Final diagnosis
Systemic Mastoyctosis

Differential diagnosis
Metastatic bone marrow replacement; storage disorders (i.e. Gaucher's disease)

Discussion
Mastocytosis is a group of disorders characterized by mast cell proliferation in different tissues of the body. Disorders of mast cell proliferation include urticaria pigmentosa,systemic mastocytosis and mast cell leukemia. Systemic mastocytosis  is a rare disorder (less than 10% of mastocytosis) that affects usually adults. Clinical symptoms resemble lymphoma or leukemia. Many organs are involved such as the liver, the spleen, the lymph nodes, the skin and the bone marrow.

Skeletal abnormalities are seen in 70% of cases. There is a special tropism for axial skeleton. This proliferation is commonly silent, although 28% of patients complain pain. Changes are observed in plain film as lytic or sclerotic lesion with focal or diffuse distribution. Mast cell proliferation into bone marrow stimulates fibroblastic activity and granulomatous reaction, which lead to trabecular destruction and replacement with adjacent new bone formation. The bone marrow represents the most commonly biopsied extracutaneous siite used in establishing the diagnosis of systemic Mastocytosis.

Mast cell proliferation in skeletal tissue is a well-known manifestation of disease that occurs in 79% of patients. First bone marrow infiltration was reported by Sagher in 1952. Axial skeleton is usually involved. This infiltration is commonly asymptomatic although may cause pain, tenderness or pathologic fractures, most frequently in the spine. Changes are observed in plain films as lytic or sclerotic lesion. In either type a focal or diffuse distribution may be seen. Diffuse pattern is predominant in axial skeleton, whereas focal lesions occur in both axial and appendicular. It is believed that lytic lesions are secondary to the releasing of a large number of chemical mediators by mast cell aggregates. The sclerotic areas are secondary to fibrotic changes related to histamine that can lead to osteoid tissue formation after the calcic sales deposit Commonly this proliferation is silent, although 28% of patients complain pain

MRI can easily detect the bone marrow abnormalities due to celular infiltration. On SE-T1 sequence, a replacement of the normal high signal due to the medular fat has been reported in neoplastic or other cellular infiltrative processes.

Typically MRI of SM shows hypointense signal on both T1- and T2-weighted sequences, similar to any other sclerotic bone lesions such as metastasis (breast and prostate) and end-stage Paget´s disease.

MRI can easily detect the bone marrow abnormalities due to celular infiltration. On SE-T1 sequence, a replacement of the normal high signal due to the medular fat has been reported in neoplastic or other cellular infiltrative processes . Typically MRI of SM shows hypointense signal on both T1 and T2-WI. MR is an excellent technique to assess the grade of medular infiltration of these patients.