Abstract: Flexible treatment catheters are configured to be inserted into a body lumen or cavity to deliver heated fluid through at least one fluid lumen therein to thermally treat or ablate a targeted site in a biological subject. The flexible treatment catheters can include improved thermally insulated regions comprising a mixture formed of liquid elastomeric (such as a polyurethane) mixture (which transitions to a solid state) and miniaturized hollow plastic bodies positioned between the outer wall of the treatment catheter and the fluid lumen. The thermally insulated region is configured such that, measured ex vivo, the treatment catheter can present a maximum temperature of below about 45° C. on the external surface of the outer wall of the treatment catheter when the treatment catheter circulates fluid having an inlet temperature of above or between about 60°-62° C.