I'm not sure why you're tumor growth was regressed; there are several factors (such as mycoplasma contamination) that can affect the overall outcome. Compare your procedure with the one for commercial CT26 xenografts (http://altogenlabs.com/xenograft-models/colon-cancer-xenograft/ct26-xenograft-model/) and see if there are any substantial differences. Otherwise, if the xenograft doesn't work you might as well outsource the study.
Please if you found the reason for tumor regression, kindly tell me, because I face the same problem. Is female is preferred to male mice? Thanks in advance.
CT26 is a colon carcinoma cell line, so there's not going to be a reason to prefer one sex or the other based on hormonal characteristics (as one would expect in a breast cancer xenograft). Sometimes male mice are more aggressive, and that's one of the reasons researchers prefer to use female ones. If you look at the growth curve from the link I put in my previous answer, you'll see that without any kind of treatment the tumor rapidly grows and within two weeks it reaches 2000 mm^3, the endpoint of most studies. BALB/c mice are not immunodeficient (unless they are athymic) so it's probable that you had very rapid growth in one week, and then the immune system of the mouse attacked the tumor. I would try doing the xenograft in athymic mice or other immunodeficient strains, and that should help improve your results. If not, then you might as well outsource your study to a commercial xenograft provider (such as the one from the link) and that should get you good results. Good luck!
Lots of thank Matt for your informative reply. Unfortunately, we dont have immunodeficient mice in Egypt yet, but whenever they are found, it will be very useful to try.