References to personal communication are placed in running text rather than as formal end references. Permission is usually required and should be acknowledged in an “Acknowledgment” or “Notes” section at the end of the document. (Scientific Style and Format, 8th ed.)
Example:
. . . and most of these meningiomas proved to be inoperable (2003 letter from RS Grant to me; unreferenced, see “Notes”) while a few were not.