A case of epidermoid cyst in which biopsy‐based diagnosis was important in determining treatment for a brainstem tumor

Of brain tumors in children, those arising in the midbrain, pons, and medulla oblongata account for 15%.1 Eighty percent of such tumors are diffuse intrinsic pontine gliomas (DIPGs)1; if an DIPG is suspected on the basis of imaging findings and the clinical course, biopsy might be avoided owing to the risk of neurologic complications. However, biopsy is required when suspected brainstem tumors are other than DIPGs. We report a pediatric case of an epidermoid cyst of the brainstem for which biopsy was useful for differentiating it from a DIPG and deciding the treatment strategy. A 4-year-old girl was referred to our hospital because of left upper limb palsy and right facial paralysis. Her mother had noticed that her left hand had been held in the supine position for the past 6 months. The patient had difficulty moving her left hand and raising the right corner of her mouth when smiling. Her medical and family histories were unremarkable. Physical examination revealed clear consciousness and no abnormalities of vital signs. Neurologic examination revealed poor elevation of the right angle of the mouth, poor internal and external rotation of the left upper limb, and increased deep tendon reflexes in the left upper and lower limbs. Laboratory examination had no abnormal findings in blood counts; biochemical factors, including endocrine tests; or tumor markers. Magnetic resonance imaging (MRI) of the brain showed a well-defined lobulated lesion, with a heterogeneous high signal on T2-weighted images, diffusion-weighted images (DWIs) with a low apparent diffusion coefficient map, and with high and low signals on T1-weighted images, which was located from a slightly right-side parenchyma at the basilar part and the tegmentum of the pons to the prepontine and medullary cisterns (Figure 1). Small nodular enhancement was seen at the margins of the lesion but not within (Figure 1). Biopsy was performed because the tumor's characteristics were atypical of a DIPG: the location on MRI in the brainstem region from the center to the right of the midline and a clinical course of more than 6 months. During the surgery, the tumor was found to be covered by the brain with a clear border between the tumor and the brainstem. Tumor contents from the small incision were aspirated as much as possible. Intraoperative analysis of a frozen section led to the diagnosis of epidermoid cysts, and the tumor was resected from wherever possible. Chemical meningitis due to leakage of tumor contents developed after surgery, but the symptoms resolved within 2 weeks. The neurologic symptoms improved and MRI examination 7 months after surgery showed shrinkage of the tumor lesion and almost disappeared. The patient is being followed up without treatment and has had no recurrence 14 months after surgery. Owing to their slow growth, epidermal cysts are often diagnosed in adults and are rarely diagnosed in children.2 The sites of epidermoid cysts are most commonly the parasellar region and the cerebellar pontine angle and are rarely the brainstem.2 Intraparenchymal epidermoid cysts are more common in the brainstem and temporal lobe, but all reported cases in children have been in the brainstem.3 Although the standard treatment of DIPG is radiation therapy, which has an extremely poor prognosis,4 the treatment of epidermoid cysts is surgical resection, which has a good prognosis.2 Epidermal cysts outside the brain parenchyma can easily be identified with MRI, but intraparenchymal lesions might be difficult to differentiation from DIPG, depending on the degree of signal on DWI. In the present patient, the differential diagnosis was complicated by an atypical nodular contrast effect in the cystic lesion, and by intraparenchymal lesions. Thus, biopsy is important in cases, such as the present case, with obtained images atypical of DIPG and a lesion that be approached laterally, even in the brainstem region. Regarding the safety of biopsies of brainstem tumors, a study of the biology and treatment of DIPGs has found that among 50 children for whom brainstem biopsies had been performed, complications of grades 3–4 (of version 4.0 of the Common Terminology Criteria for Adverse Events), were observed within 14 days but to an extent similar to that of biopsies of other sites.5 Considering the advantage of being able to make treatment decisions on the basis of tissue diagnosis, the authors of that study concluded that biopsies are acceptable for brainstem tumors.5 Diagnosing brainstem tumors via biopsy is challenging, but advances in less-invasive biopsy techniques will enable accurate diagnoses and appropriate treatment selection on the basis of genetic characteristics and improve prognoses. M.K.S., C.G.O., T.H., T.H., Y.N., and S.M. performed data correction; M.K.S. wrote the manuscript; and M.A. supervised the manuscript. All authors read and approved the final manuscript. The authors declare no conflicts of interest. Informed consent for the publication of the report was obtained from the patient's parents.