A patient presents with mediastinal mass with sheets of epithelial cells giving arborizing pattern in Asia of reactivity on keratin, alongwith interspersed lympoid cells. The apt diagnosis would be:

Thymoma Tumours made up of to two different lineage of cells i.e. lymphocytes and epithelial cells suggests the diagnosis of thymoma (Keratin positively indicates the presence of epithelial cells) Thymomas are benign tumours of mediastirum. They are composed of two types of cells i.e. epithelial cells and variable infiltrate of lymphocytes. The thymomas Type A Thymoma composed of bland spindle to oval cells with few to no lymphocyte Type AB Thymoma with a mixture of lymphocyte poor and lymphocyte rich areas Type B1 Thymoma exihibiting areas of both normal thymic coex and medulla. Type B2 Thymoma with prominent large epithelial cells within admixture of numerous lymphocytes. Type B3 Thymoma composed of sheets of epithelial cells with little atypia and scant lymphocytes. Thymic carcinoid In these cases the tumour is composed of neuroendocrine cells. On histochemistry neuroendocrine cells will be positive for chromogranin, synaptophysin.