Morphological variability of lymphohistiocytic variant of anaplastic large cell lymphoma (former lymphohistiocytic lymphoma according to the Kiel classification)

According to the WHO classification, anaplastic large cell lymphoma (ALCL) is a distinct T-cell lymphoma entity with a number of morphological variants. The characteristic feature of lymphohistiocytic variant of ALCL according to the WHO classification is the abundance of histiocytes that exceed and mask the tumour cell population. In the current, study we reanalysed a historical series of 17 lymphomas, diagnosed as lymphohistiocytic lymphoma according to the criteria of the Kiel classification, with the presence of large purple macrophages (LPM) as the decisive finding for diagnosing this lymphoma subtype. We assessed the cellular composition of the tumour and correlated the results with the definition of lymphohistiocytic variant of ALCL given in the WHO classification. Although all cases in our cohort matched the criteria of ALCL according to the WHO, in 30% of the cases, the total amount of macrophages did not exceed the number of CD30-positive tumour cells. Our results indicate that the presence of LPM might be helpful to identify this subgroup of ALCL. Because the distinction of morphological subtypes of ALCL is of clinical relevance, improved criteria for subtyping ALCL are urgently needed that might include the presence LPM as one criteria.