Dr. Daniel den Hoed in his office in the Netherlands Cancer Institute (NKI). Photo: Museum Boerhave, Leiden. On the table on the left: eggs of the fruit fly are cultured for use in radiobiological experiments.
In 1922, Dr. Daniel den Hoed is taken on as clinical assistant and laboratory co-worker. With his knowledge and liking for physics, he becomes interested in radiology and the X-ray treatments. When Dr. Valken announces that he is leaving, the head of clinic Dr. Wassink decides to release Den Hoed to allow him to specialise and succeed Valken as head radiologist. Den Hoed fulfils a training period in X-ray diagnostics with Dr. Keijzer in Groningen and in the cancer institute in Stockholm with Professor Forsell.
From 1 January 1925, he is head of the radiotherapy department in the NKI. Next to his work as radiologist and experimental radiobiologist and physicist, he continues to assist at operations. As no other radiologist of his time, he becomes acquainted with the pathology of cancer and the possibilities of surgical treatment .