In the AEC Inspector General's 1974 investigation of the
plutonium story, Dr. Shields Warren recalled a December 1947
discussion with Dr. Hamilton, in which the latter indicated that "he
had explained to the patients that they would receive an injection of
a new substance that was too new to say what it might do but that it
had some properties like those of other substances that had been used
to help control growth processes in patients, or something of that
general sort." [Patricia Durbin, Hamilton's associate, believes the
"other" substance referred to was strontium, which was administered
to eight cancer patients at UCSF between September 1941 and October
1944 as a palliative.] Hamilton, according to Warren, also noted at
this time that there "were very rigid restrictions on the use of the
word plutonium and the handling of the material and letting anyone
know that there was any such stuff."

http://www.gwu.edu/~nsarchiv/radiation/dir/mstreet/commeet/meet7/brief7/tab_g/br7g2.txt