A direct contact microwave hyperthermia applicator includes a plurality of rectangular waveguide sections each having a radiating aperture. The waveguide sections are mounted with their long sides abutting. A pair of shutter elements for individually varying the radiating aperture is associated with each of the waveguide sections. The shutters can be adjusted to form a desired composite aperture which produces a radiation pattern adapted for efficient hyperthermia treatment of a tumor having a prescribed size, shape and location. Microwave power supplied to the radiating elements can be varied in phase and power level to further control the radiation pattern.