Recognition of the differential diagnosis between lymphadenitis and malignant lymphoma requires good knowledge of the basic forms of the disease as well in depth knowledge of the structure of the individual compartments. There are defined forms of lymphadenitis where the differential diagnosis to certain lymphoma entities is known. Other reactive structural alterations show indistinct limits so that a decision is only possible after using additional techniques, such as immunohistochemistry and molecular analyses. Finally, there are marginal areas which can only be clarified by including clinical data.

Massive ovarian edema is a rare tumor-like condition found in young women resulting from accumulation of fluid mostly due to partial or intermittent torsion of the ovary or secondary, to a preexisting ovarian lesion. We report a case of a 13-year-old girl presenting with an ovarian mass measuring 16 cm. CA-12-5 levels were slightly elevated. Concerns regarding underlying malignancy led to salpingo-oophorectomy. Pathological evaluation revealed a massive ovarian edema and multiple thromboses of ovarian veins. Differentiating massive ovarian edema from malignant ovarian tumor is crucial to prevent patients from undergoing unnecessary surgery.

Differentiation of chronic gastritis from marginal zone B-cell lymphoma (MZoL) of MALT type is often difficult for the pathologist. Diagnostic tools include CD20 stain to highlight lymphoepithelial lesions, Wotherspoon grading of the infiltrate, and clonality analysis of the B-cells. MZoL may partially transform into a diffuse, large B-cell lymphoma, which the authors have named blastic MZoL. Blastic MZoL may be present with or without small cell MZoL. Without this component, blastic MzoL, while being CD10-negative, is presently difficult to positively diagnose since specific immune markers are still lacking. Blastic MZoL has a very favourable outcome compared to conventional diffuse large B-cell lymphomas (DLBCL). Moreover, there are conventional DLBCL in the stomach, mostly in a setting of a secondary organ involvement. The biology of these gastric DLBCL is identical to their extragastric counterparts. This is also true for primary gastric Burkitt lymphoma and mucosal involvement in B-CLL or mantle cell lymphoma. Unfavourable outcomes are always observed for EBV-triggered lymphoproliferations in immunodeficiency and peripheral T-cell lymphomas which might also arise or be initially diagnosed in the stomach.

Mycosis fungoides is a cutaneous T-cell lymphoma with protracted clinical course and progression in different stages with increasing aggressiveness. The clinical picture as well as the histopathology of mycosis fungoides within the early patch and plaque phase is difficult to delineate from some inflammatory skin diseases. Thus, the diagnosis of these early stages of the lymphoma is only possible when clinical, histopathological, and molecular features are integrated into the diagnosis, especially as none of the individual disease criteria is specific. Important clues towards the diagnosis of mycosis fungoides are cytologically abnormal epidermotropic CD4-positive T-cells causing only minor epidermal alterations, the formation of Pautrier-abscesses and basal alignment of the epidermotropic T-cells. The findings of an aberrant T-cell immunophenotype of the intraepidermal lymphoid component as well as the molecular proof of T-cell clonality are important further features. In the differential diagnosis between early stage mycosis fungoides and parapsoriasis, there remains nevertheless a diagnostic and maybe also a true biological grey zone.