A pilot study comparing intratumoral oxygenation using the comet assay following 2.5% and 5% carbogen and 100% oxygen

Abstract

Purpose: Tumor hypoxia has been purported to be an important biologic factor in the failure of radical radiotherapy to achieve local control in many tumor types. This study was designed to evaluate the effect of breathing high oxygen content gas mixtures (oxygen with 0%, 2.5%, or 5% carbon dioxide) on tumor oxygenation measured using the Eppendorf polarographic oxygen electrode and the comet assay in accessible, hypoxic human tumors.

Methods and materials: Using Eppendorf pO2 histography to identify hypoxic tumors (median pO2 < or = 10 mmHg), eligible patients were systematically allocated either 100% oxygen (O2) or oxygen with 2.5% or 5% carbon dioxide (CO2). Tumors were treated with 6-10 Gy during which two fine needle aspirates (FNA) were obtained from different regions of the lesion, one at midway and the other at completion of the radiation exposure. Gas breathing was initiated 4 min before radiation was commenced. A 10-min interval was specified between the first and second halves of the radiation exposure to allow near maximal DNA repair prior to the second half of the radiation treatment. FNAs were performed within 2 min of cessation of radiation and the cells immediately suspended in buffered saline at 4 degrees C for analyses of hypoxic fraction using the comet assay.

Results: Fifteen evaluations were performed in 13 patients with hypoxic tumors (median O2 tension 2.75 mmHg) treated with a median dose of 8 Gy. The median hypoxic fraction determined using the comet assay fell from 0.36 to 0.13 (p = 0.001, Wilcoxon signed rank test) due to the addition of high oxygen content gases.

Conclusions: In tumors defined as hypoxic using Eppendorf pO2 histography, a statistically significant reduction in the hypoxic fraction with the comet assay was found following administration of high oxygen content gases. These preliminary findings reveal a trend suggesting that 5% carbogen may reduce the hypoxic fraction by a greater margin than either 100% oxygen or 2.5% carbogen.