Definition
• A variant of sarcomatoid malignant mesothelioma (MM) composed of dense collagenized stroma separating sparse neoplastic cells
Clinical features
Epidemiology
• Represents about one fifth of all sarcomatoid mesotheliomas
Presentation
• Same as other pleural mesotheliomas
Prognosis and treatment
• Highly aggressive tumor with prognosis similar to sarcomatoid MM
• Usually not treated with surgical resection because of poor outcome
Pathology
Histology
• Paucicellular distribution of atypical neoplastic cells between bands of dense collagenous stroma (see Web Table 3 on the Expert Consult website)
• Subtle atypia: neoplastic cells are often deceptively bland, resembling fibroblasts; can be missed as pleural plaque or fibrous pleuritis
• Histological features to distinguish from a benign process include invasive growth, bland necrosis, frankly sarcomatoid areas, and metastatic disease
• In contrast to reactive fibrous pleuritis, desmoplastic mesothelioma often shows disorganized growth pattern and variable thickness with expansile nodules of varying sizes and abrupt changes in cellularity between nodules and their surrounding tissue
Immunopathology/special stains
• Pancytokeratin is invaluable in highlighting the invasion of malignant mesothelial cells in the connective tissue, adipose tissue, skeletal muscle, or lung parenchyma
• Mesothelial markers (such as calretinin, D2-40, and CK5/6) are not useful in distinguishing from fibrous pleurisy
• GLUT-1 may be positive
Main differential diagnosis
• Fibrous pleurisy (see Web Table 3)
• The amount of tissue from a closed pleural biopsy is often insufficient; large pleural biopsy specimens are generally needed for definite diagnosis
• Uniformity of thickness of growth, surface atypia with deep maturation, and perpendicular thin-walled vessels are typical of reactive fibrous pleuritis
• Negative GLUT-1
Fig 1 Desmoplastic malignant mesothelioma. Deceptively bland spindle cells of desmoplastic MM distributed between bands of dense collagenous stroma with a storiform growth pattern: low power (A),high power (B).
Fig 2 Desmoplastic malignant mesothelioma. Desmoplastic MM with variable cellularity, which is often present in this tumor.
Fig 3 Desmoplastic malignant mesothelioma. Desmoplastic MM invading adipose tissue (A), which is highlighted by keratin immunostain CAM5.2 (B).
Fig 4 Desmoplastic malignant mesothelioma. Desmoplastic MM invading adipose tissue with focal coagulative necrosis (center): low power (A), high power (B).
Fig 5 Desmoplastic malignant mesothelioma. Reactive fibrous pleurisy showing uniformity of growth and thickness with maturation (zonation) from surface (top) to deep (bottom): fibrinous exudates, granulation tissue, fibrosis with chronic inflammation, and parietal pleural adipose tissue.
Fig 6 Desmoplastic malignant mesothelioma. Reactive fibrous pleurisy with perpendicularly oriented thin-walled vessels.
WEB TABLE 3 Fibrous Pleurisy Versus Desmoplastic Mesothelioma
|
FIBROUS PLEURISY |
DESMOPLASTIC MESOTHELIOMA |
|
|
Stromal and fat invasion |
Absent |
Present (pancytokeratin very helpful in determining depth of invasion) |
|
Storiform growth pattern |
Not prominent |
Often prominent |
|
Necrosis |
Not present within the fibrous tissue; if present, only at the surface and often with acute inflammation |
Often present; areas of coagulative so-called bland necrosis without inflammation |
|
Thickness of the process |
Uniform thickness throughout the lesion |
Growth is variable with uneven thickness, expansile nodules, and hypercellular and hypocellular areas |
|
Zonation |
Present; surface relatively hypercellular with plump cells; deeper layers less cellular with more mature fibrosis |
Lack of zonation or maturation from the surface to deep |
|
Orientation of vessels |
Perpendicular to surface |
Poorly vascularized, without orientation |
|
GLUT-1 |
Negative |
May be positive |
|
IMP3 |
Negative |
Often positive |
|
p16 homozygous deletion |
Absent |
Often present |