ESTRO 2021 Abstract Book

S148

ESTRO 2021

Conclusion Repeatability analysis of EPI-based DW-MRI acquired on a hybrid MRL in HNC showed promising results and supports further development towards quantitative MRI for online adaptive RT. Further validation of these results including more patients and multi-center comparison is ongoing. Small systematic changes could possibly be attributed to treatment effects. Funding: DFG (ZI 736/2-1, PAK 997/1). PH-0216 Volumetric and dosimetric stability of metabolic tumor volume in dose painting of anal cancer A.M. Acosta Roa 1 , V.E. Skingen 2 , B.L. Rekstad 1 , M.G. Guren 3 , E. Rusten 1 , E. Malinen 2 1 Oslo University Hospital, Department of Medical Physics, Oslo, Norway; 2 University of Oslo, Department of Physics, Oslo, Norway; 3 Oslo University Hospital, Department of Oncology, Oslo, Norway Purpose or Objective The primary aim of the study is to evaluate if the metabolic tumor volume (MTV) in anal cancers is stable during fractionated radiotherapy and how volumetric variations translate into dosimetric variations following dose painting of MTV based on pre-therapy PET/CT. Secondary aims are to assess doses to organs at risk (OARs) and use of adaptive dose painting. Materials and Methods This study included 11 patients with squamous cell carcinoma of the anal canal who received radical