THE POTENTIAL DOUBLING TIME (T-POT) IN TH E CLINIC - WHERE DO WE STAND

The potential doubling time (T-pot) is defined as the time necessary t o double the number of proliferating tumor cells in the absence of spo ntaneous cell loss. T-pot is thought to be a better index of tumor pro liferation than clinically observed tumour volume doubling time. The i n vivo measurement of isa possible way to detect fast growing tumours which would be better controlled by accelerated radiotherapy. The publ ished data demonstrate the feasibility and the safety of the technique . Large variations in T-pot values were observed in tumors with simila r histology, indicating an accelerated proliferation rate in some tumo rs. There are technical difficulties related to intra-tumor heterogene ity, sample contamination by normal cells, and inter-laboratory variab ility, which question the biological interpretation of T-pot. Further studies are ongoing to establish whether the in vivo measurement of T- pot 1) provides information that is independent of the ''classical'' p rognostic factors, and 2) allows the early recognition of the patients likely to benefit from accelerated treatment.