Method and composition suitable for administering by direct convective interstitial infusion are disclosed. The method comprises: placing at least one direct convective interstitial infusion catheter in contact with the tissue, and pressuring the composition through the catheter. The composition is in a liquefied form having a viscosity above a predetermined value. The predetermined value is selected so as to improve infusion of the compositions into interstitial volumes of the tissue, while minimizing backflow of the compositions along an outer wall of the catheter or leakage into low resistance paths. It is demonstrated that high viscosity results in higher treatment efficiency.