Hyperthermia‐induced vascular injury in normal and neoplastic tissue

The sequential morphologic alterations in normal skeletal muscle in rats, Walker 256 tumors in rats, and transmissible venereal tumors (TVT) in dogs following microwave‐induced hyperthermia (43°C and 45°C for 20 minutes), were studied by histologic and ultrastructural examination. Normal muscle and Walker 256 tumors showed edema, congestion, and hemorrhage at 5 minutes post‐heating (PH), followed by suppuration, macrophage infiltration, and thrombosis at 6 and 48 hours PH, and finally by regeneration and repair by 7 days PH. Vascular endothelial damage and parenchymal degeneration were present 5 minutes PH. Progressive injury occurred for at least 48 hours PH. Two hyperthermia treatments separated by a 30‐ or 60‐min cooling interval, were applied to Walker 256 tumors in a subsequent study. Increased selective heating of tumor tissue versus surrounding normal tissue, and increased intratumoral steady state temperatures were found during the second hyperthermia treatment. Canine TVTs were resistant to hyperthermia damage. These results suggest that vascular damage contributes to the immediate and latent cytotoxic effects of hyperthermia in normal tissue and some types of neoplastic tissue, and that selective heating of neoplastic tissue occurs in tumor tissue with disrupted microvasculature.