Features and limitations of the in vivo evaluation of tumour response by optical means.

When a tumour is growing in a thin sheet-like fashion, many useful parameters can be directly determined by optical means. In a "sandwich" system a tumor grows in a thin, separated layer of subcutis on the back of a rodent. The tumour itself is enclosed between two transparent surfaces, one being a glass cover slip. Living tumour cells appear to be relatively transparent; therefore, the blood vessels in the tumour are the most outstanding structures. The assay methods that can be applied for determining tumour response can be divided into three groups: (1) observation and recording of the nature of the tumour vasculature, etc.; (2) dynamic investigations on blood flow; and (3) the use of optical indicators for a relative index of tissue oxygenation state. Especially with regard to the latter methods, recent developments are promising. However, as determinations with these systems do not reveal the clonogenic viability of tumour cells as such, the use of such "observation" systems should be of necessity restricted to answering questions about tumour response that cannot be evaluated by other means.