The use of stepped cannulas, wherein a step structure creates a backstop to reduce backflow along the cannula body, is known for convection enhanced delivery of agents to the brain and other structures within an animal. Described herein are novel and improved stepped cannula designs wherein the length of the cannula segment between the step and the dispensing outlet is variable and can be controllably adjusted inside the patient during delivery. This advantageously allows the operator to place the cannula step at the optimal position and deliver agents at one or more positions within the target structure.