Effective Dose Reduction Radiation Biology Paper

A patient 20 cm thick undergoes a chest x-ray. The dose to the surface of the patient is 5.0 mGy. Assume that the dose is attenuated through the patient by the narrow beam attenuation law with a mass attenuation coefficient of ??? = 0.200 cm2/g2 (corresponding to an x-ray beam with effective energy 60 keV). What is the effective dose reduction when the patient stands facing away from the x-ray tube (with breast dose attenuated at exit surface) compared to when he stands facing the tube (with breasts exposed to high dose at entrance surface). Consider that the dose to the lung and to the oesophegus remain the same in both orientations at 0.75 mGy; no other organs receive a significant dose.