An electrosurgical system includes a feedline (15) including an inner conductor (220), an outer conductor (224) and a dielectric material (222) a probe (100) operably coupled to the electrosurgical energy source (28) by the feedline (15),the probe (100) including:an antenna assembly (12) defining a feed gap ("G") between a balun structure ("B") and a distal radiating portion ("R")and a flexible, extendable/retractable fluid-cooled shaft (410) coupled in fluid communication with an interior lumen (338) defined by the probe (100), includes:an inlet sleeve (431)an outlet sleeve (435)an inner tubular member (441) coaxially disposed about and slideably coupled to the inlet sleeve (431) andan outer tubular member (445) coaxially disposed about and slideably coupled to the outer sleeve (435), so that the extendable/retractable fluid-cooled shaft (410) is selectively adjustable relative to the feedline (15) between a first length (L5), and a second length (L6).
