documentclass{scrlttr2}

% predefined variables
setkomavar{fromname}{Dr. House, MD}
setkomavar{fromaddress}{%
   Sesamestreet 5532\
   Elsewhere
}
setkomavar{subject}{Next Examination}

% own vars
newkomavar{nextdate}
setkomavar{nextdate}{5th of September 2015}
newkomavar{bodypart}
setkomavar{bodypart}{left Arm}

begin{document}
begin{letter}{%
   Jon Doe\
   Somestreet 1234\
   Somewhere
}
   opening{Dear Mr Doe,}

   your next examination will be on usekomavar{nextdate}
   and we will check your usekomavar{bodypart}.

   closing{Regards}
end{letter}
end{document}