Background. True thymic hyperplasia (TTH) is characterized as a distinct increase in both size and weight of thymus, which retains normal microscopic and immunohistochemical appearances. Massive true thymic hyperplasia (MTTH) is an extremely rare but significant subtype of TTH in pediatric ages due to its potentially serious consequences. It was reported that the age of cases with MTTH was predominantly between 1 and 15 years, while those before 1 year rarely occurred. By presenting the diagnosis and treatment process of our case as well as reviewing the related literature, we aimed to analyze the clinical characteristics of MTTH for patients younger than 1 year. Case. A 3-month-old male infant was admitted to our department with a chief complaint of gradually increasing polypnea over 9 days, whose preoperative imaging examination showed a large intrathoracic soft tissue shadow predominantly on the right side. The percutaneous fine-needle biopsy guided by ultrasonography was performed to identify its diagnosis. However, proliferating lymphocytes and Hassall’s corpuscles were seen microscopically in the biopsy tissues, which were immunohistochemically positive for CD3, CD19, CD20, CD99, TdT, PCK and Ki67 (>90%). Due to the aggravating symptoms, a second operation with total thymectomy was carried out successfully for this infant, which confirmed the diagnosis of TTH again by both morphological study and immunohistochemical staining from the surgical specimen. Conclusions. By reviewing the literature, there were only 10 cases with MTTH reported between 1975 and 2020 for children aged
___
1. Riazmontazer N, Bedayat G. Aspiration cytology of an enlarged thymus presenting as a mediastinal mass. A case report. Acta Cytol 1993; 37: 427-430.
2. Regal MA. Gigantic enlargement of the thymus gland. Saudi Med J 2007; 28: 1587‐1589.
3. Judd RL. Massive thymic hyperplasia with myoid cell differentiation. Human Pathol 1987; 18: 1180- 1183.
4. Pedroza Meléndez A, Larenas-Linnemann D. Thymus hyperplasia, differential diagnosis in the wheezing infant. Allergol Immunopathol (Madr) 1997; 25: 59-62.
5. Tan Z, Ying LY, Zhang ZW, Li JH, Gao Z, Qi JC. True thymic hyperplasia in an infant. J Pediatr Surg 2010; 45: 17111713.
6. Linegar AG, Odell JA, Fennell WM, et al. Massive thymic hyperplasia. Ann Thorac Surg 1993; 55: 11971201.
7. Weissferdt A, Moran CA. Thymic hyperplasia with lymphoepithelial sialadenitis (LESA)-like features: a clinicopathologic and immunohistochemical study of 4 cases. Am J Clin Pathol 2012; 138: 816-822.
8. Tadiotto E, Clemente M, Pecoraro L, Piacentini G, Degani D, Pietrobelli A.Massive thymic hyperplasia in a 15monthold boy: case report and literature review. Clin Case Rep 2019; 7: 27-31.
9. Lee Y, Moallem S, Clauss RH. Massive hyperplastic thymus in a 22-month-old infant. Ann Thorac Surg 1979; 27: 356-358.
10. Szarf G, Mussi de Andrade TC, De Oliveira R, Ota LH, Lederman HM. Massive thymic hyperplasia presenting with respiratory insufficiency in a 2yearold child. Thorax 2010; 65: 555-556.
11. Katz SM, Chatten J, Bishop HC, Rosenblum H. Report of a case of gross thymic hyperplasia in a child. Am J Clin Pathol 1977; 68: 786790.
12. Lamesch AJ. Massive thymic hyperplasia in infants. Z Kinderchir 1983; 38: 16‐18.
13. Lee YM, Koh MT, Omar A, Majid A. Hyperplasia of thymic gland. Singapore Med J 1996; 37: 288290.
14. Woywodt A, Verhaart S, Kiss A. Massive true thymic hyperplasia. Eur J Pediatr Surg 1999; 9: 331-333.
15. Sayed S, Sharma V, McBride CA, Levitt D, Alphonso N. Massive thymic hyperplasia in a neonate with BeckwithWiedemann syndrome. J Paediatr Child Health 2016; 52: 90-92.
16. Weis CA, Märkl B, Schuster T, Vollert K, Ströbel P, Marx A. “Echte Thymushyperplasie” Differenzialdiagnose der thymusvergrößerung bei säuglingen und Kindern. Pathologe 2017; 38: 286293.