A shaft assembly (20) for a microwave ablation applicator which includes a shaft assembly (20) and an antenna assembly (26) located within the shaft assembly (20) is disclosed. The shaft assembly (20) comprises an elongate shaft (22) which extends from a first end (201) to a second end (202) thereof, and which defines therein a hollow inner volume and a longitudinal axis of the antenna assembly, and an applicator tip (24) mounted on the second end (202) of the elongate shaft. The shaft assembly (20) further includes a coolant delivery (80) tube having a side wall which defines an elongate hollow interior (82). The coolant delivery tube (80) extends along the inner volume of the shaft (22), coaxial therewith, and has a first end towards the first end (201) of the shaft (22) for reception of coolant fluid therein, and a second end towards, and spaced from, the second end (202) of the shaft (22), the second end of the coolant tube providing a first fluid flow path from the interior of coolant tube into the inner volume of the shaft adjacent the applicator tip. The coolant delivery tube (80) defines a return aperture (91) through the side wall thereof, which return aperture (91) is located between the first and second ends of the coolant delivery tube (80) within the elongate shaft, and which return aperture (91) provides a second fluid flow path (92) from the interior (82) of the coolant delivery tube (80) into a return volume (84) of the inner volume of the shaft.
