Bing WPF radius of a longitude and Latitude point

I have multiple locations which are pin pointed on my Bing Map WPF by longitude and latitude coordinates, I know how to measure distance between two set locations and display all pins at once. But in order to be more efficient and reduce needed lag on the map I want to show only points within a certain distance of a fixed point.

For example Big ben at 51.500733, -0.124658 was my fixed point and if another one of my locations was in lets say in a 5 mile radius e.g. London eye 51.503364, -0.119522 it would show but anything out side of radius would not. Currently the process is activated when the user click a single button to one of my points which it then displays a pushpin. Currently I concentrating on the theory of using a If statement within VB to filter out a range of Longitude and Latitude coordinates for set locations.

Any of ideas on improving my method or different takes would very beneficial.