We measured and compared the oxygen partial pressure (pO2) profiles in experimental tumors after irradiation with carbon ions and with X-rays. The NFSa fibrosarcomas grown in the hind legs of C3H male mice received isoeffect single doses of carbon ions or X-rays. Coaxial oxygen microelectrodes of high spatial resolution were inserted into the tumor with 20 μm steps by a computerized micromanipulator. The number of pO2 peaks that reached 15 mmHg were at least 0.45 per 3,000 μm in unirradiated tumors and significantly increased to 1.55 per 3,000 μm as early as day 1 of carbon-ion irradiation (p < 0.001). The tumors that received X-ray irradiation also significantly increased pO2 peaks, but as late as day 3. The time course of pO2 peak appearance in the present study coincides with a previous report where reoxygenation was measured by paired growth delay assay. The pO2 peaks appeared selectively in peripheral regions of X-ray irradiated tumors, but they appeared rather homogeneously in the tumor after carbon-ion irradiation. It is concluded that carbon-ion irradiation reoxygenated the NFSa fibrosarcomas earlier in time and deeper in space than the X-ray irradiation did.
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