No. NSG mice are already immunologically null (B cell null, T cell null, NK cell cytotoxic activity null) , so it is not necessary to irradiate them to get successful engraftment of xenografts. Of course, if you are trying to measure the effects of irradiation on the grafted tumor in vivo, then you would have to irradiate the engrafted tumor in the NSG host. You should measure the number of cells engrafted, the size of the tumor freshly engrafted, and then the tumor size growth curve as a measure of tumor growth. NB, only a small fraction of the tumor cells will be cancer stem cells (CSCs), so initial tumor growth will be slower than that of the CSCs. The growth curve(s) should be plotted on a semilog scale.

Thank you @Christopher Lange for your answer. I think I would go with not irradiating them as well as my purpose is just to test the in-vivo efficacy of test compounds on these mice and the effect on their overall survival and disease progression.

Radiation is not necessarily. The xenotransplantation model of these cell lines by IV or IF transplantation will give you a systemic leukaemic model with bone marrow engrafment, enlarged spleen and also you will find leumaemic cells in the liver and leptomemimgies. MV4,11 tends to give sometimes leg tumours as well. So the effect of your drug should be in those relevant leukaemic compartments if you need to analyse them and not only rely on the IVIS or the peripheral blood. If you were transplanting a primary AML sample from a patient or a PDX then Christopher is correct that a small fraction will be a cancer stem cell. But in your case each cell should be a cancer stem cell because it is AML cell line so each cell is a leukaemic initiating cell (LIC).

Thank you Hasan Issa for your reply.