Ribonuclease. Abnormal elevation of serum ribonuclease (RNase) occurs in patients with various cancers." Although this elevation is also observed in the presence of severe renal insufficiency, a creatinine test excludes this possibility. This activity was determined using polycytidylic acid (Yamasa Co., Ltd., Choshi, Japan) as a substrate according to the method described by Reddi and Holland.15

Sialic acid. High levels of sialic acid'"' have been found in the sera of patients with various cancers. Sialic acid was assayed using a Hitachi 705 autoanalyzer (Hitachi, Tokyo, Japan) with the Sialic Acid Reagent Kit (KT Sial Rate 50, Kyokuto Pharmaceutical Industrial, Co., Ltd., Tokyo, Japan). The determination is based on the enzyme assay as described by Comb and Roseman."

Alkaline phosphatase isoenzymes. Alkaline phosphatase (ALP) isoenzymes were separated by cellulose acetate membrane (Taitan III kit, Helena Ltd., Tokyo, Japan) electrophoresis. We had already found that in patients with cancer with ALP activity in the normal range, the serum ALP isoenzymes showed significant variation depending on the condition of the disease."" Three parameters (ALP,, ALP2,3, alkaline phosphatase isoenzyme angle) were then calculated from the densitometric patterns of the ALP isoenzymes, as previously described.'

Carbohydrate antigen 19-9. Carbohydrate antigen (CA) 19-9 was discovered by Koprowski et al." The determination of this marker was performed by radioimmunoassay using an ELSA CA 19-9 TM RIA Kit (Green Cross Co., Osaka, Japan).

Tissue polypeptide antigen. Tissue polypeptide antigen (TPA), a general tumor marker,22 was determined by radioimmunoassay using a TPA ('251) RIA Kit (Daiichi Radio Isotope Laboratories, Ltd., Tokyo, Japan).

Additional tests. Fe, the ratio of albumin to globulin, glutamic-oxaloacetic transaminase, glutamic pyruvic transaminase, tolbutamide tolerance test, ALP, and creatinine were determined with a Hitachi 705 autoanalyzer. These tests were used to exclude false-positive results caused by liver damage and renal insufficiency. The above-mentioned tumor markers were classified into three groups: tumor-specific tumor markers (CEA, CA 19-9, HSAP, TPA), tumor-associated tumor markers (FT, FT/Fe, IAP, sialic acid), and growth-related tumor markers (ALP isoenzyme [ALP,, ALP2/3, APA], RNase).

Tumor Staging According to a Model of the Natural History of Preclinical Cancer

Diagnosis for tumor stages was made using a tumor marker combination assay of the above 12 items (CEA, HSAP, FT, FT/Fe, IAP, RNase, sialic acid, three ALPisoenzyme parameters, CA 19-9, and TPA) according to the cutoff values of each tumor marker described in Table 3. The cutoff values that determine the tumor stages in the preclinical cancer were determined by extrapolation of the results of the correlation between tumor marker and tumor size 23 among clinical patients with cancer admitted to our hospital between 1980 and 1985. A hypothetical model of cancer in preclinical development is described in the Discussion section. As a rule, each tumor stage was defined by two or more abnormal levels, identified according to the cutoff values of each tumor marker at each stage.

Follow-up Studies

Subjects identified as having Stage IV or V tumors were given additional morphologic tests, such as radiograph, computed tomography scan, endoscopy, and biopsy, in follow-up studies lasting 5-7 years.

In the current report, the detection rate expressed is the percentage of subjects who were diagnosed as having cancer among presumably healthy screenees.
Results

Distribution of Each Tumor Stage

Figure 1 shows the distribution of subjects grouped by tumor stage. The number of subjects at each tumor