CELL LOSS FROM SEVERAL TYPES OF SOLID MURTNE TUMOUR: COMPARISON OF 125I‐IODODEOXYURIDINE AND TRITIATED THYMIDINE METHODS

Abstract

ABSTRACTThe method of measuring tumour cell loss rates in situ following radioactivity loss after a single injection of 125I‐iododeoxyurudine (125I‐UdR) was tested for its accuracy in five different types of murine tumour. To achieve this the method was compared with two others: (1) using 125I‐UdR, but excising tumours before the radioactivity determinations, with or without extracting DNA; (2) using tritiated thymidine and autoradiography. A third method was used on three of the tumours, in which 125I‐UdR‐labelled tumours were grown in unlabelled hosts, followed by whole body counting of the tumour‐bearing mice.In two of the tumours an increase was observed in total tumour radioactivity with time after 125I‐UdR injection. This prevented the estimation of cell loss parameters in these tumours. Approximately half the increase was due to reutilization of 125I‐UdR supplied from tissues within the mouse; approximately a third to an influx of labelled inflammatory cells (probably in response to infection accompanying ulceration of overlying skin); and the remainder to an increase in non‐DNA radioactivity. In these tumours cell loss rates could be obtained from the whole body counting technique in which influxes of labelled cells and reutilizable radioactivity were eliminated.A comparison of either 125I‐UdR technique with the 3H‐TdR technique showed good agreement of the cell loss factors for the low cell loss tumours. However, for tumours with high cell loss factors the 125I‐UdR technique gave lower values for cell loss. This implied that reutilization of 125I‐UdR within the tumour (i.e. from internal, not external sources) occurred in the high cell loss tumours.It is concluded that equating radioactivity loss with cell loss after an injection of 125I‐UdR is reasonable for some tumours, but will result in significant underestimates in others. For high cell loss tumours the 3H‐TdR technique will give the