Microwave thermal ablation therapy is a minimally invasive technique introduced in the interventional oncology practice to treat a range of cancerous pathologies. Whereas satisfying results are obtained with the treatment of large and relatively homogeneous areas (e.g. hepatic tumours), treatments of small and inhomogeneous targets are currently under investigation. Minimizing the transversal dimension of applicators represents a crucial aspect in the case of sensitive structures (e.g. blood vessels) surrounding target area. Despite several improvements being proposed, a minimal invasive applicator suitable for small targets adjacent to crucial structures remains an unsolved issue, so far. A proposal to achieve a compromise between a minimally-invasive applicator geometry and a focused thermal pattern is presented in this work. The idea concerns exploiting insulator properties of fat layer, which normally coats the organs in the abdominal cavity. In this scenario, fat tissue is used to focus the heating pattern in the target tissue. Low effective conductivity of fat tissue induces a low absorbed power of the tissue and a consequent low heating of the area. Based on these evaluations, fat layer is also used to avoid unnecessary and potentially dangerous overheating of surrounding healthy structures.