This paper extends the previous work on scatter problems by Barret and Swindel [ Radiological Imaging, Vols 1 and 2. Academic Press, New York (1981)], using formal transport theory based on the Boltzmann equation: we focus on the various aspects of photon scattering of a thin pencil beam incident on a uniform slab of a scattering medium (water, soft tissue, bone, etc.), where the observational plane (image plane in radiology) is positioned behind the exit face of the slab. Expressions are derived for the number of scattered photons, the scatter fraction and the scattered mean photon energy, and numerically evaluated for various input parameters.