I have not used an addition of feeder cells but the growth of the tumor and the efficiency of the tumor formation after injection can be increased by mixing tumor cells with a small volume of Matrigel (isolated extracellular matrix from EHS sarcoma from BD Biosciences). It will polymerize and form a small plug of extracellular matrix improving the conditions for the development of the tumor cells increasing the probability of the tumor growth and the initial ratio of its growth.