In mice with spontaneous or MTV-induced breast tumors, neonatal thymectomy lengthens the induction period and decreases their frequency. Similarly, the incidence of extrathymic neoplasms and particularly of breast cancer in women with myasthenia gravis is extremely high but falls precipitiously following thymectomy. Thus thymic activity may in some instances contribute to the development of breast cancer. We biopsied the thymus through a small neck incision in 25 nonmyasthenic women with breast cancer prior to mastectomy, in 9 patients with breast lesions suspected to be malignant but subsequently proved to be benign, and in 7 women during thyroidectomy for goiter. Various histological parameters were compared with the clinical stage of disease and the menopausal status of the patients. Comparisons between study and control groups were made only between age-matched patients and controls. We found that the mean diameter of the thickest lobule and the thickness of thymic cortex are appreciably greater in breast cancer patients than in controls; the thickness of the cortex tends to increase with advancing stage of the disease. The thymic medulla of breast cancer patients is significantly thinner than that of age-matched controls ( p < 0.005), and this relative atrophy of the medulla is equally pronounced in premenopausal and postmenopausal cancer patients. Since the thymic medulla appears to be responsible for the secretion of thymic hormones, the substantial atrophy of that region consistently found in o u r cancer patients is compatible with the view that a relative deficiency of thymic hormone(s) may be present in these patients. Other possible interpretations of these findings are also discussed. fact be detrimental. 32 Neonatal thymectomy, for example, reduces the known high frequency and lengthens the induction period of murine breast tumors due to the mammary tumor virus (MTV), 2 0 ,~ 5 1 and treatment with antithymocyte sera decreases the frequency of spontaneous mammary Thymic transplantation restores capacity of thymectomized mice to develop breast cancer. 51 Likewise, observations in patients with myasthenia gravis (M.G.) suggest that some component of thymic function may contribute to the development of breast cancer. Women with M.G. have a high incidence of extra-thymic neoplasms, and particularly of breast cancer, which is drastically reduced if these patients undergo thymectomy; the propensity for the development of malignant tumors, however, persists if thymectomy is not done.35 It should be also noted that the majority of patients with M.G. have low urinary 17-ketosteroid excretion which return to normal following thymectomy. sq44 This endocrine abnormality is similar to that found in breast cancer
