Surgery Today Jpn J Surg (1994) 24:72-74

@ SURGERYT~Y © Springer-Verlag 1994

Focal Thymic Hyperplasia in an Adult: Report of a Case

TSUYOSHI YOSHITAKE, 1 SHINGI ITOYAMA, 2 ATSUKO MASUNAGA, 2 and ISAMU SUGAWARA 2

The Departments of 1 Surgery and 2 Pathology, Saitama Medical Center, Saitama Medical School, 1981 Tsujido, Kamoda, Kawagoe, Saitama, 350 Japan

Abstract: We report herein an unusual case of a 35-year-old Japanese man who underwent thoracotomy for a mass in the mediastinum which was found to be a well circumscribed localized tumor within the thymus. Focal thymic hyperplasia in adults is extremely rare and the clinicopathological features of this case were particularly interesting and posed great difficulty in establishing a diagnosis.

Key Words: focal thymic hyperplasia, adult, clinicopathology

Introduction

Focal thymic hyperplasia without any associated dis- ease is thought to be extremely rare. Its diagnosis requires careful examination of the excised material, especially of the cellular components and histological architecture. We present herein the case of a 35-year- old man with focal tumor-like thymic hyperplasia, focusing on the histopathological features and clinico- pathological characteristics of this interesting case.

Case Report

A 35-year-old man was referred to our hospital in July 1987, 1 month after presenting at a local clinic for a cough, where a chest X-ray showed an abnormal mediastinal mass. He was one of monozygotic twins but his brother had no disease and there was no family history of any autoimmune disorder.

On admission, the patient was asymptomatic and physical examination revealed no abnormal findings. The white cell count was 8,000/mm 3 with 50% lympho-

Reprint requests to: T. Yoshitake (Received for publication on Aug. 19, 1992; accepted on Mar. 12, 1993)

cytes. Roentgenogram and computed tomographic scan of the chest revealed a mass shadow adjacent to the right cardiophrenic angle (Figs. 1 and 2). Based on these graphical data, a preoperative diagnosis of either a pendulous thymoma or pericardial cyst was made.

Thoracotomy through a right anterolateral incision in the sixth intercostal space revealed an encapsulated oval and parenchymatous mass pendulous to the right thymic lobe which was easily resected. The excised specimen measured 10.5 x 7.5 x 2.5cm, weighed 135 g, and was well encapsulated with a small area of normal thymic tissue at the periphery. The cut surface showed a slight yellowish and soft parenchymatous appearance with indistict lobulation (Figs. 3 and 4).

Histologically, the tumor contained all the funda- mental components of the thymic gland, although dis- tortion and disproportion was observed. The thymic epithelium was generally hypertrophic with a relatively loose network, occasionally extended as fine strands or minute cystic dilatation. The cortex was also well developed and defined, but the density of lymphocytes did not exceed that of the normal thymic gland of this patient. The order between the parenchymal and interstitial areas was as distinct as the normal thymic gland, but the interstitial tissue was composed of loose fibrous or fibroadipose tissue in contrast to the mature adipose tissue of the normal thymic gland (Fig. 5). Considering these macroscopic and microscopic find- ings, a diagnosis of focal thymic hyperplasia was made.

The patient had an uneventful recovery and the lymphocytosis disappeared soon after surgery. He is currently in excellent health 5 years after his operation.

Discussion

True thymic hyperplasia without any associated disease is very rare and seen mostly in infants or children before adolescence, a-2 although there have been a

T. Yoshitake et al. : Focal Thymic Hyperplasia 73

Fig. 1. Posteroanterior chest roentgenogram shows a jib-like homogenous shadow obliterating the border of the right cardiophrenic angle. The mass shadow disappeared in the left recumbent position

Fig. 3. The gross resected specimen (posterior view) showing an encapsulated parenchymal tumor, oval in shape, con- necting with the normal thymic tissue

Fig. 4. The cut surface shows soft parenchymal tissue with indistinct lobulation

Fig. 2. Computed tomographic scan of the chest at the cardiac level shows a circumscribed oval mass with a soft tissue density separated from the heart shadow

few reports in the English literature of true thymic hyperplasia in adults following chemotherapy for malignant diseases. 3-4 The patient reported herein had no disease besides the mass lesion in the thymic gland.

True thymic hyperplasia is characterized by an in- crease in both the size and weight of the thymus with a normal microscopic appearance for age. 5 However, the thymic hyperplasia of our patient showed some different characteristics from true thymic hyperplasia,

in the form of a well circumscribed tumor-like hyper- plasia distingushed from the adjacent thymic gland which seemed to be well preserved for its age.

The surgical specimen showed all the fundamental components of the thymic gland, but it was not simple hypertrophy and a certain disproportion and distortion of the elements was seen. The parenchymal/interstitial ratio was slightly higher than that of the surrounding thymic tissue and the epithelial tissue was hypertrophic and frequently found to be very thin with long strands in a mesh-like pattern. The thymic cortex was not as hypertrophic and the cortex/medulla ratio was slightly lower than the normal thymus. Hassall's corpuscles were as numerous as those in the normal thymus, but

74 T. Yoshitake et al.: Focal Thymic Hyperplasia

Fig. 5. A microscopic photograph shows normal thymic com- ponents with a well developed thymic cortex, medulla, and dilated Hassall's corpuscles (H&E, original magnification x 120)

slightly more dilated, while the interstitum was com- posed of loose fibrous or fibroadipose tissue in contrast to the mature adipose tissue of the normal thymus. Such circumscribed thymic hyperplasia is extremely rare and we were unable to find any other case in the literature.

The morphological criteria of thymic hyperplasia distinguish between two types, true hyperplasia and lymphoid hyperplasia. True hyperplasia is characterized by an increase in both the size and weight of the thymus gland with histologically normal architecture. The present case partially corresponds to this type. Lymphoid hyperplasia is defined by the development of lymph-follicles with germinal centers regardless of the size or weight of the thymus. 2 This type of thymic hyperplasia is usually associated with autoimmune dis- orders such as myasthenia gravis.

The tumor from our patient did not show hypertrophy of the whole thymus, but a localized and circumscribed

mass within the thymus. This characteristic was of great clinical significance for differentially diagnosing it from a mediastinal neoplasm or cystic lesion. However, a preoperative differential diagnosis would be difficult unless a biopsy specimen is obtained.

Most cases of true thymic hyperplasia are associated with lymphocytosis of the peripheral blood. ~ Although the etiology of lymphocytosis associated with true thymic hyperplasia is now obscure, O'Shea et al. 6 have speculated from immunohistological research of thymic hyperplasia in infants that a disorder in which T-cell suppressor activity might be a predisposing factor to lymphocytosis, which disappears after thymectomy. 7 The presence of lymphocytosis in the peripheral blood was also found in our patient and it resolved soon after excision of the thymic mass.

In conclusion, the prognosis of true thymic hyper- plasia appears to be favorable 5 and the concept of sudden infant death syndrome being associated with thymic enlargement has been disproven by several studies, z

References

1. Arliss J, Scholes J, Dickson RR, Messina JJ (1988) Massive thymic hyperplasia in an adolescent. Ann Thorac Surg 45:220-222

2. Hofmann WJ, M611er P, Otto HF (1987) Thymic hyperplasia 1. True thymic hyperplasia. Review of the literature. Klin Wochenschr 65:49-52

3. Shin M, Ho K (1983) Diffuse hyperplasia following chemotherapy for nodular sclerosing Hodgkin's disease. Cancer 51:30-33

4. Carmosino L, Di Benedetto A, Feffer S (1985) Thymic hyperplasia following successful chemotherapy: A report of two cases and review of the literature. Cancer 56:1526-1528

5. Lack EE (1981) Thymic hyperplasia with massive enlargement. Report of two cases with review of diagnostic criteria. J Thorac Surg 81:741-746

6. O'Shea PA, Pansatiankul B, Fames F (1978) Giant thymic hyper- plasia in infants: Immunologic, histologic, and ultrastructural observations (abstract). Lab Invest 38:391

7. Levine GD, Rosai J (1978) Thymic hyperplasia and neoplasia: A review of current concepts. Human Pathol 9:495-515