Dosimetric analysis of a compartmental model for radioligand uptake in tumor lesions.

Radioligand therapy is a targeted cancer therapy that delivers radiation to tumor cells based on the expression of specific markers on the cell surface. It has become an important treatment option in metastasized neuroendocrine tumors and advanced prostate cancer. The analysis of absorbed doses in radioligand therapies has gained much attention and remains a challenging task due to individual pharmacokinetics. As an alternative to the often used sum of exponential functions in intra-therapeutic dosimetry, a basic compartmental model for the pharmacokinetics of radioligands is described and analyzed in this paper. In its simplest version, the model behavior is determined by the uptake capacity and the association constant and can be solved analytically. The model is extended with rates for excretion from the source compartment and externalization from the lesion compartment. Numerical calculations offer an insight into the quantitative effects of the model parameters on the absorbed dose in the tumor lesion. This analysis helps understanding the importance of clinically relevant factors, e.g. the effect on absorbed doses of modified radioligands that bind to albumin. Using clinical data, the potential application in intra-therapeutic dosimetry is illustrated and compared to the bi-exponential function which lacks a mechanistical basis. While the compartmental model is found to constitute a feasible alternative in these examples, this has to be confirmed by further clinical studies.