Hacettepe University Faculty D~Partment of Radiology Computed Tomography

T.C. HACETTEPE UNIVERSITYFACULTY OF EDICI, E D~PARTMENT OF RADIOLOGY DIAGNOSIS OF THYMOMA BY COMPUTED TOMOGRAPHY Dr. 11. Naz t r Far 1d 19YL Ankara ~5394 - .... '::~·.. - . -~ .· : . Contents 1- Introduction. 1 2- Anatomy of the tl1y::::us. .. ................ 5 3- Subjects and met11ocs. 19 4- Results. 22 5- Dlscussion. 24 6- Conclusion. • ..•............... -... ' ...• ' ..... 3 1 7- References. ' .......................... ,' ... 3 3 (t) Introduction Thymomas are solid neoplasms that occur in association with myasthenia gravis or others less common conditions ( red cell aplasia,hypogammaglob~linemia) or sporadically .. Because of the proximity of the thymus gland to the major mediastinal vascular structure, small thymic lesions often are not detectable on plain chest radiography or conventional tomography. So as a result computed tomography of the mediastinum sho~ld be the imaging procedure of choice ·when thymic pathology is suspected (1). The 10-15% incidence of thymomas in patients with myasthenia gravis has led to routine CT screening of the ·anterior superior mediastinum in many of these patients, especially if these patients recently exacerbated the symptoms. Comput~d tomography also "is r~commended to search for a thymic·carcinoid whenever there is clinical suspicion of ectopic adrencorticotrphic hormone (ACTH) production causing cushing syndrome (2). The presence of thymoma must always be suspected in patients with myasthenia gravis because about 10-15% will have such a tumor (4). Accuracy of radiologic detection of thymoma in association with myasthenia gravis has varied widely (2-7). The evaluations is complicated because fats cpntents of thymus increase with the age- and thymic size is highly variable (8-12). Other method including atteriography (13) Thymic venography (14) Pneumomediastinography (15) ultrasonography (16) and several other methods have been ( 2) found useful in some cases but none of these methods have been ~idely accepted for screening patients with myasthenia gravis. The ability to predict a thymoma noninvasively in such patients would be beneficial .. Since all these patients are not referred for thymectomy , atest that could reliably predict the presence or absence of a thymoma would be of great assistance in ·the management of these patients. For the patients having thymectomy for the treatment of the disease, operative planing and counselling would benefit from a pre operative diagnosis of thymoma. Computed to~ography is a sensitive and specific diagnostic procedure for detection of thymoma in patients with myasthenia gravis. Lesions as small as 1 em in diameter can be depicted. (1,3). In this study the patients suffering from myasthenia gravis were sent to the depart.ment: of radiology for computed tomographic examination, before undergoing to thymectomy. Histopathologic re~ults we~e compared ~ith those obtained at CT. !he results indicatA that the computed tomography is an imRortant diagnostic tool in detecting thymic abnormalities. ·Thymus G 1 and The thymus is one of the two central organs of the lymphoid system and is responsible. for the provision of the thymus processed lymphocytes (T-lymphocytes) to the whole body. In varies considerably in size and activity depe~ding upon age, (3) disease, and physiological state of the body, but.remains active even upto old age (17). A~ birth it weighs 10-15 gm gro~ing until puberty upto 30-40 gm. There after it gradually diminishes and infiltrated by adipose tissues. Although.it is described as a single median organ each lob8 develops from the third pharangeal pouch of th8 same side, so strictly they are left and right thymic bodies. The thymus lies in the superior and anterior inferior mediastinum extending inferiorly to the fourth costal cartilages. Its upper. part taperin& into the neck and sometime reaching to the inferior pole of the thyroid gland or even higher. Its shape is largely due to the adjacent structure to which it is moulded C17J BLOOD SUPPLY. The arteries are derived mainly from the internal thoracic and inferior thyroid arteries branches which also supply the surrounding mediastinal connective tissue. After entering the thymus gland.proper these vess~ls course along the corticomedullary junction giving off irregular branches to the medulla and small radially arranged arterials. Venules retrace the art~riolar paths converging to principal veins i'n the intralobar septa, from there the venous return is partly via capsular plexes, the venules from the small thymic veins which ultimately drain into left brachiocephalic vein C17J. (4) THE BLOOD THYMUS-BARIER. When colloidal substances are injected intravenously, they fail to penetrate the ·extrav~scuiar spaces in the thymic cortex, where lymphocytes are proliferating, sugge~~ing that the walls of the thymic ~lood vessels may bar antigens and other substances from passing into the thymic ~issue. This tissue would thus be an immunologically sequestrated site for the differentiation of lymphocytes [17]. LYMPHATIC DRAINAGE Afferent lymphatics are absent from the thy~us but the efferent lymphatics arising from the medulla and corticomedullary junction drain though the extravascular spaces in company with the arteriep and veiris entering and leaving the thymus [17]. NERVE SUPPLY The thymic innervation is derived from the sympathetic chain via the cervicothoracic (Stellate) ganglion and vagus. Branches from the phrenic nerve and descendents ~ cervicaliS are distributed mainly· to the capsule of the thymus [ 1 7 J. / ( .s) ..... ANATOMY OF THE THYMUS GLAND BY COMPUTED TOMOGRAPHY On computed tomography the normal.. thymus can be recognised in a large percentage of cases especially those younger than 40 years of age C10J. Upto 30% of the individual have a fat cleft distinctly dividing the thymus into two lcbes on computed tomog~aphy. In most of the remainders the lobes are confluent and thymus assumes an arrowhead (or triangular) configration Cfig) The width and and thickness of each lobe can be measured on 6omputed tomography. \.. ' ~' Where T stands for thickness and W for width. ( 6) 'Although the width of the thymus is quite variable among different age group, The thickness of the thymus shows a definite decrease in size· with advancing age. The maximum thickness of either lobe is 1 .8cm prior to age 20 Thereafter, 1.3 cm:is a maximal thickness although most normal lobes are less than o.a em in thickness. In pediatric and adolescent age group the density of the thymus is similar to that of chest wall muscles C10,18J. After puberty, a gradual progressive infiltration of the fat occur into thymic paranchyma. Usually after the age of fo~ty the organ approaches/pure fat in attenuation value. Only the sparse small nodular densities oi thymic paranchyma may remain. In thin patients no recognised thymic remnant may be visible. ( 7) PATHOLOGY OF THE THYMUS GLAND 1- Thymic hyperplasia 2- T~ymic hypoplasia 3- Thymic tumors .. Traditionally all thymic tumors have been n'arned as thymomas. However the term thymoma is restricted to the neoplasms of thymic epithelial cells. At the same time thymoma may have lymphoid component of non neoplastic origin , presumably reactive lymphocytes, on this basis the thymoma have been classified as follow. a. Predominan~ly lymphocytes (40%) b. Predomina~~ly epithelial (40%) c. Mixed epithelial and lymphocytes (20%) The other variety of thymic tumors are as £allow. 1- Hodgkin disease 2- Lymphomas 3- Teratomas Thymic Hypoplasia. ( Digeorge's Syndrome) In this diSbrder the thymus is usually rudimentary and T&B cells are deficient or ;bsent in the circulation. At the same time these are deficient in the thymus dependent area of lymph nodes and spleen [19]. The infants with this defect are (8) vulnerable to viral and fungal infections, requiring T&B cells cooperation for the synthesis ?f protective antibodies. THE THYMIC HYPERPLASIA AND TUMORS Hyperplasia of the thymus is characte~ized by ~he appearance of lymphoid follicles within the thymus .. The thymus hyperplasia or tumors or both have been associated with many seemingly unrelated systemic disorders. These disorders can be classified as follow, 1- Neuromuscular disorders 2- Hematological disorders 3- Immunity disorders Perhaps the best known association is with myasthen~a gra~is. The involvement of thymus in my~sthenia gravis is 75%. Out of ' ' this 75% cases 85% will have thymic hyperplasia and 15% will have thymomas [5,20,21]. Conversely about 44% Of the patients I with thymoma have myasthenia gravis C21J. Anemia, granulocytopenia, thrombocytopenia and agammaglobulinemia, all have'been associated with thymic hyperplasia and more often with thymoma. Improvement or even remission have been known to follow thymectomy (22]. MYASTHENIA GRAVISj(MG). (9) ~YASTHEN!A GRAVIS;(MG). Myasthenia gravis is a disorder of neuromuscular junction with a tendency to remission and relapses. Increased fatiguability and weakness OT ~he' voluntary muscles are the dominant finding. Antibodies directed against acetylcholin receptors interfering at the skel~tal muscles motor end-plate . have been demonstrated.It has been also demonstrated that in myasthenia gravis there is severe reduction ·in acetylcholin receptors doe to an autoimmune reaction directed against the receptors protien C24J. IgG antibodies against acetylcholin have been found in 70% cases of these patients C25J. Thoug0 there is no correlation between the percentage bf these receptors and severity of the disease suggesting that the factors other than antibodies may also

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