Germinomas, the most common type of central nervous system (CNS) germ cell tumors, is highly sensitive to chemotherapy and radiation and usually has very good outcomes. The clinical manifestation is largely dependent on tumor location and size. Suprasellar germinomas initially arise from the hypothalamus, then extend to the infundibulum as they enlarge, mimicking pituitary macroadenoma. The usual presenting symptoms of suprasellar germinoma are hypothalamic-pituitary dysfunction, visual impairment and diabetes insipidus (DI)(Mesquita Filho, Santos et al. 2018). The serum tumor markers, β-human chorionic gonadotropin and α-fetoprotein, are informative in only a few patients. Preoperative imaging plays an important role in diagnosing intracranial germinoma but does not reliably differentiate germinoma from other tumors; therefore, histological evaluation following stereotactic brain biopsy or craniotomy is commonly recommended for a definitive diagnosis and to determine treatment.
MRI is usually helpful in differential diagnosis. The typical MRI findings of germinoma include a heterogeneous, contrast-enhancing mass with space-occupying effect, cystic formation or calcification, mainly localized in the pineal and suprasellar regions. In our patient, MRI showed a huge suprasellar tumor invading the cavernous sinus and surrounding the internal carotid artery, which is a typical MR imaging feature of pituitary macroadenoma. Clinically, the chief complaints of this patient included impaired vision, headache and panhypopituitarism. Therefore, the first consideration was pituitary adenoma which accounts the vast majority of sellar tumors. In addition, based on the clinical manifestation, craniopharyngioma was also a diagnostic consideration despite finding no cystic formation or calcification on MRI.
Histopathological examination is necessary for the diagnosis of germinomas. In our case, the tissue consisted of large round cells with clear cytoplasm and round nuclei with prominent vesicular nuclei resembling primordial germ cells. Moreover, immunohistochemistry revealed tumor cells positive for placental alkaline phosphatase, c-kit, SALL4 and Oct3/4, which are highly expressed in germinomas (2), supporting the diagnosis of germinomas. Postoperatively, our patient had obvious recovery of visual acuity and subsequently underwent systemic chemotherapy and radiotherapy.
Two aspects make this case very educational. Firstly, when considering sellar or suprasellar mass, it is important to raise the suspicion of a diverse diagnosis from pituitary adenoma, as some tumor such as germinomas can mimic pituitary adenoma in MRI (1). Secondly, it is noteworthy that DI is one of the most common symptoms in patients with suprasellar germinomas. In our case, although DI didn't seriously endanger patient's life like worsening vision, it preceded other symptoms for several years. Thus, suprasellar germinoma must be highly suspected in any young patient with acquired DI. As for treatment, the tumor was considered a nonfunctioning pituitary macroadenoma preoperatively and was subtotally resected. In fact, complete surgical removal of germinoma is not commonly recommended because of potential recovery following radiation and chemotherapy and because of the risk of intraoperative complications (3). However, acute neurological deficits secondary to obstructive hydrocephalus caused by pineal masses or suprasellar masses require immediate neurosurgical intervention. In our patient, given the aggressive visual impairment, subtotal resection of the tumor was beneficial. In summary, we report a rare case that suprasellar germinomas mimic pituitary adenoma almost identically on MRI, intending to raise attention that germinomas should be suspected when confronting a sellar or suprasellar mass, accompanying with DI, particularly in an adolescent patient.
Contributed by Zhiwei Shen, PhD; Xinjie Bao, MD; Renzhi Wang, MD