Germinomas typically arise in regions adjacent to the third ventricle, such as the pineal body and neurohypophysis, but they may be present also in the basal ganglia. Germinomas originating in the cerebellum are quite rare [5–11] (see Additional file 2: Table S1); in one study, only three of 153 intracranial GCTs were located in the cerebellum [12]. It is not difficult to diagnose intracranial GCTs when they have a typical location and radiographic features. However, an intracranial GCT located in the basal ganglia often is difficult to detect because enhancement is usually slight or absent [2]. Recent reports have emphasized the effectiveness of SWI in early detection of basal ganglia GCTs [13]. SWI can be used to detect blood products and accumulation of biologic metal, thereby providing additional information in evaluation of brain tumors. SWI may be useful also in the detection of intracranial GCTs, because this type of tumor is characterized by a high incidence of intra-tumor micro bleeds. Since T2*WI is also extremely sensitive for detecting micro bleeds in the brain, both techniques are considered useful for early detection of intracranial GCTs [14].
In the present case, low-intensity spots were present in the left cerebellar hemisphere with both T2*WI and SWI at early stages of tumor growth, when the tumor showed no enhancement after gadolinium administration. To our knowledge, this is the first report describing low-signal intensity lesions on T2*WI without any enhancement. Interestingly, low-signal lesions on T2*WI and SWI were found to concomitantly expand with tumor progression. Although having not been performed in our case, 11C-methionine positron emission topography provides valuable information for making the diagnosis of germinoma, and it is useful also for evaluating treatment effects and monitoring for tumor recurrence [4, 15].
Since intracranial GCTs are extremely sensitive to radiation, the possibility remains that our patient’s tumor became temporarily invisible by conventional MRI techniques due to the radiation exposure of the CT scan. Since diagnostic CT scans are required in clinical practice, the patient had received a CT scan prior to the initiation of the MRI studies. Although conventional MRI showed no unremarkable findings, low-signal lesions were observed with T2*WI and SWI, suggesting that low-signal lesions may not be affected by radiation and may be useful in detecting intracranial GCTs after radiation exposure associated with CT. This view is supported by the fact that the low-intensity lesions remained visible after chemotherapy, while gadolinium-enhanced regions disappeared.
The symptoms of intracranial GCTs depend on tumor location, and typical symptoms include hydrocephalus, diabetes insipidus, hypopituitarism, hemiparesis, or higher cognitive dysfunction [16]. Intracranial GCTs located near the fourth ventricle are also known to present with trigeminal, abducens, and facial nerve paresis and cerebellar signs [17]. In the present case, the patient presented with slowly progressive ataxia, followed by cranial nerve palsy and brainstem dysfunction, such as difficulty breathing. In such situations, patients are more likely to be consulted by neurologists or general physicians, because neurodegenerative diseases, infectious diseases, or autoimmune diseases are considered to be more likely. Therefore, not only neurosurgeons but neurologists, neuroradiologists, and general physicians should know that CNS germinomas could have the radiographic findings and symptoms described above. Although measuring the serum alpha-fetoprotein and human chorionic gonadtropin is important, they are often negative in pure germinomas, as in our case. The only test that conclusively makes the diagnosis is biopsy, and it should be performed without hesitation if the symptoms are progressive and if there are any abnormal lesions on MRI. Our study suggests that hypointensity changes in T2*WI and SWI are helpful for making the early diagnosis of intracranial GCTs arising in atypical locations.