I-125-labelled human chorionic gonadotrophin (hCG) as an elimination marker in the evaluation of hCG decline during chemotherapy in patients with testicular cancer

The rate of reduction in the concentration of serum human chorionic gonadot rophin (hCG) following chemotherapy for germ cell tumours may follow a comp lex pattern, with longer apparent half-life during later stages of chemothe rapy, even in patients treated successfully. The commonly used half-life of less than 3 days for hCG to monitor the effect of chemotherapy in patients with germ cell tumours of the testis may represent too simple a model. I-1 25-labelled hCG was injected intravenously in 27 patients with germ cell tu mours and elevated hCG during chemotherapy. The plasma radioactivity and hC G concentrations were followed. During chemotherapy, the plasma disappearan ce of hCG showed a biphasic pattern, with an initial fast and a later slow component in all patients. Using the steep part of the hCG plasma disappear ance curve, five patients who achieved long-term remission had half-lives l onger than 3 days (3.6-6.8 days), whereas four out of five patients not ach ieving long-term remission had half-lives shorter than 3 days. After the th ird treatment cycle, eight patients who achieved long-term remission had hC G half-lives longer than 3 days (7.4-17.0 days). In these patients, the pla sma disappearance of [I-125]hCG was equivalent to that of hCG. Thus, the sl ow decline of hCG represented a slow plasma disappearance rather than a hCG production from vital tumour cells and could, consequently, not be used to select patients for additional or intensified chemotherapy. The concept of a fixed half-life for plasma hCG during treatment of hCG-producing germ ce ll tumours is inappropriate and should be revised. Difficulties in interpre ting a slow decline of hCG may be overcome by comparing the plasma disappea rance of total hCG with the plasma disappearance of [I-125]hCG.