I have seen some dvts too in patients with big extremity sarcomas and I'm pondering if we should be giving these patients heparin as a prophylactic measure when treating in the preoperative setting.
Swelling of the limb often occurs due to necrosis/inflammation in the tumor, add to that the cytokines released from dying tumor cells plus the endothelial damage to the vessels being irradiated and you are looking at many factors that lead to dvt.