Electron cyclotron waves enable precise control of fusion plasmas through resonant modification of the electron distribution function, with applications including heating, current drive, and control of magnetohydrodynamic instabilities such as neoclassical tearing modes and sawteeth. This work employs canonical perturbation theory to derive analytical expressions for electrons’ extreme variations of canonical momenta, resonantly interacting with spatially localized radio frequency (RF) beams in magnetized plasmas. The underlying analytical framework captures the complete dependence of the wave-particle interaction on both electron kinetic characteristics and RF beam parameters, including wave mode, frequency, polarization, beam waist, and angle of incidence with respect to the magnetic field.