ESTRO 2021 Abstract Book

S101

ESTRO 2021

Conclusion ZiFix TM was the most effective AC solution. It reduced motion without detriment to MRL image quality or registration accuracy and it was well tolerated in this non-patient volunteer population. For some, it reduced abdominal motion below the detectability threshold of the RGSC system. Results support this workflow as a solution to enable non-gated pancreas MRIgRT. Further review in patient volunteers is planned.

Poster highlights: Poster Highlights 6: CNS 1

PH-0165 Differentiation of Pseudoprogression vs. True Progressive Disease using Contrast Clearance Analysis R. Bodensohn 1 , R. Forbrig 2 , S. Lietke 3 , J. Reis 2 , A. Boulesteix 4 , U. Mansmann 4 , I. Hadi 1 , D.F. Fleischmann 1 , K.H.J. von Henning-Yoo 1 , J. Mücke 5 , A. Holzgreve 6 , N.L. Albert 6 , V. Ruf 7 , M. Dorostkar 7 , S. Corradini 1 , C. Belka 1 , N. Thon 3 , M. Niyazi 1 1 University Hospital LMU Munich, Department of Radiation Oncology, Munich, Germany; 2 University Hospital LMU Munich, Institute of Neuroradiology, Munich, Germany; 3 University Hospital LMU Munich, Department of Neurosurgery, Munich, Germany; 4 Faculty of Medicine LMU Munich, Institute for medical information- processing, biometry and epidemiology, Munich, Germany; 5 Department of Radiation Oncology, University Hospital LMU Munich, Munich, Germany; 6 University Hospital LMU Munich, Department of Nuclear Medicine, Munich, Germany; 7 University Hospital LMU Munich, Center for Neuropathology and Prion Research, Munich, Germany Purpose or Objective After cranial radiotherapy pseudoprogression (PsP) may frequently occur displaying a similar imaging pattern as progressive disease (PD) with contrast enhancement and perifocal edema. These equivocal findings make it challenging even for experienced radiologists to differentiate between both disease states. Contrast clearance analysis (CCA) could provide additional and important information by analyzing the contrast media (CM) washout; in theory, tumor tissue shows a higher CM washout than reactive tissue. This study evaluates the accuracy off CCA in distinguishing PsP from PD and tests its potential as a diagnostic tool. Materials and Methods For this prospective study 33 patients were to be included, who received cranial radiotherapy and required a stereotactic biopsy for further differentiation of an unclear progression on follow-up MRI. The planning-MRI with CM for the biopsy was complemented with the late phase T1-sequence one hour after the regular MRI T1- sequence with CM; for CCA both sequences are coregistered and a differential map is calculated by subtracting both imaging studies. Two blinded experienced neuroradiologists viewed the CCA independently and rated the progression between “real” tumor progression and PsP: The radiological assessment was then compared with the pathological results from the biopsy, and its accuracy calculated statistically. Results The aimed number of 33 was reached; 16 (48.5%) patients were treated on a primary brain tumor, and 17 (51.1%) on a secondary brain tumor. 17 lesions (51.5%) were diagnosed by CCA as PD and 15 (45.5%) as PsP; for one patient with a non-contrast-enhancing low-grade glioma CCA did not deliver any output and therefore was not analyzable. CCA showed an accuracy in predicting the histological result of 0.844 (95%-CI 0.672-0.947) with a nearly significant p-value of 0.051 for N=32. If the one case, which was not analyzable by the CCA, had been predicted correctly, the accuracy would be 0.848 (95%-CI 0.681-0.949) and the p-value 0.041. Sensitivity and specificity of CCA was 0.929 (95%-CI 0.661-0.998) and 0.778 (95%-CI 0.524-0.936), respectively. 25 patients received due to regular diagnostics additionally a FET-PET. The accuracy hereby was 0.75 (95%-CI 0.533-0.902), the study was not powered for this examination. Conclusion Despite of slightly missing the threshold of statistical significance, CCA was highly accurate in this cohort, and could prove to be an easy and helpful method for distinguishing PsP from PD after radiotherapy. It seems comparable in accuracy to the FET-PET.