pleural desmoplastic mesothelioma

A subset of sarcomatoid mesotheliomas, desmoplastic mesothelioma, typically produces a dense and bland collagenous matrix that can be exceedingly difficult to distinguish from active pleural fibroplasias.

Synopsis

 desmoplasia
 bland basket-weave fibrosis.
 The entrapped spindle cells show plump nuclei and prominent nucleoli.
 Possible invasion of the soft tissues of the chest wall.

Differential diagnosis

Desmoplastic sarcomatoid mesothelial growth may represent the primary or exclusive mode of growth in a malignant mesothelioma, whereas, at other times, it is a component of an otherwise cellular sarcomatoid malignancy or of a biphasic mesothelioma, ie, epithelioid and sarcomatous.

When spindle cell mesotheliomas are highly cellular and cytologically atypical, it is generally not difficult to distinguish them from a benign fibroinflammatory process.

But the diagnosis of desmoplastic mesothelioma is one of the greatest challenges in surgical pathology. In these cases, the cytological benignity of the cells may not allow accurate distinction from reactive pleural fibroplasia. Mitotic activity is uncommon.

The most reliable features in order of reliability include:
 (a) the presence of nests or nodules of spindle cells interspersed in an otherwise paucicellular stroma,
 (b) a well-developed storiform pattern of growth,
 (c) focal necrosis,
 (d) fibrosis toward the pleural surface with increased cellular activity at the leading edge approaching the chest wall,
 (e) unequivocal invasion into endothoracic fascia and fat.

The vascular pattern in desmoplastic malignant mesothelioma can help in differentiating this lesion from a pleural reactive fibroproliferative process as the vessels are arranged in a haphazard manner, rather than in an orthogonal configuration with respect to the pleural surface. A CD31 immunostain can emphasize the haphazard arrangement of tumor vessels in a desmoplastic malignant mesothelioma.