post is reported.

Conclusion Based on weekly MRI scanning during nCRT for esophageal cancer, a considerable linear decrease in tumor volume was observed. Volume regression and consequential anatomical changes suggest the possible benefit of adaptive radiotherapy. PO-1680 DW-MRI of parotid glands in head and neck cancer patients treated with radiotherapy M. Bruvo 1 , R.H. Hansen 2 , F. Mahmood 3 1 University College Copenhagen, Institute of Technology/ Department of Radiography, Copenhagen N, Denmark ; 2 Herlev Hospital, Department of Radiology, Herlev, Denmark ; 3 Odense University Hospital, Department of Oncology, Odense, Denmark Purpose or Objective In recent years, diffusion-weighted imaging (DW-MRI) has been applied to examine microstructural changes in the parotid glands during radiotherapy (RT) with apparent diffusion coefficient (ADC) values. Still, in these studies the ADC values before and after RT, respectively, have varied widely. These differences in ADC values could be due to variability in DW-MRI acquisition settings, for example the choice of b-values. The time point of the acquisition post treatment is another parameter that can influence ADC measurements. The radiation damage of salivary gland is most pronounced in the acute phase (0–10 days) and early phase (10–60 days) after RT, and after that a partial restitution of parotid gland function can appear during the first two years after RT. In addition, region of interest (ROI) selection methods can possibly also influence on ADC measurements. This study presents an overview of published studies where DW-MRI has been used to evaluate the parotid glands in relation to RT. The objective is to report the used b-values, ROI selection methods, and post treatment time points and the corresponding ADC values before and after RT in patients with head and neck cancer (HNC).

Conclusion The limited number of studies about DW-MRI of parotid glands in HNC patients treated with RT, together with a lack of standardization regarding choice of b-values, ROI selection methods and time point of the post treatment, are important limitations for a valid inter-study comparison of ADC values. In conclusion, investigators must pay attention when interpreting the ADC values in DW-MRI of the irradiated parotid glands. PO-1681 Daily diffusion weighted MRI during SBRT on an MR-linac for oligometastatic disease P. Van Houdt 1 , E.S. Kooreman 1 , V.W.J. Van Pelt 1 , U.A. Van der Heide 1 , M.E. Nowee 1 1 the Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Until recently patients with oligometastatic disease have been treated with palliative intent. However, randomized trials have shown a survival benefit to treat