In recent years there have been published a great many investigations of problems relating to malignant tumours, carried out with the. aid of artificial radioactive isotopes. These investigations may be said to follow two different lines. On the one hand, radioactive isotopes have been applied in studies of metabolic processes. Very small quantities of. a radioactive isotope have then been given together with "larger amounts of the same inactive element, their distribution to the organs at different times can then be measured by physical methods. Studies of this kind have been surveyed by HEVESY (1), HAMILTON (2) and others. On the other hand, attempts have been made to use artificial radioactive isotopes in tumour therapy. Some degree of success seems so far to have been attained only in the treatment of forms of leukemia, polycythemia and lymphosarcoma (3).The present paper deals with a study of the Brown-Pearce rabbit carcinoma. Of the animal tumours generally used in cancer research, this one possesses a rather outstanding capacity for forming metastases in almost any part of the body, and thus affords good opportunities of studying the distribution of injected isotopes to metastatic and normal tissues. On the other hand the real nature of the Brown-Pearce rabbit carcinoma seems to be a matter of dispute. The metastases form rapidly growing masses, the living cells of which are mostly situated peripherally, or around the blood vessels. The rest of the tumour often consists of a necrotic mass, infiltrated by lymphocytes and plasma cells. For histological details the reader is referred to BROWN and PEARCE (4) and CASEY (5). According to CASPERSSON and SANTESSON (6) there are also
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