Is serous cystadenoma is a tumour or a cyst ? do they all look exactly the same inside and outside?

Ovarian Tumors: A serous cystadenoma is a benign ovarian tumor which is a sac filled with fluid, hence it is a cyst, although a tumerous cyst. They vary considerably in size, but histologically they are all similar. Depending on their growth progression, they should probably be removed surgically, but it sounds like your's may have already been excised. If the pathologist found only benign cells, then is cured.