An electrosurgical apparatus for treating a biological growth that contains a fluid-filled sac, the apparatus comprising: an electrosurgical instrument for delivering microwave energy into biological tissue, the electrosurgical instrument comprising: a coaxial cable for conveying microwave energy; and a radiating tip portion disposed at a distal end of the coaxial cable to receive microwave energy from the coaxial cable; and a fluid delivery mechanism for transporting fluid to and from a treatment zone located around the radiating tip portion, wherein the fluid delivery mechanism comprises: a flexible fluid conveying conduit that extends along the coaxial cable, and a rigid insertion element in fluid communication with a distal end of the fluid conveying conduit and arranged to extend into the treatment zone, wherein the fluid delivery mechanism is arranged: to aspirate fluid from the treatment zone, and to inject a substance into the treatment zone to replace the aspirated fluid, and wherein the substance has dielectric properties selected to facilitate uniform delivery of treatment energy to biological tissue in the treatment zone, and wherein the coaxial cable comprises an inner conductor, an outer conductor, and a first dielectric material separating the inner conductor and outer conductor, and wherein the inner conductor is hollow to provide a passageway for the fluid delivery mechanism. an electrosurgical instrument for delivering microwave energy into biological tissue, the electrosurgical instrument comprising: a coaxial cable for conveying microwave energy; and a radiating tip portion disposed at a distal end of the coaxial cable to receive microwave energy from the coaxial cable; and a coaxial cable for conveying microwave energy; and a radiating tip portion disposed at a distal end of the coaxial cable to receive microwave energy from the coaxial cable; and a fluid delivery mechanism for transporting fluid to and from a treatment zone located around the radiating tip portion, wherein the fluid delivery mechanism comprises: a flexible fluid conveying conduit that extends along the coaxial cable, and a rigid insertion element in fluid communication with a distal end of the fluid conveying conduit and arranged to extend into the treatment zone, a flexible fluid conveying conduit that extends along the coaxial cable, and a rigid insertion element in fluid communication with a distal end of the fluid conveying conduit and arranged to extend into the treatment zone, wherein the fluid delivery mechanism is arranged: to aspirate fluid from the treatment zone, and to inject a substance into the treatment zone to replace the aspirated fluid, and to aspirate fluid from the treatment zone, and to inject a substance into the treatment zone to replace the aspirated fluid, and wherein the substance has dielectric properties selected to facilitate uniform delivery of treatment energy to biological tissue in the treatment zone, and wherein the coaxial cable comprises an inner conductor, an outer conductor, and a first dielectric material separating the inner conductor and outer conductor, and wherein the inner conductor is hollow to provide a passageway for the fluid delivery mechanism.