Intracisternal enhancing lesions can be a feature of leptomeningeal metastasis (LM) or part of dural based brain metastasis, especially in breast cancer patients [1]. Typically, breast cancer metastases within the brain are more likely to develop in the brain parenchyma, predominantly in the subcortical regions of the cerebrum or cerebellum. However, in cases of LM or dural based metastasis, lesions can spread along the surface of the brain or spinal cord, and frequently develop into an enhancing nodule [2]. For the diagnosis of LM, cerebrospinal fluid (CSF) cytology is an essential examination for detecting malignant cells. And either definite or suggestive findings of gadolinium enhancement MRI are supportive for the diagnosis [3]. Dural-based metastasis, although relatively uncommon, can also be observed in breast and prostate cancer. This form of metastasis involves the dura mater, the outermost layer of the meninges, and appears as a well-defined, enhancing mass on MRI, often exhibiting a homogeneous enhancement pattern [4]. Excisional biopsy has been recommended for both the diagnosis and effective treatment [5]. However, invasive open biopsy is not easily performed and CSF cytology can be false negative. Thus, differential diagnosis with other enhancing intracisternal lesions such as resolving hematoma, infection, or pure cisternal schwannomas is necessary. After excluding metastatic causes, it is essential to consider other differential diagnoses, such as meningioma, cranial nerve schwannoma, or arachnoid cyst. Pure cisternal schwannomas are rare and frequently remain asymptomatic making the diagnosis to be difficult [6].

Here, we experienced asymptomatic prepontine intracisternal enhancing lesion with radiological diagnosis of abducens schwannoma in a patient with multiple recurrent metastatic breast cancer. We would like to suggest that the differential diagnosis is crucial in guiding the appropriate management and therapeutic approach in breast cancer patients with brain lesions.

Go to :

The patient was diagnosed with breast cancer (invasive ductal carcinoma, hormone receptor (+), grade 2, T2N0M0) 11 years ago and underwent a right mastectomy, subsequent adjuvant chemotherapy, and local radiation therapy at outside hospital. Tamoxifen had been given for 5 years, which was completed in 2018. In 2023, 5 years after the completion of breast cancer treatment, she found a lung nodule on her routine examination chest X-ray (Fig. 1A). Based on her past medical history, positron emission tomography CT was taken and it revealed multiple hyper-uptake in the T10 vertebra, lungs, and mediastinal lymph nodes (Fig. 1B). For confirmation, endobronchial ultrasound-guided trans-bronchial needle aspiration of the nodule (Fig. 1C) was performed and the biopsy revealed the metastatic breast cancer. All these lesions are asymptomatic, but as she complained of occasional mild, hammer-like headaches, which had been present for several years, a brain MRI was taken and it showed a well-defined, round enhancing 0.9-cm nodule in the right prepontine cistern (Fig. 2). Under the suspicion of intracranial metastatic lesion, the patient was referred to hospital.

After review of the biopsy slide and the subtype of estrogen receptor (+), progesterone receptor (+), and HER2 (-), the oncologist regarding the well-enhancing lesion to be brain or LM, referred the patient to our neuro-oncology clinic. After careful neurological examination, we confirmed that the lesion was asymptomatic and decided to proceed with other supportive examinations for differential diagnosis of pure cisternal cranial nerve schwannoma or LM. At the neuro-oncology tumor board meeting, our neuroradiologist evaluated the lesion as more likely to be asymptomatic abducens nerve schwannoma rather than metastasis based on radiological features including brainstem surfaced location, slightly high signal intensity on T2-weighted image and no diffusion restriction. To rule out the diagnosis of LM, we explain the need for serial (at least a week apart and repeated possibly 3 times) CSF cytology examination and a whole spine gadolinium-enhanced MRI as a staging work-up. After the confirmation of the 1st CSF cytology result (negative for malignant cells) which revealed no evidence of LM at the whole spine MRI, the oncologist started the systemic chemotherapy with letrozole and ribociclib. Upon agreement of the patient, we planned 2 months later follow-up brain MRI instead of immediate stereotactic radiotherapy, which was a secondary plan in case of the mass growing.

Two months later (3 months apart from the initial) brain MRI showed no change in the size of the mass (Fig. 3A and B). Seven months after the systemic chemotherapy, a follow-up chest CT showed a decrease in the metastatic lesion, which still remained asymptomatic. And follow-up brain MRI at the same time (9 months after the initial MRI) showed no change of the prepontine cistern lesion (Fig. 3C and D). Based on these findings, we suggest that the intracranial lesion is a pure cisternal abducens nerve schwannoma. A wait-and-see policy with regular brain MRI follow-up is recommended for this asymptomatic lesion while the patient continues systemic chemotherapy.

Go to :

Since brain metastases can occur in up to 30% of breast cancer patients, accurate detection and appropriate therapeutic intervention are crucial [7]. Among brain masses found in follow-up breast cancer patients, some cases of intracisternal enhancement, like in this case, may suggest benign tumors. However, according to Nayak et al. [5], about 8%–9% of breast cancer patients experience dural-based metastasis, and 5%–16% of patients may develop LM, which can sometimes lead to confusion during diagnosis. Dural-based metastasis typically exhibits pachymeningeal enhancement on T1-weighted contrast-enhanced images. This appears as irregular thickening of the dura with high-intensity signals. In some cases, it presents as flat nodular lesions, which resemble meningiomas, making differentiation crucial. Additionally, the dural tail sign, a feature often associated with meningiomas, can also be observed. Another key feature is that dural metastasis does not extend into the sulci or fissures, which serves as an important distinguishing factor from LM. In severe cases, dural metastasis can involve the area surrounding the ventricles.

As the mass of this case is located in cisternal space, the possibility of nodular thickening in LM should be evaluated. In this case, the patient underwent three times of CSF studies, and the results were negative including on atypical cells. However, according to Frechette et al. [8], up to 30% of patients with solid tumors, including breast cancer, may have occult LM that is not detected through imaging or clinical symptoms, reinforcing the need for continuous follow-up.

On the other hand, intracranial schwannomas mostly originated from sensory cranial nerves (i.e., vestibular and trigeminal nerve), and pure motor nerves (3rd, 4th, and 6th) are rarely affected by schwannomas [6]. Typical appearance on MRI is a well-defined nodular tumor extending into neural foramen, displaying low signal intensity on T1-weighted images and high signal intensity on T2-weighted images, with robust contrast enhancement. However, some cystic tumors or small abducens schwannoma were found to be pure cisternal mass as in this case. Lee et al. [9] reported a pure cisternal abducens schwannoma, which was surgically excised as it caused facial numbness in a 72-year-old male patient. Li et al. [10] surgically removed a 1.4-cm well-enhancing pre-pontine lesion, for which pre-operative ipsilateral gaze palsy was suggestive of abducens schwannoma.

In summary, the mass on MRI shows homogeneous enhancement upon contrast administration, with no evidence of pachymeningeal thickening in other areas of the dura, and follow-up imaging shows no growth. These findings suggest that this case is more likely to represent a cranial nerve schwannoma possibly abducens nerve origin. Since the patient does not exhibit symptoms such as diplopia or facial sensory disturbances, there is no evidence of compression of the surrounding structures. Therefore, regular neurosurgical and ophthalmologic examinations, along with follow-up MRI, are recommended.

Go to :