There are three principal histological types of thymoma, depending on the appearance of the cells by microscopy:

Type A if the epithelial cells have an oval or fusiform shape (less lymphocyte count);
Type B if they have an epithelioid shape (Type B has three subtypes: B1 (lymphocyte-rich), B2 (cortical) and B3 (epithelial). );
Type AB if the tumor contains a combination of both cell types.Thymic cortical epithelial cells have abundant cytoplasm, vesicular nucleus with finely divided chromatin and small nucleoli and cytoplasmic filaments contact adjacent cells. Thymic medullary epithelial cells in contrast are spindle shaped with oval dense nucleus and scant cytoplasm
thymoma if recapitulates cortical cell features more, is thought to be less benign. Diagnosis
When a thymoma is suspected, a CT/CAT scan is generally performed to estimate the size and extent of the tumor, and the lesion is sampled with a CT-guided needle biopsy. Increased vascular enhancement on CT scans can be indicative of malignancy, as can be pleural deposits. Limited biopsies are associated with a very small risk of pneumomediastinum or mediastinitis and an even-lower risk of damaging the heart or large blood vessels. Sometimes thymoma metastasize for instance to the abdomen.The diagnosis is made via histologic examination by a pathologist, after obtaining a tissue sample of the mass. Final tumor classification and staging is accomplished pathologically after formal surgical removal of the thymic tumor. Selected laboratory tests can be used to look for associated problems or possible tumor spread.
These include: full blood count, protein electrophoresis, antibodies to the acetylcholine receptor (indicative of myasthenia), electrolytes, liver enzymes and renal function. Staging
The Masaoka Staging System is used widely and is based on the anatomic extent of disease at the time of surgery:
I:   Completely encapsulated
IIA: Microscopic invasion through the capsule into surrounding fatty tissue
IIB: Macroscopic invasion into capsule
III: Macroscopic invasion into adjacent organs
IVA: Pleural or pericardial implants
IVB: Lymphogenous or hematogenous metastasis to distant (extrathoracic) sites

Treatment
Surgery is the mainstay of treatment for thymoma. If the tumor is apparently invasive and large, preoperative (neoadjuvant) chemotherapy and/or radiotherapy may be used to decrease the size and improve resectability, before surgery is attempted. When the tumor is an early stage (Masaoka I through IIB), no further therapy is necessary. Removal of the thymus in adults does not appear to induce immune deficiency. In children, however, postoperative immunity may be abnormal and vaccinations for several infectious agents are recommended. Invasive thymomas may require additional treatment with radiotherapy and chemotherapy (cyclophosphamide, doxorubicin and cisplatin).. Recurrences of thymoma are described in 10-30% of cases up to 10 years after surgical resection, and in the majority of cases also pleural recurrences can be removed. Recently, surgical removal of pleural recurrences can be followed by hyperthermic intrathoracic perfusion chemotherapy or intrathoracic hyperthermic perfused chemotherapy (ITH). Prognosis
Prognosis is much worse for stage III or IV thymomas as compared with stage I and II tumors. Invasive thymomas uncommonly can also metastasize, generally to pleura, bones, liver or brain in approximately 7% of cases.