Radiation therapy has been in use as a cancer treatment for more than 100 years, with its earliest roots traced from the discovery of X rays in 1895 by Wilhelm Rontgen. The field of radiation therapy began to grow in the early 1900s, largely due to the groundbreaking work of Nobel Prize-winning scientist Marie Curie, who discovered the radioactive elements polonium and radium.This began a new era in medical treatment and research. Radium was used in various forms until the mid-1900s, when cobalt and caesium units came into use. Medical cobalt units and linear accelerators have been used to as sources of radiation since the late 1940s. Swedish doctors showed a great interest in this medical specialty from the beginning, making major contributions in the fields of radiobiology, radiophysics and radiotherapy are contributed to doctors of Swedish origin, working mainly those early days in Stockholm. Immediately after the discovery of X rays, the first treatment of patients with these'mysterious rays' took place, with two patients with skin carcinomas being treated by Stenbeck and Sjogren in Stockholm. This article makes a detailed reference to historical data regarding the gradual transformation of a small private outpatient clinic into an academic department with a world-wide recognition.