A system for monitoring ablation size is provided and includes a power source including a microprocessor for executing at least one control algorithm. A microwave antenna is configured to deliver microwave energy from the power source to tissue to form an ablation zone. A radiation detection device is operably disposed on the microwave antenna. The radiation detection device is configured to generate a voltage corresponding to a radius of the ablation zone, wherein the radiation detection device is in operative communication with at least one module associated with the power source. The at least one module triggers a signal when a predetermined threshold voltage is measured corresponding to the radius of the ablation zone.