Would you irradiate the breast of a patient with an unknown primary malignancy manifesting as bulky axillary adenopathy and non-specific poorly differentiated carcinoma histology?  

Incidences of occult breast primary has reduced with the advent of MRI imaging. If the IHC of the node doesn’t support breast primary (GATA 3 or mammoglobin are breast specific markers), then I would not treat the breast.

@Sushil Beriwal's answer is on point, particularly given the non-specific nature of the pathology in the present case. Additional implied points are to ensure that contralateral breast is assessed (less of an issue in MRI era) & that disease is non-metastatic (in which case breast XRT for occult...