Would you include the entire hardware as part of ISRT for a patient with Stage IE DLBCL of the distal femur treated with upfront prophylactic nailing for impending fracture, who had a CR to chemotherapy?  

Quite uncommon to encounter such a patient, but based on data for non hematologic tumors, no. Treat just the involved site with a generous margin, the latter never precisely defined, but depends on the tolerance of surrounding normal tissue. Parenthetically, it's often difficult to determine a CR to...