ALK-rearranged Inflammatory Myofibroblastic Tumor of the Placenta, With Observations on Site of Origin

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To the Editor:
In their article 1, Banet and colleagues described 2 interesting cases of inflammatory myofibroblastic tumor (IMT) of the placenta, a neoplasm previously unreported at this site. We wish to report our experience with a case, particularly for its insight into how IMTs potentially involve this organ of fairly limited neoplastic potential.
The consultation case was received for an isolated 1.5-cm well-demarcated lesion found in the parenchyma of an otherwise unremarkable 34 weeks’ gestation placenta. Histologically, the lesion was comprised of a nodular and fascicular proliferation of spindle cells covered in a thin layer of basal fibrin and emerging continuously from the decidua basalis (Fig. 1A, arrow). The cells featured elongated and wispy eosinophilic to amphophilic cytoplasm, spindled to ovoid nuclei containing vesicular nucleoli, and were set in a myxoid matrix (Fig. 1B). A prominent mixed inflammatory infiltrate was present as were tumor cells demonstrating deciduoid change (Fig. 1C). Cytoplasmic immunoreactivity to ALK was intense, but patchy in both the spindled and deciduoid cells (Fig. 1D), with desmin and smooth muscle actin also showing heterogenous patterns of expression. S100 protein was negative. Fluorescence in situ hybridization identified rearrangement of ALK by break-apart probe (Fig. 1D). The lesion was classified as an IMT.
Although IMT is known to arise in a wide variety of organs, including endometrium and myometrium 2,3, the finding of direct continuity with the decidua basalis in this case suggests that IMTs of the placenta may arise from the uterus and secondarily involve the placenta. Both cases in the series by Banet and colleagues display the earlier steps of this cascade: the first tumor abutted the parenchyma without protruding into the villous compartment, whereas the second tumor was separated from villi by a border of intermediate trophoblast. Our case concludes this sequence by the tumor fully extending into villous parenchyma, but still maintaining its anchoring to the endometrium. We interpret this sequence as reasonable evidence that placental IMTs be considered truly uterine in origin.
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