To assess the oxygenation status of human tumors, needle probes were first used in the 1950s, and changes in oxygen tension were monitored in patients breathing ai r or oxygen (33-39). Many data have been published on the subject, some of them demonstrating a clear correlation between pO2 and treatment outcome (34,37-39). However, most of these results have been criticized, because of the lack of reproducibility with the various systems used, and because of some technical limitations (37,38). Improvements were made in the late 1980s with new equipment, the KIMOC-6650 histograph (Eppendorf, Hamburg, Germany). Most recently published results on direct oxygen tension measurements in humans have been obtained with this computerized polarographic system. This subheading will focus on the results obtained with that equipment, which can now be considered as giving base line information on normal tissues and tumors oxygenation.
This histograph uses fast responding electrodes programmed to minimize the effects of tissue compression and massive oxygen consumption by the electrode. The technical data on this equipment have been published (40-45). Briefly, a gold microcathode (12 ^m in diameter) is placed within an unbreakable stainless steel needle probe (300 ^m in diameter). The cathode is polarized against an Ag/AgCl anode placed on the skin of the patient. The resulting current is proportional to the partial oxygen pressure in the tissue. All electrical connections to the patient are insulated at a level of 8 kV. Values are corrected for the barometric pressure and temperature of the tissues (recorded with a thermocouple). The electrodes are cleaned, dried, and sterilized. For most patients, one track is recorded in normal tissues surrounding tumors, followed by independent tracks in the tumor. The probe is automatically moved through the tissues as defined at the beginning of each experiment, with each forward movement followed by a backward step, and, at the end of the measurement, the needle probe is automatically removed. The results appear as a histogram, together with the mean, median, and 10 and 90 percentiles of recorded values along tracks. Tissue architecture is not significantly modified by oxygen-tension measurement, as assessed by histology (40). However, it has been shown that some systematic overestimation of partial oxygen pressure was found with this equipment (which could be caused by bleeding along the probe track) when compared to measurements made with microelectrodes.
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