His major field of scientific interest are gastrointestinal cancers, and here specifically modern therapeutic approaches in biomarker-driven decision processes, and multimodal treatment strategies, also including other specialities. Here, he has served as chairman of Colorectal Cancer Study Group of the German AIO, a major collaborative group in clinical oncology. Within the AIO framework, he served as founding member of the Central Clinical Trials Office, underlining his expertise also in methodology of clinical research. Furthermore, his interest is the organisation of larger collaborative research structures and of integrated cancer centre networks.
Prof. Arnold is also member of the American Society of Clinical Oncology and Task Force Member of the National Cancer Institute, as well as Board member of the EORTC GI study group.
Within ESMO, he is the Chair of the Compliance Committee serving also as a member of the ESMO Council. He is an Associate Editor for Annals of Oncology, member of the ESMO Open Editorial Board and member of the ESMO Gastro-Intestinal Tumours Faculty Group.