Abstract
The comet assay was used to estimate radiobiological hypoxic fraction across a full range of tumour oxygenations in C3H mammary tumours implanted into the feet of female CDF1 mice. Tumours were either clamped before irradiation or mice were allowed to breath air, 100% oxygen, carbogen or carbon monoxide for 5-35 min before and during exposure to 15 Gy. For the alkaline comet assay, tumours were excised after irradiation and individual tumour cells were analysed for DNA single-strand breaks. Hypoxic cells were defined as those cells with approximately three times fewer single-strand breaks than aerobic cells. Radiobiological hypoxic fraction was calculated by fitting DNA damage histograms to two normal distributions, representing the response of the aerobic and hypoxic populations. The percentage of hypoxic cells estimated using the comet assay was then compared with hypoxic fraction measured using a clamped tumour control assay. Carbogen and oxygen breathing reduced the normal hypoxic fraction from 14% to 2-3% in this tumour, whereas 75-660 p.p.m. carbon monoxide progressively increased the hypoxic fraction from 18% to 82%. The slope of the line comparing the two methods was 1.23 with 95% confidence limits of 1.12-1.33 (r2 = 0.994). In the SCCVII squamous cell carcinoma growing subcutaneously in C3H mice, a similar correlation was observed between hypoxic fraction measured using the comet assay and hypoxic fraction measured in the same tumour cells using the paired survival curve assay (slope = 1.20 with 95% confidence limits of 1.03-1.37). These results confirm the ability of the comet assay to provide an accurate estimate of radiobiological hypoxic fraction over a wide range of tumour oxygenations and between two tumour types.
Highlights
The proportion of hypoxic cells decreased as oxygen concentration in the inspired gas increased or as carbon monoxide concentration
Combined results for six to eight tumours per group are shown in Figure 2 and are compared with the hypoxic fraction measured using a clamped tumour control end point
Experiments with carbon monoxide may be subject to greater interanimal variation, and a smaller variation might have been seen for tumour control experiments performed at the same time as comet experiments
Summary
The majority of experiments were performed on 10 to 14-weekold male CDF1 mice bearing a C3H mouse mammary carcinoma whose derivation and maintenance have been described previously (Overgaard, 1980). Tumours were implanted in the dorsum of the right rear foot and treated when they had reached about 200 mm in size. The SCCVII squamous cell carcinoma was grown in the dorsum of the foot (Aarhus) or implanted subcutaneously in the back (Vancouver). SCCVII tumours implanted in the back were used for experiments when they reached a size of 350-500 mg (Olive, 1994). Mice were restrained in lucite jigs and allowed to breathe 100% oxygen or carbogen (95% oxygen, 5% carbon dioxide) for 5 min before and during irradiation, or various percentages of carbon monoxide in air for about 35 min before and during irradiation. All mice were unanaesthetized and those with foot tumours were exposed to 15 Gy 250 kV X-rays at a dose rate of 2.3 Gy min-', or with back tumours at a dose rate of 3.3 Gy min-'
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