  Microwave hyperthermia aims at selectively heating cancer cells to a
supra-physiological temperature. For non-superficial tumors, this can be
achieved by means of an antenna array equipped with a proper cooling system
(the water bolus) to avoid overheating of the skin. In patient-specific
treatment planning, antenna feedings are optimized to maximize the specific
absorption rate (SAR) inside the tumor, or to directly maximize the temperature
there, involving a higher numerical cost. We present here a method to effect a
low-complexity temperature-based planning. It arises from recognizing that SAR
and temperature have shifted peaks due to thermal boundary conditions at the
water bolus and for physiological effects like air flow in respiratory ducts.
In our method, temperature focusing on the tumor is achieved via a SAR-based
optimization of the antenna excitations, but optimizing its target to account
for the cooling effects. The temperature optimization process is turned into
finding a SAR peak position that maximizes the chosen temperature objective
function. Application of this method to the 3D head and neck region provides a
temperature coverage that is consistently better than that obtained with
SAR-optimization alone, also considering uncertainties in thermal parameters.
This improvement is obtained by solving the bioheat equation a reduced number
of times, avoiding its inclusion in a global optimization process.
