The Proliferative Status And Clonogenic Capacity Of Tumour Cells In A Transplantable Rhabdomyosarcoma Of The Rat Before And After Irradiation With 800 Rad Of X‐Rays

R‐1 Rhabdomyosarcomas Transplantable In The Inbred Strain Of Wag/Rij Rats Were Labelled Semicontinuously With 3 H‐Tdr And Subsequently Irradiated With X‐Rays In Order To Estimate The Rate Of Recruitment Of Q Into P Cells And To Obtain More Insight Into The Importance Of Hypoxia With Respect To The P And Q Status Of Clonogenic Cells. Labelling Was Performed By Giving Animals From One To A Total Number Of Up To Seven Intraperitoneal Injections With 3 H‐Tdr Within 30 Hr. Half Of The Number Of Tumours Received An Acute Dose Of 800 Rad Of 300 Kv X‐Rays At 45 Min After Completion Of The Last Labelling In Order To Reduce The Fraction Of Well Oxygenated Cells Among The Surviving Clonogenic Cells To Less Than 1%; Labelled Control Tumours Were Sham Irradiated. Fractions Of Labelled Tumour Cells, Li Tumour ’Were Evaluated From Autoradiographs Prepared From Sections Of Tumours That Were Fixed 15 Min After Irradiation: Fractions Of Labelled Cell Foci, Li Fo , Were Determined In Autoradiographs Prepared From Cultures Of Cells Isolated From The Remaining Parts Of The Tumours. Analysis Of The Variation In Li Tumour As A Function Of Time And Of The Number Of Injections Showed That The Rate Of Recruitment Of Q Cells Into P Cells Is Most Likely Less Than 1%/Hr. Comparison Of Data On Li Fo Derived For Irradiated And Non‐Irradiated Tumours Indicates That Clonogenic Q Cells Are More Frequently Present Among Cells Which Survived The Dose Of 800 Rad Of X‐Rays. These Results Suggest That, In R‐1 Tumours Of About 800 Mm 3 The Clonogenic Q Cell Population Contains A Larger Fraction Of Hypoxic Cells Than The Fraction Of Hypoxic Cells Present In The Clonogenic P Cell Population. Several factors are discussed which complicate the analysis and may influence the results, e.g. reutilization of labelled debris of dead cells from irradiated tumours.