An electrosurgical instrument having a microwave ablation antenna dimensioned to be suitable for insertion into a pancreas via a surgical scoping device, to provide a rapid and accurate alternative to known RF ablation techniques. The electrosurgical instrument comprises: a proximal coaxial transmission line for conveying microwave electromagnetic (EM) energy； a distal radiating portion； and an intermediate impedance transformer arranged to match an impedance of the coaxial transmission line to an impedance of the distal radiating portion, wherein the distal radiating portion comprises a microwave antenna for emitting the microwave EM energy conveyed by the coaxial transmission line, wherein the distal radiating portion has a maximum outer diameter less than an outer diameter of the coaxial transmission line. With these features, the instrument is able to deliver microwave energy via a small diameter structure.
