A 71-year-old woman presented with left thigh pain and subjective weakness that had first appeared about 20 days previously. She had been diagnosed with MCL 6 months previously, and had been treated six times with bortezomib, rituximab, cyclophosphamide, and doxorubicin. She had felt coldness and tingling in both legs since receiving chemotherapy. A neurological examination revealed decreased vibration sensation symmetrically with normal motor power and symmetric hyporeflexia on the ankle. A nerve conduction study (NCS) showed early polyneuropathy. About 10 days later she experienced voiding difficulty following diplopia. A neurological examination revealed additional findings that included bilateral sixth cranial nerve palsy, mild motor weakness (Medical Research Council grade 4) of both legs, and hyporeflexia in both knees and ankles. A CSF study showed a yellowish color, leukocytosis with predominant mononuclear cells (340/mm³), and elevated total protein (566 mg/dL). No malignant cells or infectious causes including tuberculosis were observed in the CSF. A bone-marrow biopsy revealed complete remission of the previous lymphoma and no malignant cells. Leptomeningeal enhancement with bilateral thickening of trigeminal nerves was observed in brain magnetic resonance imaging (MRI) (Fig. 1A-D). Leptomeningeal enhancement of the thoracolumbosacral area was found in spine MRI (Fig. 1F), as well as diffuse enhancement of multiple spinal nerve roots (Fig. 1G). Compared to the previous study that was performed after diagnosing MCL (Fig. 1H), FDG PET-CT performed at presentation showed that the previous lesion had completely resolved and revealed a new linear area of high uptake in the lower thoracolumbar spinal cord and cauda equine (Fig. 1I). Based on a diagnosis of relapse of MCL as NL, the patient was started on ibrutinib with high-dose methotrexate. The patient expired due to pneumonia one month later.

A 62-year-old man who had been diagnosed with MCL 2 months previously was referred to us due to neuropathic pain on both thighs during the third phase of chemotherapy with cyclophosphamide, vincristine, doxorubicin, and dexamethasone. He felt severe neuropathic pain on both posterior thighs, and this pain was aggravated when supine or sitting. A neurological examination revealed bilateral hypesthesia in the S1 and S2 dermatome areas and hyporeflexia on the ankle. Acute-stage radiculopathy from L5 to S2 was suspected from the NCS and electromyography. He complained of worsening neuropathic pain and difficulties in urination and defecation 1 week later. A CSF study showed a yellowish color, leukocytosis with predominant mononuclear cells (1,210/mm³), and elevated total protein (>3,000 mg/dL). There were no malignant cells or infectious causes including tuberculosis in the CSF. Spine MRI showed diffuse leptomeningeal enhancement from L1 to S1 (Fig. 2C) with diffuse enlargement of spinal nerves (Fig. 2D, E). FDG PET-CT showed a linear hypermetabolic lesion in the spinal cord from T11 down to the cauda equine (Fig. 2G) and complete remission of previous multiple lymph-node lesions (Fig. 2F). He was diagnosed with NL presenting as cauda equine syndrome and treated with a methylprednisolone pulse and rituximab. However, the patient did not improve, and expired 2 days later.