The two photon exchange amplitude in $ep$ and $e\mu $ elastic   scattering: a comparison
  In this note we give arguments in favor of the statement that the contribution of the box diagram calculated for electron muon elastic scattering can be considered an upper limit to electron proton scattering. As an exact QED calculation can be performed, this statement is useful for constraining model calculations involving the proton structure. 
E. A. Kuraev and E. Tomasi-Gustafsson