The clinical picture and CXR is highly suggestive of a primary lung malignancy with mediastinal lymphadenopathy. 

The patient became significantly hypoxic so underwent CTPA to investigate for potential pulmonary embolism (PE). There was no PE but instead massive mediastinal lymphadenopathy causing compression of the left main bronchus.

Subsequent core biopsy confirmed small cell carcinoma.

Histology

MICROSCOPIC

The sections show multiple small fragments of a cellular neoplasm, much of
which is necrotic. There is a small amount of residual viable tumor, in
perivascular locations, which is composed of cells with pleomorphic,
hyperchromatic oval nuclei with speckled chromatin and indistinct cytoplasm.
There is marked crush artefact.

A series of immunohistochemical stains show that the tumor cells are:
Positive: AE 1/3, CD 56
Negative: Chromogranin, synaptophysin
Ki-67 stains greater than 50% of nuclei in the well preserved areas.

The features are consistent with a small cell carcinoma.

SUMMARY:   Mediastinal mass, core biopsies: SMALL CELL CARCINOMA.