Possibility of D2-40 as a diagnostic and tumor differentiation-suggestive marker for some of phosphaturic mesenchymal tumors.

Phosphaturic mesenchymal tumor (PMT) has been established as a tumor that causes tumor-induced osteomalacia (TIO) associated with mesenchymal neoplasm. Its lineage of differentiation has not been elucidated. Previously, the presence of lymphatic vessels inside PMTs has been documented using an anti-podoplanin antibody; the tumor cells of PMTs were reported to not react with it. In this study of 14 cases of PMTs, we used immunohistochemistry of D2-40, a relatively specific lymphatic endothelial marker, to see if they stained PMTs or not, with particular interest in its reaction with microcystic structures containing lymph-like fluid. We report that most of the PMTs (12 out of 14 cases; 86%) were immunostained by D2-40 in their tumor cells; D2-40-positive lymphatic vessels inside the tumor were also observed. We used a proportion score (0-4+), an intensity score (0-3+), and a total score (the sum of the proportion score and the intensity score) to quantitate our results. We report that 50% of cases (7 out of 14 cases) had a total score ≥ 4+; immunostaining of D2-40 in cases with a total score ≥ 4+ was easy to observe at a glance. Most of the tumor cells lining the microcystic structures were immunostained with D2-40. Thus, D2-40 could be a useful diagnostic marker of PMTs and it might also indicate that PMTs take a lymphatic endothelial immunophenotype.