The theoretical efficiency of solar thermophotovoltaic (STPV) systems is much greater than their efficiencies achieved in practice. Optical cavities can improve the performance of STPV systems by increasing the emitter-to-PV cell view factor and by facilitating photon recycling, whereby photons are reflected back to the emitter. Photon recycling reduces losses and increases the temperature of the emitter, thereby increasing efficiency. Our study presents STPV systems comprising optical cavities in the form of oblate and prolate spheroids. The geometry of the optical cavity can be tuned to control the degree of photon recycling, emitter temperature, emission losses, and the emitter-to-PV cell effective view factor and separation distance without using complex nano- or microstructured materials or optical filters. Numerical analysis shows an optical cavity in the form of a prolate spheroid, prolate spheroid with a middle annular aperture specular reflector, and integrated oblate- and prolate-spheroid can be used to achieve efficiencies of 17.7%, 18.9%, and 22%, respectively, under solar irradiation at a concentration factor of 1500X. These robust spheroid-based optical cavities can be used to design improved STPV systems with increased durability and higher performance.