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Tumor Size Dependency in the Radiation Response of the Lewis Lung Carcinoma¹

Department of Biophysics, Institute of Cancer Research and Department of Radiotherapy, Royal Marsden Hospital, Sutton, Surrey, England

Tumor cell survival characteristics were assessed following ⁶⁰Co -irradiation of the Lewis lung carcinoma as 500-cu mm s.c. tumors or as 0.5-cu mm (1 mm in diameter) pulmonary metastases. Cells in the small pulmonary tumors were markedly more radiosensitive (D₀ = 106 rads; hypoxic fraction < 0.005) than were those in large s.c. tumors (final D₀, 315 rads; hypoxic fraction, 0.36). When pulmonary metastases were excised and irradiated intact under well-oxygenated conditions in vitro, the hypoxic fraction rose to 0.30. This implies that, under normal in situ conditions, these nodules contain a microvascular system that achieves adequate oxygen supply to the great majority of tumor cells. Thus, the tumor cells within these small metastatic implants were more sensitive to irradiation, largely due to better oxygenation, and may be more sensitive to chemotherapy, due to better drug availability.

¹ The work contained in this report was partly supported by National Cancer Institute Contract NCI-CM-23717.

² Work in this report done while on a Moseley Fellowship from Harvard Medical School, Boston, Mass. To whom reprint requests should be addressed, at Department of Radiation Medicine, Massachusetts General Hospital, Harvard Medical School, Boston, Mass. 02114.

Received 12/12/74. Accepted 5/21/75.

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