Lung Cancer (Bronchogenic Carcinoma): This is the most common cause of a perihilar mass, given the proximity to the major airways and the high incidence of lung cancer. The perihilar region is a common location for lung tumors to develop, especially those originating from the main bronchi or the upper lobes.

Lymphoma: Both Hodgkin's and non-Hodgkin's lymphoma can present with a perihilar mass due to involvement of the mediastinal or hilar lymph nodes.
Sarcoidosis: This autoimmune disease can cause granulomatous inflammation in the lungs, leading to the formation of masses or nodules in the perihilar region.
Tuberculosis: Although less common in some regions, tuberculosis (TB) can cause perihilar masses due to caseating granulomas or lymphadenopathy.

Pulmonary Embolism: While not typically considered a "mass," a large pulmonary embolism can cause significant hilar enlargement or a mass-like appearance on imaging, which could be mistaken for a tumor. Missing this diagnosis could be fatal.
Mediastinal Cysts: Congenital or acquired cysts in the mediastinum can present as perihilar masses and, although usually benign, their location can cause significant symptoms or complications if ruptured or if they compress vital structures.

Pulmonary Arteriovenous Malformation (AVM): These are abnormal connections between the pulmonary arteries and veins, which can appear as a perihilar mass on imaging.
Hamartomas: Benign lung tumors that can occasionally present as a perihilar mass, although they are more commonly found in the peripheral lung.
Castleman Disease: A rare lymphoproliferative disorder that can cause perihilar masses due to lymph node enlargement.
Intrapulmonary Teratoma: A rare type of germ cell tumor that can occur within the lung and present as a perihilar mass.