Caco2 xenograft model in radiotherapy

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Caco2 xenograft model in radiotherapy

I am looking to check the radiosensitivity of my Caco2 cell line with a growth factor knockout. I had stably transfected with shRNA. This knockout resulted in increased radiosensitivity in vitro (cell survival assay and clonogenic assay).

Now I am going to check this in xenograft model (CD1 Nu/Nu). I am looking for some suggestions on how much cells (Caco2 cell line) to inject. As previously reported I had injected 5 million cells in Rag1 mouse. But even after 1month it didn't implant any tumour. 
So now I have following questions:

1). How much cells to inject? Knockout cell line grows very slowly compared to empty vector transfected (control)? Do I need to inject more cells compared to control?

2) We have Cs-137 whole animal gamma irradiator. We have a special lead shield to cover the whole mouse and expose only flanks. I had come across various research articles using either fractionated irradiation or single irradiation.
Can anybody tell me which suits my purpose? Fractionated (at what dose) or single irradiation (at what dose)

Thanks in advance,