ESTRO 2020 Abstract Book

S925 ESTRO 2020

oligometastatic disease with local aggressive treatment. 1,2 Patients with oligometastatic disease present a heterogeneous group with different tumor types and different tumor biology. Stereotactic body radiotherapy (SBRT) on MR-guided radiotherapy systems (MR-linac) is an upcoming approach to treat oligometastatic disease 3 . The possibility for daily functional imaging on an MR-linac provides a unique opportunity to gain more insight in the differences in tumor biology between patients. The purpose of this study was to assess the feasibility of using diffusion-weighted imaging (DWI) as an imaging biomarker in patients with oligometastatic disease and to study changes in DWI parameters during treatment. Material and Methods Five patients were treated with 5 fractions of 7 – 10 Gy on a 1.5T MR-linac (Unity, Elekta AB, Stockholm, Sweden). Patients received additional imaging as part of a prospective trial for which they signed informed consent. After each fraction a DWI scan was made with b-values of 0, 30, 150, and 500 s/mm 2 . We rigidly registered the DWI b = 0 s/mm 2 images to the T2-weighted image to correct for movement between the scans and applied this to the other b-values as well. The apparent diffusion coefficient (ADC) was calculated from the high b-values images (150 and 500 s/mm 2 ). The GTV was delineated on the T2- weighted image of each fraction. The size of the GTV was calculated from the number of voxels within the delineation. The GTV contour was propagated to the ADC map. The mean and standard deviation of ADC values within the GTV were obtained for each fraction and compared over time. Results Three patients were treated with SBRT for lymph node oligo-recurrences from prostate cancer, one patient for a lymph node oligo-recurrence of a renal cell carcinoma, and a stage IV melanoma patient was treated for an oligoprogressive lymph node. The volume of the GTV was different between patients, ranging from 0.03 cm 3 to 58 cm 3 . An example of the delineation on the T2-weighted image, DWI images and ADC map is shown in Fig. 1 for the first and last fraction. This example illustrates that the GTV was well visible on the DWI image with a b-value of 500 s/mm 2 , which was the case for all patients. The tumor volume decreased from 0.8 to 0.4 cm 3 in this patient (Fig 2), which coincided with an increase in ADC value (Fig. 2). In the other patients, the tumor volume and ADC did not change during treatment.

Conclusion This study shows the potential of DWI as an imaging biomarker in different types of oligometastatic disease treated with SBRT on an MR-linac. References 1. Palma et al. Lancet 2019; 393: 20151-2058. 2. Gomez et al. J Clin Oncol 2019; 37: 1558-1565. 3. Winkel et al. Clin Transl Radiat Oncol 2019; 18: 46-53. PO-1682 MR-based adaptive IGRT for prostate cancer: Results of an exploratory cohort on DWI A. Othman 1 , D. Wegener 2 , D. Zips 2,3 , F. Paulsen 2 , C. De Colle 2 , D. Thorwarth 3,4 , J. Bedke 5 , A. Stenzl 5 , S. Afat 1 , J. Weiss 1 , M. Notohamiprodjo 1 , K. Nikolaou 1 , A. Müller 2,3 1 Eberhard Karls University Tübingen, Department of Radiology, Tübingen, Germany ; 2 Eberhard Karls University Tübingen, Department of Radiation Oncology, Tübingen, Germany ; 3 German Cancer Consortium DKTK- partner site Tübingen, and German Cancer Research Center DKFZ, Heidelberg, Germany ; 4 Eberhard Karls University Tübingen, Section for Biomedical Physics- Department of Radiation Oncology, Tübingen, Germany ; 5 Eberhard Karls University Tübingen, Department of Urology, Tübingen, Germany Purpose or Objective We studied whether multiparametric MRI-based treatment planning and threshold-/constraint-based adaption of the radiation treatment plan using a weekly MRI could reduce G2+ gastrointestinal (GI) and genitourinary (GU) side effects within a prospective phase II study (M-base Pro 1.0 study). In this analysis, change of diffusion weighted imaging (DWI) in the dominant region is evaluated for patients of the exploratory cohort. Within this prospective study it is aimed to develop potential biomarker for treatment response on the way to biologically-adapted IGRT. Material and Methods Patients with MRI-staged PC (cT1-3b N0 M0) were included in this prospective study. (ClinicalTrials.gov Identifier: NCT02724670). Treatment consisted of 78 Gy (39 frx.) to prostate and seminal vesicles (low/intermediate/high risk or cT3b: 0/1/2cm or complete SV). Neoadjuvant androgen deprivation therapy (neoADT) was given according to national guidelines. Diagnostic 3T MRI was performed 4 time points: at diagnosis, after neoadjuvant ADT (at time of treatment planning), at week 2 and 7 of IGRT in the exploratory cohort (n=25). ADC-level (apparent diffusion coefficient) was evaluated at a standard image postprocessing software (Syngovia, Siemens Healthineers). Results Twenty-five patients were evaluated as participants of the exploratory cohort. Mean (range) ADC-level in the dominant lesion at baseline reached 710·10 -6 mm 2 /s (446- 1075 mm 2 /s). Diffusion increased after 2-3 months of neoADT to a mean ADC of 830 ·10 -6 mm 2 /s (483-1301 mm 2 /s). Radiation treatment was associated with a further