A radioimmunoassay for ng quantities of DNA was developed. [¹²⁵I]Iododeoxyuridine-labeled DNA was used as the antigen, and the serum of a lupus erythematosus patient served as the source of antibody. The level of free DNA in the serum of 173 patients with various types of cancer and in 55 healthy individuals was determined by this radioimmunoassay. DNA concentration in the normal controls had a range of 0 to 100 ng/ml with a mean of 13 ± 3 ng/ml (S.E.). For comparison purposes, the range of 0 to 50 ng/ml was designated as normal, and 93% of controls were found in this range. In the cancer patients, the DNA concentration ranged from zero to µg levels with a mean of 180 ± 38 ng/ml. Fifty % of the patients' values were found in the range of 0 to 50 ng/ml; the other 50% were between 50 and 5000 ng/ml. No correlation could be seen between DNA levels and the size or location of the primary tumor. Significantly higher DNA levels, however, were found in the serum of patients with metastatic disease (mean of 209 ± 39 ng/ml), as compared to nonmetastatic patients (mean 100 ± 30, p < 0.02).

After radiation therapy in lymphoma, lung, ovary, uterus, and cervical tumors, the levels decreased in 66 to 90% of the patients, whereas in glioma, breast, colon, and rectal tumors, the DNA levels decreased only in 16 to 33% of the patients. Generally, the decrease in DNA concentration in the serum correlated with improved clinical condition, such as decrease of tumor size and reduction of pain. Conversely, when DNA levels either increased or remained unchanged, a lack of response to the treatment was noted. Of 17 patients who died within a year, 13 showed DNA levels that remained high or unchanged, whereas only 4 showed lower levels during treatment. Persistent high or increasing DNA levels in the circulation, therefore, may signal a relapse and are probably a poor prognostic sign.

The relatively high percentage (50%) of cancer patients with apparently normal DNA levels would suggest that this test may have low diagnostic value. It should be pointed out, however, that all these patients represent a selected group considered for radiation therapy, usually after surgery and/or chemotherapy. It is possible that a better correlation between DNA levels and cancer will be obtained prior to the initiation of treatment. On the other hand, DNA in the serum may be an important tool for the evaluation of therapy or the comparison of different regimens.