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Pepperpot or not?

A 62-year-old woman had magnetic resonance imaging (MRI) of the head after presenting with a possible transient ischaemic attack. The MRI demonstrated no evidence of a recent event but showed small T2 intense foci consistent with mature ischaemic lesions. Incidentally, ‘a heterogeneous signal throughout the marrow space of the cranial vault’ was reported on the axial T2 scan (left) and a lateral skull X-ray was suggested. This showed multiple ‘punched out’ lytic lesions throughout the skull vault (right) and she was referred to the haematology department urgently with a probable diagnosis of multiple myeloma. On review, she was asymptomatic and had a normal full blood count, creatinine and calcium. She had an IgMj paraprotein that was too low to quantitate and a normal serum free light chain ratio; her urine was negative for Bence–Jones protein. A skeletal survey demonstrated additional lesions in the mandible, some with endosteal scalloping and less convincing lesions in the right distal femur. Bone marrow aspiration and trephine biopsy were performed, these showing just 3% plasma cells. At this point, the differential diagnosis was reconsidered. The skull has two cortical layers with marrow between them. Skull lesions with lytic appearances can arise from the bone itself or occur either by distant metastasis or local invasion. The differential diagnosis for multiple lytic lesions on a skull X-ray in a patient of this age includes: multiple myeloma, metastases (particularly breast, lung, renal and prostate carcinoma), leukaemia, ‘brown tumours’ (osteitis fibrosa cystica secondary to hyperparathyroidism), venous lakes and vascular channels, Pacchionian granulations (hypertrophied arachnoid villi), benign lipomas and osteoporosis. Lipomas are relatively rare in the bone marrow despite the fact that it is rich in fatty tissue. MRI is useful for the detection of lipomas because they have a homogenous high signal on T1 and suppression on short T1 inversion recovery (STIR) sequences. This distinguishes them from metastases and inflammatory lesions. Our patient went on to have a further MRI showing high signal on T1, suppression on STIR and no enhancement post-contrast, in keeping with a signal of fat in a lobular pattern, that is multiple lipomas giving the X-ray appearances. A computerized tomography (CT) of the body cavities revealed no primary disease. Some lucencies in the axial skeleton on CT were also confirmed to be the signal of fat on axial MRIs. She was reassured and discharged from the haematology clinic.