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In this study, a highly sensitive surface plasmon polariton (SPP)-based sensor with a metal-insulator-metal (MIM) configuration is proposed and numerically investigated for multifunctional detection, including refractive index sensing, breast cancerous cell discrimination, and temperature sensing. The sensor consists of an air-filled MIM waveguide integrated with a triangular cavity, further optimized by embedding an internal elliptical silver island to enhance light-matter interaction. Finite element method simulations reveal that the elliptical cavity design significantly improves transmission characteristics, extinction ratio, and RI sensitivity (RIS) due to the increased effective optical path length. The optimized structure achieves a RIS of 1207.14±4.12nm/RIU and a detection resolution as low as 8.28×10^-7RIU. Furthermore, the sensor demonstrates the capability to distinguish between normal and malignant breast cells, making it an advantageous platform for biomedical diagnostics. Additionally, the sensor exhibits a significant temperature sensitivity of 0.945±0.003nm/K. The proposed design also shows strong potential for nanofabrication via techniques such as electron beam lithography, enabling its integration into compact optoelectronic and biosensing devices.