Case of the Month: Right Lung and Pleura

This case first appeared as Performance Improvement Program in Surgical Pathology (PIP) 2014, case 07 and is a biphasic mesothelioma.

Histologic sections show nests of tumor cells, which appear to be quite monotonous with abundant cytoplasm, prominent nucleoli, and associated dense desmoplastic response. In some areas, the tumor cells are forming papillae and glands lined by a single layer of cuboidal cells with similar morphology. Adjacent areas show spindle cells with high cellularity, pleomorphism, and nuclear atypia, resembling sarcoma. Tumor shows both epithelial and sarcomatous components, each component being more than 10%, and hence classified as biphasic mesothelioma. Depending on the slide received, the proportion of either component may be minimal.

Malignant mesothelioma is a primary diffuse tumor of the serosal membranes that bears resemblance to normal and reactive mesothelial and submesothelial cells, and needs to be differentiated from benign and borderline mesothelial proliferations. Mesothelioma is an uncommon neoplasm, occurring more commonly in males. There is a large variation in the incidence of mesothelioma in different countries. Mesotheliomas are known to occur in the pleural, peritoneal, and pericardial cavities, as well in the tunica vaginalis testis. The majority of mesotheliomas occur in the pleura. Interestingly, mesotheliomas in women occur more commonly in the peritoneum than those in men.

Mesotheliomas have a complex etiology in which environmental carcinogens (asbestos and erionite), ionizing radiation, therapeutic radiation (thorotrast), viruses (simian virus, SV40), chronic pleural irritation, and genetic factors act alone or in concert to cause malignancy. Asbestos exposure is the most common cause of mesothelioma. Asbestos fibers occur in two mineralogic forms: commercially used amphiboles (amosite and crocidolite), and chrysotile. Amosite and crocidolite are considerably more persistent in tissue than chrysotile. This difference is responsible for the higher incidence rates of mesothelioma in persons exposed to amosite and crocidolite types of asbestos fibers. The latency period for the development of mesothelioma after asbestos exposure varies from 30 to 40 years.

The median age of presentation of mesotheliomas is around 60 years. However, women with peritoneal mesotheliomas tend to present at younger ages. Mesotheliomas have rarely been reported to occur in children and adolescents. Patients usually present with chest pain and shortness of breath, and occasionally present with weight loss, cough, and fever. Radiographic examination shows a pleural effusion in the majority of the patients, along with the presence of diffuse pleural thickening or multiple pleural tumor nodules or pleural plaques. Rarely a tumor can present as a mediastinal mass or a well-circumscribed mass.

Epithelioid mesothelioma shows epithelioid cytomorphology, mostly bland, but more anaplastic forms are occasionally seen. Epithelioid mesotheliomas show a wide range of morphologic patterns, including tubulopapillary, micropapillary, trabecular, acinar, adenomatoid, solid, clear cell, deciduoid, adenoid cystic, signet ring cell, small cell, rhabdoid, and pleomorphic. Epithelial malignant mesotheliomas, especially the papillary form growing along the serosal surface, can mimic reactive mesothelial proliferations. In these cases, definitive invasion into fat and adjacent tissues should be seen before making a diagnosis of mesothelioma. The spindle cells of sarcomatous mesotheliomas can be plump or long and thin with sparse cytoplasm. Cytologically they can be quite bland, but can also have areas with high-grade cytology. The lesions are often composed of a poorly defined storiform pattern of densely packed spindled cells with high cellularity, nuclear atypia and frequent mitotic figures resembling malignant fibrous histiocytoma, synovial sarcoma or fibrosarcoma. Heterologous elements including osteosarcoma, chondrosarcoma or other sarcomas may be present. Most sarcomatous mesotheliomas are positive for keratin immunostains. The variants of this subtype include transitional, lymphohistiocytoid and desmoplastic mesothelioma. Desmoplastic mesothelioma consists of paucicellular storiform or "patternless" pattern with elongated, often attenuated tumor cells present between collagen bundles. As these lesions can mimic organizing pleuritis or plaques, a definitive diagnosis requires either presence of frankly sarcomatoid areas, foci of bland collagen necrosis, invasion of adipose tissue, skeletal muscle or lung, or distant metastases.

There is an enormous amount of data on a variety of immunostains proposed for mesothelioma. The distinction of epithelioid mesotheliomas from metastatic adenocarcinomas is one of the most challenging diagnostic problems faced by surgical pathologists. Since epithelioid mesotheliomas can exhibit many histologic patterns, they may be confused with a variety of carcinomas arising from the lung or from a distant organ (eg, kidney). Markers identified as potentially useful in the diagnosis of epithelioid mesothelioma include positive markers (namely, calretinin, keratin 5/6, D2-40, podoplanin, mesothelin, and WT-1) and negative markers (namely, carcinoembryonic antigen, MOC-31, B72.3, and Ber-EP4). TTF-1 can assist in determining the lung origin of a carcinoma, and renal cell carcinoma marker (RCC) may help establish its renal origin. A panel of four markers (two positive and two negative) selected based upon availability and which ones yield acceptable staining results in a given laboratory is recommended. The best combination appears to be calretinin and cytokeratin 5/6 (or WT-1) for the positive markers and CEA and MOC-31 (or B72.3, Ber-EP4, BG-8) for the negative markers. The International Mesothelioma Panel also recommends that at least two mesothelial and two carcinoma markers be used in addition to a pancytokeratin.

Karyotypic and comparative genomic hybridization (CGH) analyses have shown that most mesotheliomas have multiple chromosomal alterations. The homozygous deletion of p16 detected by fluorescent in situ hybridization (FISH) has been shown to be a reliable test to distinguish the benign/reactive and malignant mesothelial proliferations. Studies have shown that majority of mesothelioma cases are positive for the p16 gene deletion by FISH. However, this test can be positive in almost any malignancy and, thus, an immunohistochemical panel to prove mesothelial origin should still be the first step in diagnosis. Epithelioid mesothelioma shows the presence of long, often branching, surface microvilli that are devoid of glycocalyx on electron microscopy. These often have a length to diameter ratio that exceeds 10 to 1. Other features include perinuclear tonofilament bundles, the presence of basal lamina and long desmosomes. Most adenocarcinomas have short, stubby microvilli covered with a glycocalyx. Their length to diameter ratio is usually less than 10 to 1. Electron microscopy is less useful in sarcomatous mesothelioma.

Differential diagnosis includes high-grade vascular tumors such as primary angiosarcomas, epithelioid hemangioendotheliomas and lymphangiosarcomas. Immunohistochemical staining is extremely helpful in these cases as these tumors are weakly positive for keratin, strongly positive for CD31, CD34, and factor VIII. Well-differentiated papillary mesothelioma shows papillary architecture with superficial spread and without invasion. They are usually found in the peritoneal cavity of women, but may also occur in the pleura, pericardium and tunica vaginalis testis.

Despite recent advances in chemotherapy, surgery, and radiotherapy, prognosis remains poor for these patients. It is possible that some very early stage tumors have been cured by so-called triple modality therapy: extrapleural pneumonectomy followed by chemotherapy and radiation therapy, but this remains to be proven.

1. Which of the following is true regarding well-differentiated papillary mesotheliomas?
   
   1. Invasion is frequently identified.
   2. Microscopically these tumors are distinctive with papillary architecture and a tendency towards superficial spread without invasion.
   3. Patients frequently present with symptoms and mass-like lesions.
   4. Patients have a poor prognosis and complete surgical resection is not curative.
   5. These tumors are usually found in the pleural cavity of men.
2. Which feature is most helpful in separating benign and malignant mesothelial proliferations?
   
   1. Cytologic atypia
   2. Positive keratin staining
   3. Presence of necrosis
   4. Presence of zonation with areas of high cellularity
   5. True stromal invasion (including fat and adjacent tissues)
3. Which of the following tests is most helpful in the distinction of epithelioid mesotheliomas from metastatic adenocarcinomas?
   
   1. Demonstrating invasion by cytokeratin stain
   2. Performing 2 mesothelial and 2 carcinoma markers
   3. Performing 3 mesothelial markers
   4. Presence of homozygous deletion of p16 detected by FISH
   5. Presence of short, stubby microvilli by electron microscopy

1. Churg A, Cagle PT, Roggli VL. Tumors of the Serosal Membranes: AFIP Atlas of Tumor Pathology. fourth series, fascicle 3. Washington, DC: ARP Press; 2006Fletcher CD. Diagnostic Histopathology of Tumors. 3rd ed. Philadelphia, PA: Elsevier Limited; 2007;7-40.
2. Churg A, Colby TV, Cagle P, et al. The separation of benign and malignant mesothelial proliferations. Am J Surg Pathol. 2000;24(9):1183-1200.
3. Husain AN, Colby TV, Ordonez NG, et al. Guidelines for pathologic diagnosis of malignant mesothelioma: a consensus statement from the International Mesothelioma Interest Group. Arch Pathol Lab Med. 2009;133(8):1317-1331.
4. Ordonez NG. The immunohistochemical diagnosis of mesothelioma: a comparative study of epithelioid mesothelioma and lung adenocarcinoma. Am J Surg Pathol. 2003;27(8):1031-1051.

2014
Kirtee Raparia, MD
Surgical Pathology Committee
Northwestern University
Chicago, IL

1. Microscopically these tumors are distinctive with papillary architecture and a tendency towards superficial spread without invasion. (b). 
2. True stromal invasion (including fat and adjacent tissues) (e). 
3. Performing 2 mesothelial and 2 carcinoma markers (b).