Mechanisms in Acquired Radioresistance of Cancer

Acquired radioresistance in cancer of various types is a phenomenon all too common in the radiotherapist's experience. The lymphomas as a group appear to be among the most outstanding examples of human cancers which may exhibit this finding. To explain it, many theories are recorded in the literature; few experimentally established facts, however, are available. The purpose of this paper is to present negative data obtained by studying the effects of in vitro irradiation on a transplantable lymphoma in order to determine whether acquired radioresistance is a property of this tumor cell or the host.

Methods

The Gardner mouse lymphosarcoma 6C3HED was used throughout the experiment.² The tumor was carried subcutaneously in young adult C3H mice obtained from the Roscoe B. Jackson Memorial Laboratory in Bar Harbor, Maine. All transplants were made under sterile conditions by the trocar implantation of one small (2 mm. in diameter) pellet of freshly excised tumor into the subcutaneous tissue of the lateral abdominal wall. All x-irradiation treatments were given with a G. E. Maximar Therapy Unit at 200 kv., with 3 mm. aluminum inherent filtration, target distance of 35.6 cm., and an output of 165 r per minute. The tumor to be transplanted was excised from a freshly killed animal and cut into small fragments, which were placed in Wassermann tubes with 2 ml. of Hank's balanced salt solution. The controls were set aside and the treated fragments were irradiated in the Wassermann tubes. Both controls and the irradiated fragments were then implanted subcutaneously into normal animals. In 200 control transplants made in this fashion the tumors invariably grew well, attaining a size which necessitated transfer in twelve days.

The animals were kept in separate cages and received the standard diet employed in this institution.

The experiment was divided into two parts. The object of the first was to establish the lethal dose for this tumor under the conditions outlined above, i.e., the in vitro dose of radiation sufficient to prevent growth of the tumor fragment when transplanted into a normal animal. This was determined by giving tumor fragments single in vitro doses of radiation varying from 2,500 to 5,000 r and then implanting them into normal animals.

The second part of the experiment consisted in an attempt to demonstrate under these in vitro conditions of x-irradiation the phenomenon of acquired radioresistance. To accomplish this, the originally unirradiated tumor was given increasing in vitro doses of radiation with each successive transplanation. The initial dose was well below the lethal dose for the tumor, and increasing doses were given at each transplantation in order to accustom the tumor to the effects of radiation, with the hope that the lethal dose for the previously unirradiated tumor could thus be exceeded. No attempt was made to determine the possible influence of time upon the induction of radioresistance.