TNF and fibrosarcoma: If most of the TNFα in these tumors was within interstitial fluid, and this constituted around 10% of the tumor volume (as was reported for subcutaneous fibrosarcomas [72]) our data suggest that the tumor cells in this implantation model may be exposed to around 6–10 pg/ml of TNFα in vivo, a concentration that achieved approximately half-maximal cooperation with Smac mimetics to kill osteosarcoma cells in vitro [26].