This paper presents an analytical model to approximate the probability distribution function of the aggregate interference that a primary user receives from multiple secondary transmitters. In particular, we consider heterogeneity in spatial distribution of secondary users such that there are several sites with densely populated secondary users in the whole area. The concentration of secondary users is modeled as an annulus sector with higher user density, which is termed hot zone. The mathematical framework presented in this paper can readily be adapted to various existing interference models. It is observed that the heterogenous user distribution has a considerable impact on the aggregate interference if the hot zone is near the primary receiver, while hot zones over a certain distance is well approximated by a homogeneous secondary user distribution. The aggregate interference also depends on the shape of the hot zone and the interference threshold imposed on the secondary users.