Objective: To develop adjuvant therapies to improve tumor control induced by thermal ablation, which as a stand-alone treatment has high recurrence rates in clinical settings.

Methods: We hypothesized that incomplete ablation of tumor tissue near the margin and around major blood vessels allows reoxygenation and repopulation of the tumor. We studied microvessel functionality after partial thermal ablation of murine FSaII tumors. In addition, we investigated using carbogen breathing and radiation to sterilize tumor cells in sub-lethally heated tumor. A conductive thermal-surgical probe was used to ablate tumors using a tip temperature of approximately 60°C until the temperature in the margin of the tumor reached 42°C for 1 min. A fiber optic pO₂ probe with incorporated thermocouple was implanted 1-2 mm into the opposite side of the tumor. DiOc7, a carbocyanine fluorescent marker of vessels with active blood flow, and the hypoxia marker pimonidazole were injected at 24, 48 and 72 h post- ablation and tumors were excised, frozen and sectioned for imaging. In addition, tumor cell survival was studied with the in vivo/in vitro clonogenic assay from tumors exposed to radiation 30 min before or 30 min after thermal ablation or at 24 h after ablation, with and without carbogen (95% oxygen/5% CO₂) breathing during the radiation.

Results: The oxygenation in the margin was found to increase on average from 1.2 ± 0.98 mmHg to 10.4 ± 6.3 mmHg in the first 30 min after ablation. Vessels with active blood flow in the peripheral regions of the tumor were seen to persist at 24 h post-ablation, diminish at 48 h and begin to return at 72 h post-ablation. The level of hypoxia detected by pimonidazole binding was inversely related to the number of DiOc7 stained vessels detected. Radiosensitivity was found to be maximal when radiation was applied 30 min after ablation as compared to before ablation and radiosensitivity was increased when carbogen breathing was administered at 24 h after ablation.

Conclusions: Because of the dynamic cellular and physiological changes induced by thermal ablation, radiation therapy with carbogen breathing may be effective in extending the duration of tumor response.