A method of selectively treating interstitial tissue following microwave thermal therapy includes an intraurethral Foley-type catheter shaft. The catheter shaft contains a microwave antenna which is capable of generating a cylindrically symmetrical thermal pattern in tissue, within which temperatures are capable of exceeding 45.degree. C. The antenna is surrounded by means within the shaft for asymmetrically absorbing electromagnetic energy emitted by the antenna so as to concentrate thermal energy on a first side of the shaft. This asymmetrical absorption alters the thermal pattern generated by the microwave antenna, making it possible to focus microwave thermal therapy toward undesirous tissue and away from healthy tissue. The absorption means is also capable of absorbing heat from adjacent tissue by thermal conduction. Operation of the absorption means upon the conclusion of microwave thermal therapy aids in reducing edema of the treated tissues caused by the elevated temperatures of those tissues.