SVC syndrome and pancoast tumor

I'm kind of confused about pancoast tumor and what it causes. FA says it's a common cause of SVC syndrome, but there was a related UW question, i picked pancoast and it was wrong. Their answer was mediastinal mass. (UW #565). What tumors cause mediastinal mass? I can just think of like T cell lymphoma or maybe other types of lymphomas. What does pancoast tumor cause then if SVC syndrome isn't very common? I know about horner's.
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Source Type: forums