A B3 Type-Thymoma in a 7-Year-Old Child with Myasthenia Gravis
Authors: Raffaele Iorio, Amelia Evoli, Libero Lauriola, Anna Paola Batocchi
Journal: Journal of Thoracic Oncology (2012)

Acquired myasthenia is uncommon in childhood and its onset in the first decade of life accounts for less than 3% of all myasthenia gravis (MG) cases.1Evoli A Batocchi AP Bartoccioni E Lino MM Minisci C Tonali P Juvenile myasthenia gravis with prepubertal onset.Neuromuscul Disord. 1998; 8: 561-567Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar Thymoma is infrequent below the age of 10; the association of the two diseases is exceedingly rare in children.2Dickermann JD The late effects of childhood cancer therapy.Pediatrics. 2007; 119: 554-568Crossref PubMed Scopus (149) Google Scholar In September 2005, a 7-year-old boy began to complain of fatigable limb weakness and dysphagia. MG was diagnosed on the basis of positive Tensilon test, significant decrement (>20%) in the amplitude of the compound muscle action potential on deltoid muscle at low-rate repetitive nerve stimulation, and increased serum acetylcholine receptor (AChR) antibody (8.6 nM). Treatment with oral prednisone (1mg/kg/day) and acetylcholinesterase inhibitors was started with benefit. A chest computed tomography scan disclosed a large anterior mediastinal mass (56 × 36 mm), which was removed by extended thymectomy. Histological examination revealed a partially capsulated thymoma of B3 type,3Rosai J Sobin LH Histological Typing of Tumors of the Thymus. International Histologic Classification of Tumors. 2nd ed. Springer, New York1999Crossref Google Scholar with perithymic fat invasion (Masaoka stage II)4Masaoka A Staging system of thymoma.J Thorac Oncol. 2010; 5: S304-S312Abstract Full Text Full Text PDF PubMed Scopus (166) Google Scholar (Fig. 1). Myasthenic symptoms improved and corticosteroid therapy was tapered off within 6 months after surgery. One month later, the boy was referred to our institute for MG worsening. Prednisone therapy was reintroduced (1mg/kg/day), several courses of immunoglobulin were administered intravenously, and azathioprine (1.5mg/kg/day) was added. To date, 6 years after thymectomy, the boy, still under azathioprine and prednisone shows only mild facial weakness. The dose of prednisone was tapered to 0.3mg/kg/every other day; MG symptoms recurred when further tapering was attempted. Chest magnetic resonance imaging and Octreotide-positron emission tomography revealed neither local nor extramediastinal recurrences of thymoma. AChR antibody test was not repeated during the follow-up. Our patient had a B3-type thymoma that has never been described in infantile MG. These data suggest that in the presence of serum anti-AChR abs, a careful radiological investigation of the mediastinum to rule out the presence of a thymoma should also be done in prepubertal MG. Surgical resection is the treatment of choice for stage-I and -II thymomas, whereas chemotherapy is performed in patients with stage-III or -IV tumors. However, therapeutic indications about adjuvant radiation therapy for stage-II thymoma are still controversial. Current indications range from postoperative radiation for all patients with thymoma, to adjuvant radiotherapy for only patients with tumors larger than 5 cm. A recent study suggested that complete resection alone might be sufficient for stage-II thymoma.5Rena O Papalia E Oliaro A et al.Does adjuvant radiation therapy improve disease-free survival in completely resected Masaoka stage II thymoma?.Eur J Cardiothorac Surg. 2007; 31: 109-113Crossref PubMed Scopus (68) Google Scholar In the absence of settled guidelines for thymoma treatment in children, we decided not to perform adjuvant therapies. We did not consider chemotherapy on the basis of the low invasiveness of the tumor, and excluded radiotherapy for its well-known late-local morbidity associated with mediastinal and lung irradiation, and for its possible side-effects on developing organs.2Dickermann JD The late effects of childhood cancer therapy.Pediatrics. 2007; 119: 554-568Crossref PubMed Scopus (149) Google Scholar However, it remains to be settled how long and with which modalities tumor follow-up is to be performed in children. In our patient, as in most MG cases, the onset of muscle weakness led to the discovery of the thymic tumor. The association of MG and thymoma, although rare, does occur in the pediatric population. Corticosteroids should be slowly tapered after thymectomy to avoid the worsening of MG.