The right costo-phrenic angle is blunted. Could this be due to overlying breast shadow?
Certainly not! The lateral half of the right hemi-diaphragm is obliterated. That indicates something that is sitting or abutting the diaphragm. Notice that there is fluid in the right horizontal fissure. Thus, the blunted CPA is most probably due to pleural effusion.
Why can't it be breast shadow? Breast and diaphragm are in different planes. Breast shadow will never obliterate the diaphragm.