It is commonly believed that the only tumors which can appear as bilateral ("butterfly") lesions of the corpus callosum are central nervous system lymphomas or malignant gliomas.

     We performed a stereotactic biopsy in an 80 year-old woman who presented with an expanding symmetrical mass in the splenium of the corpus callosum who presented with short-term memory loss.  During her evaluation, a hilar chest mass was found. Biopsy was performed because the imaging was inconsistent with her pulmonary lesion. The stereotactic brain biopsy showed metastatic small cell carcinoma, likely of lung origin. All glial markers and T-cell or B-cell markers were negative.

     The differential diagnosis of bihemispheric callosal mass lesions should include metatstatic small cell carcinoma.This finding supports consideration of a metastatic work-up for patients with similar lesions.

     The finding on imaging of a bi-hemispheric callosal mass lesion, either in the genu, body, or splenium heralds the diagnosis of either a glioma or CNS lymphoma. Histologic confirmation is performed to distinguish between these entities or to grade a glial tumor.  Other histologies are considered "almost impossible" to result in this imaging finding, and this "dogma" is a part of training.

     We performed a stereotactic biopsy in an 80 year-old woman who presented with an expanding symmetrical mass in the splenium of the corpus callosum who presented with short-term memory loss.  During her evaluation, a hilar chest mass was found. She was kept off corticosteroids to reduce the chance that a lymphoma might be hard to define with imaging.  Her first magnetic resonance imaging scan (MRI) was performed without contrast.  She was admitted for an MRI-guided stereotactic biopsy, with contrast that showed the callosal lesion as well as a small 1 cm ring enhancing lesion in the anterior right frontal lobe.  The frontal lobe lesion remained consistent with a CNS lymphoma or a multicentric glioma, in addition to other pathologies.
The stereotactic brain biopsy of the right splenial mass showed metastatic small cell carcinoma, likely of lung origin. All glial markers and T-cell or B-cell markers were negative.

     This observation has not been previously reported.

     This work was approved by the University of Pittsburgh Institutional Review Board.

     This study was not funded by any external source.

     The observations in this study were made with two MRI scans and with tissue obtained via stereotactic biopsy.

     The corpus callosum is composed of dense myelinated fibers that connect regions of the two cerebral hemispheres.  It is believed that this density serves as a blockage to tumor spread or flow of edema and, thus, only high-grade, aggressive tumors such as lymphoma of glioblastoma multiforme, cross the corpus callosum and cause the typical "butterfly" appearance. 1,2.  The importance of this finding is that clinicians should be aware of the possibility for metastatic small cell carcinoma to mimic the appearance of a CNS lymphoma or glioblastoma of the corpus callosum.

     This finding still remains a rare possibility in the differential diagnosis of bihemispheric callosal tumors.

     Not all lesions of the corpus callosum are necessarily GBM or lymphoma.  Demylinating diseases can manifest in this region and appear as a pseudo-butterfly glioma. 3,4.  Rare diseases, such as granuloctyic sarcoma or myeloid sarcoma taking the form of an enhancing "butterfly" lesion of the corpus have been reported.5,6.  Infection, in particular, toxoplasmosis mimicking lymphoma in an HIV patient, can appear as a butterfly lesion. 7.  We present a case of biopsy-proven metastatic non-small cell lung cancer presenting in the same manner.  Therefore, as part of the evaluation of a bihemispheric callosal tumor, the patient should undergo a typical metastatic work-up, which includes imaging (x-ray or CT of the chest, CT of the abdomen and pelvis) to ensure that a primary cancer is not present.  Typically a metastatic work-up is not performed in such patients.