Effects of Radiation Mediating Agents on the Response of a Murine Ependymoma to Proton Irradiation

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Abstract
Proton radiation is currently employed in a few centers in the United States and Sweden for the treatment of brain tumors in man. There is, however, relatively little quantitative information in the literature on the radiobiological response of experimental brain tumors. Some studies of x-radiation effects on gliomas in mice have been reported (1–3), but the difficulties associated with the use of intracerebral tumors led us to choose a subcutaneously transplantable ependymoma. This test system is a highly malignant neoplasm which transplants readily and grows rapidly. In the normal routine sequential transplants over several years, 100 per cent tumor growth has been obtained (4). We have investigated the in vivo response of this tumor to x and proton radiation under a variety of conditions, in terms of reduction in successful transplantation immediately after irradiation of the tumor-bearing animal. Materials and Methods The two mouse gliomas (Nos. 15 and 48) which we have studied had been induced in C3H mice by intracerebral implantation of methylcholanthrene pellets and then carried by subcutaneous transplantation in male C3H mice six to eight weeks old. The tumors were induced by Perese and Moore2 (5) in 1957 and histologically they have remained unchanged and are both classified as ependymomas. The same general transplantation procedure was employed in the radiation experiments and for routine stock maintenance. Single or bilateral injections of 20 mg of homogenized tumor tissue in a volume of 0.1 ml were made subcutaneously in the flank of the recipient (unirradiated) animals. Tumor growth (“take”) was considered positive when a detectable, palpable mass continued to increase in size. Irradiation Procedure: Proton radiation was administered with a 3∕4-in. diameter beam from the Harvard synchrocyclotron. Absorbers and a water collimator reduced the beam energy to 55 MeV, with a mean linear energy transfer (LET) of 11.5 MeV cm2∕g. The dose calibration was obtained from Faraday cup measurements, and a secondary nitrogen-filled transmission ion chamber was used as integrating monitor. The beam cross-section profile curve at the radiation position was obtained with a silicon diode probe detector (6). The mice were irradiated individually, under light Nembutal anesthesia, with the tumor positioned axially in the beam. Doses in the range of 1,500 to 6,000 rads were given at a dose rate of 1,000 rads∕min. The dose was essentially uniform throughout the tumor volume of approximately 800 cubic millimeters. A smaller series of x irradiations was carried out under comparable conditions. A 2 MeV Van de Graaff accelerator with an x-ray dose rate of 350 rads∕min.

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