It is often difficult to come up with a well-principled approach to the selection of a spatial indexing mechanism for medical image databases. Spatial information about lesions in medical images is critically important in disease diagnosis and plays an important role in image retrieval. Unfortunately, the images are rarely indexed properly for clinically useful retrieval. One example is the well-known R-tree and its variants which index image objects based on their physical locations in an "absolute" way. However, such information is not meaningful in medical content-based image retrieval systems, and the approaches above suffer from problems caused by variations in object size and shape, imprecise image centering, etc. A more appropriate approach, which does not require object registration, is to model the spatial relationships between the lesions and anatomical landmarks. To convey diagnostic information, lesions must exist in certain locations with regard to the landmarks. In this paper, we show that the histogram of forces (which represents the relative position between two objects) provides an efficient spatial indexing mechanism in the medical domain.