Studies on the Walker Tumor: I. Standardization of the Growth of a Transplantable Tumor

1. Factors contributing to the variability in growth rate of a transplantable tumor have been analyzed. 2. The Walker tumor was selected because it presents a malignant growth, to which there is 96 per cent susceptibility in either sex of albino rats of the Sprague-Dawley strain. Regressions are extremely rare, and the growth rate is reliable, reproducible, and rapid. 3. The most important factor contributing to variations in growth rate is the site of implantation. Subcutaneous implants show a large variability in size, depending upon whether the tumor becomes vascularized from muscle or skin. Uniform symmetrical growths may be obtained by resorting to bilateral intramuscular implantation into the flexor muscles of the thighs. 4. The growth rate of the tumor is not significantly affected by relatively large changes in the size of the inoculum. 5. In weaned rats, the size and sex of the animal do not influence the growth of the tumor but the carcass weight is greatly affected by the growth phase of the host animal.