A 25‐Year‐Old Male with a Subgaleal Mass

Clinical History

A 24‐year‐old man presented with new onset of generalized tonic clonic seizure. Prior to admission, the patient reported an enlarging painless scalp mass for the previous two months, without having sought medical attention. MRI of the brain revealed a 7.7 cm heterogeneously enhancing mass with a large subgaleal extracalvarial/extracranial component and a smaller intracranial enhancing component along the dura on T1 weighted images (Figure 1A and B). Noncontrast CT showed abnormal calvarial sclerosis concerning for osseous involvement (Figure 1C). The patient underwent biparietal craniectomy and sub‐total resection of the mass with residual tumor involving the superior sagittal sinus. The cranium, dura and subgaleal mass were submitted for evaluation.

Figure 1.

Microscopic Pathology

Histologic examination revealed a dural‐based tumor growing into the overlying skull table, composed of spindle cells arranged in large nodules and sheets with interdigitating thin collagen bundles (Figure 1D). The tumor cells featured variably open and clumpy chromatin with brisk mitotic activity (Figure 1D). There was abundant admixed lymphocytic inflammation and scattered macrophages with eosinophils present. No whorls, psammoma bodies, herringbone pattern or intranuclear cytoplasmic pseudoinclusions were present. CD20 (Figure 1E) and CD45 (Figure 1F) showed strong, diffuse positivity. The tumor cells were positive for Bcl‐6 (Figure 1G) and CD10 (Figure 1H). Immunostains for CD3, Bcl‐2, CD34, EMA, S100, SMA, desmin, CD56, CD1a and ALK1 were negative in tumor cells. Ki‐67 proliferation index was 70%. Mitotic count was 10 per 10 HPF. What is your diagnosis?

Diagnosis

Spindle/sclerosing variant of follicular lymphoma.

Discussion

Spindle cell features in hematopoietic neoplasms are unusual and must be differentiated from true sarcomas. Cases have been reported in the skin, subcutaneous tissue, mediastinum and nasal mucosa/paranasal sinus 4, and more conventional lymphomas can occur in the dura 3. The current tumor most likely arose in the skull. Such rare cases have been described previously but none have involved the dura.

Radiologically, the differential diagnosis for skull‐dural lymphoma is broad and includes high grade meningioma, metastasis, solitary fibrous tumor, leiomyosarcoma, inflammatory pseudotumor, neurosarcoidosis and Castleman's disease.

Histologically, these spindle cell features do not raise suspicion of a lymphoma, but rather suggest a sarcoma. Metastatic spindle cell neoplasms such as spindle cell carcinoma, spindle cell melanoma or dendritic cell sarcoma, could also resemble this tumor. Neoplastic lymphocytes are known to assume a spindled morphology when infiltrating into bone and soft tissues 2, but to have an entire tumor show such prominent spindling, without any cytologic hint of lymphoma, is very unusual. Immunohistochemistry is therefore critical to confirm the lymphocytic origin of these tumors. The spindle cell variant of B‐cell lymphoma exhibits both phenotypic and genotypic markers of germinal B cells, with mutations of immunoglobulin and BCL‐6 and co‐expression of CD‐10, BCL‐6, MUM1 and CD138 1.

Spindle cell lymphoma is a rare morphological variant of B‐cell lymphoma and, to the best of our knowledge, has not previously been described in the dura. This case underscores the importance of keeping a very broad differential in mind, and the importance of using hematopoietic immunohistochemical markers, even when microscopic examination does not suggest such a tumor.