Studies on the Walker tumor. I. Standardization of the growth of a transplantable tumor.

In this paper a standardized reproducible technic for obtaining non-necrotic transplantable tumors of uniform size and growth rate will be described, and the principal factors contributing to variability in growth rate will be analyzed. A number of potential objections have been raised to the extrapolation of results obtained with trans plantable tumors to tumors of spontaneous origin (e.g., extreme malignancy and anaplasia, rapid growth, extensive necrosis, and rarity of me tastasis) ; nevertheless, transplantable tumors ex hibit all the salient biological and biochemical characteristics of malignant tissue, and resemble these to a far greater extent than normal tissues. Moreover, they provide the only practical method of studying reproducible malignant growths under controllable conditions during a reasonably short time period. The transplantable tumor selected for this study was the Walker carcinosarcoma 256. This tumor was discovered on the lower abdomen of a pregnant albino rat in 1928. It regressed during lactation but again began to grow rapidly after weaning. The early history of the tumor and its histology have been extensively reviewed by Earle (1). The initial histological appearance of this tumor was that of an adenocarcinoma (presum ably arising from mammary tissue) with a large proportion of connective tissue elements. Organ ized epithelial elements are no longer evident, and now the histological appearance is that of a highly anaplastic growth (Fig. 1). The Walker rat tumor was selected for these studies because of its uni formly rapid growth rate, the rarity of early re