Thanks for the information. I did not do any Orthopedic Oncology, particularly Pediatric Orthopedic Oncology, as a practitioner. This would be highly specialized surgery at a high level center. So, I am not sure how to answer the question of coding it. I looked at several procedural codes, but what it really comes down to is an amputation through the thigh/femur, 27590. The next part is the harvesting of the tibia from the lower leg portion of the amputated leg, then preparing it for re-implantation as a bone "autograft" that is fixed to the femoral stump (I presume with some sort of internal fixation), then the preserved soft tissues of the thigh are "reattached" around it to incorporate it into the now longer thigh. Since the tibia is a bone "graft," in particular a "structural" graft, then +20938 would apply for the harvesting and preparation of the tibia. Beyond that, I can't find any very good or accurate codes for the last part of the procedure (the re-implantation of the tibia to the femur/thigh), so you would probably have to use an Unlisted Code for the Femur (thigh), 27599, plus some Modifiers, 22 at least, possibly 51, or a variant of 59 if one would fit correctly.
I don't know how much this helps you, but I gave it my best shot.
Sincerely, Alan Pechacek, M.D.