Radioisotopes produced gamma rays in the megavolt range .
Prior to World War II virtually the only radioisotope available for radiotherapy was naturally occurring radium ( producing 1 - 2 MeV gamma rays ) , which was extremely expensive due to its low occurrence in ores .
Radioisotopes produced gamma rays in the megavolt range , but prior to World War II virtually the only radioisotope available for radiotherapy was naturally occurring radium ( producing 1 - 2 MeV gamma rays ) , which was extremely expensive due to its low occurrence in ores .