**9. Conclusions**

By applying the "integrated" approach to the revision of our OA series, we conclude that, OA could no longer be regarded as a separate tumor entity. However, rare cases might retain the OA denomination [74, 75, our case], indicating that tumors can differentiate in oligodendro‐ glial and astrocytic sense, even maintaining the same molecular genetics [71], or that they can arise from progenitors before differentiation in the two lineages.

Two points must be emphasized: one is the importance of the double immunostaining to recognize the astrocytic nature of tumor cells, especially the association between IDH and ATRX expression, considering that IDH1/2 mutations prevail in oligodendrogliomas in comparison to astrocytomas [52, 79]. In supposed mixed tumors with the two separate components, it has been observed that these share the same molecular asset and that astro‐ cytes in the tumor are reactive cells [71]. However, in tumors with the two cell types inter‐ mingled, tumor cells can be distinguished from reactive astrocytes by the double ATRX/GFAP immunostaining. It must be remarked that in rare instances 1p/19q co-deletion can be associated with ATRX mutations/ATRX protein loss [64] and that, since a quota of astrocyto‐ mas do not harbor IDH1/2 mutations, tumor astrocytes with wild type IDH may be found.

Minigemistocytes show definitely the oligodendroglial nature due to their ATRX-positive nuclei (**Figure 7B**).

The other point is the great influence that microglia/macrophage occurrence may exert in the recognition of tumor oligodendrocytes due to their nuclear ATRX positivity. In cases where

the number of microglia/macrophages is very high to reach the number of ATRX-positive nuclei, if tumor astrocytes are demonstrable, the tumor should be reclassified as astrocyto‐ ma. In order to recognize an oligodendroglial tumor infiltration, often result of a biopsy, it is still mandatory that the number of ATRX-positive nuclei in a given area encompasses the number of microglia/macrophages (personal data). ATRX-negative nuclei in infiltration areas correspond to normal oligodendrocytes. Only the double ATRX/Iba-1 immunostaining can reveal the occurrence of normal oligodendrocytes, beside their Cyclin D1 expression. In IDH1R132H mutant cases, the double ATRX/IDH1R132H immunostaining unequivocally identi‐ fies oligodendroglial tumor cells. The coexistence of normal and tumor oligodendrocytes by ATRXIHC can bedemonstratedin perineuronal satellitosis: in its initial phases, ATRX-positive and ATRX-negative nuclei are found around neurons; in later phases, all nuclei are ATRXpositive (**Figure 7C**–**E**). The same can be said for pericapillary satellitosis (**Figure 7F**).

The last point to be discussed is the possibility to find a false "honeycomb" appearance mimicking the typical perinuclear "halo" with ATRX-positive nuclei. This aspect cannot be interpreted as suggestive of an oligodendroglial origin of the tumor, being expressions of a water disturb/edema of unspecified nature, as it may happen for instance, in pilocytic astro‐ cytoma [81].
