Minimally Invasive Thermal Therapy (MITT) is an effective way for the treatment of localized cancer and could replace surgery, chemotherapy or radiation. During MITT, high temperatures in the range of 55-95 °C are produced locally in the target tissue or tumour, resulting in localized protein coagulation. A real-time imaging method is required to guide the procedure of thermal therapy. Ideally, this imaging modality should be noninvasive, inexpensive and easily used and interpreted. It is known that acoustic attenuation is sensitive to both the tissue temperature and the structural changes due to protein coagulation (the endpoint of any thermal therapy treatment) during thermal therapy. Transmission ultrasound imaging is a real-time imaging modality which measures the attenuation property of ultrasound. The goal of this work is to demonstrate the potential of ultrasound attenuation imaging during MITT to quantitatively monitor lesion formation dynamics. An important finding of the present study is that the temporal changes in acoustic attenuation during MITT follow a reproducible pattern in albumen phantoms and bovine liver tissue within the range of thermal therapy temperatures. After heating, the measured attenuation remains higher than the initial amount, suggesting that this irreversible increase is a result of the structural change due to protein coagulation.