For the development of active millimeter wave imaging systems, e.g. to detect concealed ob- jects on the human body, it is important to be able to simulate some representative scattering con¯gurations. Typically, Gaussian beams are used in active imaging systems. Since these beams only illuminate a spatially limited region, the human body and various objects can be treated as two-dimensional (2D) (in)homogenous cylinders. However, the incident Gaussian beam has a 3D character. Therefore, a 2.5D full-wave Volume Integral Equation (VIE) forward solver is developed: only the cylinder's cross-section is discretized, reducing the number of unknowns strongly, while the incident ¯elds (e.g. oblique plane waves and 3D Gaussian beams) maintain their full 3D character. In this paper, a vectorial Gaussian beam is constructed by using a dipole source in a complex point. This elegant implementation is valid in the near and far ¯eld of the beam. Furthermore, simulation results are compared to measurements to validate the 2.5D numerical scheme. In a ¯rst measure- ment set-up, the scatterer is a long inhomogeneous dielectric cylinder, illuminated by plane waves under di{\textregistered}erent elevation angles at microwave frequencies in the range 1 - 18 GHz. Simulations agree well with the experimental results for normally incident plane waves and plane waves with a small elevation angle, for all measured frequencies. For larger elevation angles, the ¯niteness of the cylinder in°uences the results and decreases the agreement. The second measurement set-up consists of a long te°on cylinder, illuminated by a normally incident Gaussian beam at 94 GHz. The measured incident and total ¯eld amplitudes correspond well to the simulated ones. Hence, the 2.5D algorithm is proven to be a valuable simulation tool to study scattering of long inhomogeneous dielectric objects, illuminated by 3D plane waves or 3D Gaussian beams under di{\textregistered}erent elevation angles.