A model for the behavior of β-hCG after evacuation of hydatidiform moles

Abstract

Objectives. The objective of this study is to develop a physical model of the behavior of β-hCG following the complete evacuation of a hydatidiform mole. Because hCG is an excellent marker for continued trophoblastic activity, the model can be used for early detection of persistent sites. Method. The model was developed from analysis of the post surgical hCG decrease in a patient with Stage III gestational trophoblastic neoplasia. As found in previous molar pregnancy studies, hCG follows a log-linear decrease after resolution. In contrast to those studies, however, we assume that the decrease can be explained by the dilution of the residual hCG from two different tissue reservoirs, a tissue reservoir with a half-life of ∼ 4 days and a reservoir with a longer half-life, in this case ∼ 18 days. Results. Simple dilution of two tissue reservoirs explains behavior of hCG following tumor removal. The model also explains the hCG decrease in a larger study of Japanese and Dutch women following the evacuation of uneventful hydatidiform moles. Conclusions. Following an initial rapid drop in hCG after resolution of the mole, the patient should experience a slower drop associated with the dilution of residual hCG in the deep tissue reservoir. This is normal. The physical model suggests that even earlier detection of chemotherapy resistant persistent trophoblastic disease is possible if the patient's decrease in hCG is slower than a log-linear fit to the patient's previous data. The results also suggest an alternative approach to processing patient statistics in analysis of carcinomas with large variations in the tumor marker concentrations.