Integrated sensing and communication (ISAC), near-field communication, and extremely large-scale antenna arrays are transformative and indispensable technologies that revolutionize 6G communications. This work proposes a monostatic ISAC architecture for near-field systems employing extremely large-scale uniform planar arrays (XL-UPAs). A theoretical analysis of multi-user interference and sensing-over-communication interference is provided for the considered near-field ISAC system, where the sensing and communication channels are modeled using a non-uniform spherical wave (NUSW) model. In addition to communication, the monostatic ISAC system must ensure simultaneous target detection with a constant probability of false alarm, independent of noise statistics. A grid-based two-dimensional constant false alarm rate (2D-CFAR) detection scheme is proposed to address the issue. Furthermore, an analytical expression is derived to quantify the achievable range resolution in near-field ISAC systems.