There is provided a new use of microspheres comprising a water-insoluble, water-swellable polymer which is anionically charged at pH7, and electrostatically associated with the polymer, in releasable form, a cationically charged chemotherapeutic agent, in the manufacture of a composition for use in the treatment of a brain tumour, wherein in the treatment the composition is introduced into the brain and the chemotherapeutic agent is released from the microspheres, wherein the microspheres, when equilibrated in water at 37° C., comprise at least 40 wt % water based on weight of polymer plus water. Compositions comprising the microspheres and methods for the treatment of brain tumours are also provided.