Radiosensitization of two murine tumours with mild temperature hyperthermia and carbogen breathing.

PURPOSE: To test the individual and combined effects of local mild temperature hyperthermia (MTH) at 41.5 degrees C for 60 min and carbogen breathing on tumour radiosensitivity.
MATERIALS AND METHODS: The FSall fibrosarcoma of C3H mice and the SCK mammary carcinoma of A/J mice were used. The effect of various treatments on tumour cell survival was determined using the in vivo/in vitro tumour excision assay, and the radiobiological hypoxic fraction was calculated. The tumour radiation sensitivity was tested with the tumour growth delay assay.
RESULTS: The radiobiological hypoxic cell fraction (HF) in control FSall and SCK tumours was 0.45 and 0.78, respectively, and these values decreased to 0.12 in FSall tumours and 0.22 in SCK tumours when determined immediately after the tumours were treated with MTH. The HF was 0.32 in FSall tumours and 0.33 in SCK tumours after carbogen breathing was applied. When tumours were treated with MTH and the animals breathed carbogen the HF decreased to 0.03-0.04 in both FSall and SCK tumours. MTH treatment alone had only a small effect on tumour growth, but MTH treatment applied before irradiating the tumours significantly increased the radiation-induced tumour growth delay. Carbogen breathing modestly improved the radiation-induced tumour growth delay while the combination of MTH treatment and carbogen breathing caused the largest increase in radiation-induced tumour growth delay.
CONCLUSIONS: MTH treatment alone and combined with carbogen breathing substantially increased the tumour radiation response probably through an increase in the tumour oxygenation status.