Is this cyst mediastinal or intrapulmonary in location?

Although not in the characteristic location of either (carina for mediastinal, right lower lobe for pulmonary), this cyst still demonstrates adequate characteristics to be identified as a mediastinal bronchogenic cyst.

# Both axial and coronal images show obtuse angulation between the cyst at the medial surface of the lung, which denote extrapulmonary location.
# Coronal images also show the cyst coming in direct contact with the superior vena cava and upper part of right atrium, as well as with lymph nodes.
# There is no lung tissue (thinned / atelectatic / ventilated) overlying the medial surface of the cyst — rather it comes in direct contact with mediastinal structures as noted above.

The foremost differentials for mediastinal cysts at this age could be germ cells tumors (seminoma, teratoma etc). Absence of solid components, calcifications etc make teratoma a weaker possibility. Female gender almost excludes teratoma.

Distance from esophagus excludes duplication cyst, and distance from vertebral column excludes neuroenteric cyst. It is also away from thymus and pericardium.

CT is often adequate imaging for diagnosis, and treatment may not be necessary unless the cyst creates critical compressive symptoms, or demonstrates malignant changes.