Estimation of potential doubling time (Tpot) in vivo, by flow cytometry: Practical problems and possible solutions.

Kinetic parameters of tumour growth yield predictive values and allow an optimisation of the treatment schedule, especially for fractionation in radiotherapy. Among those parameters, the labelling index (LI) and the potential doubling lime (Tpot) may be measured by flow cytometry, a semi-quantitative analysis, after in vivo administration bf iododeoxyuridine (IdUrd) to humans. This Begg's recommended methodology needs the selection of thresholds and gates whose boundaries are arbitrary. They can be positioned on a more objective basis using a negative control (aspecific fixation). Moreover a previous identification of the different cell populations of the tumour samples according to the method of Vindelov allows a better determination of these cell populations processing IdUrd-DNA staining. This procedure was used with 11 tumour biopsies including mainly head and neck cancers. This method displayed results similar to the literature concerning LI and Tpot determinations as well as shortened Tpot when the patients recurred. One sample has no labelling at all. A small function, likewise up to 10% of cells exhibiting an S DNA content were not labelled by IdUrd. These cells leave the S phase or progress too slowly in order to display IdUrd uptake. Intra-tumoral hypoxia is a possible explanation of these findings. DNA ploidy and the percentage of cells in S phase could be worth while to precise the relationship between DNA index and tumour kinetic. The measurement of Tpot could also be used in other cancers and for optimisation of dose of chemotherapy.