ALK and anaplastic large cell lymphoma: Most studies have shown patients with systemic ALK+ anaplastic large-cell lymphoma have a favourable prognosis compared with the ALK– group.98,99,117,118 Patients with ALK+ tumours at extranodal sites, particularly the bone, may not always have a good prognosis.128 Other poor prognostic factors include small cell variant histology and peripheral blood involvement and CD56 expression.118,129–131 Primary CNS anaplastic large-cell lymphoma is very uncommon, being reported in less than 20 cases.