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* From the Departments of Pulmonary Medicine (Drs. Khan and Berman), Oncology (Dr. Tlemcani), Nuclear Medicine (Dr. Shanmugam), Pathology (Dr. Y), and Surgery (Dr. Keller), Albert Einstein College of Medicine, Montefiore Medical Center, Bronx, NY.
Correspondence to: Amir M. Khan, MD, MSc, 3600 Fieldston Rd, No. 7C, Bronx, NY 10463; e-mail: dramirkhan{at}hotmail.com
A 77-year-old man was referred for evaluation of a left chest mass found on a routine radiograph during hospitalization for a myocardial infraction. The patient underwent successful coronary stenting for occluded right coronary and left circumflex coronary vessels and remained asymptomatic.
His medical history was significant for hypertension, hyperlipidemia, benign prostatic hypertrophy, and peripheral vascular disease. He had undergone a carotid endarterectomy 2 years prior to this hospital admission. His medications included atenolol, aspirin, clopidogrel, pantoprazole, simvastatin, tamsulosin, and amlodipine. His social history was remarkable for cigarette smoking (1 pack a day for > 50 years) until he quit 4 years ago. He was retired from the merchant marine, and his work involved ship repair for several weeks at a time. No clear asbestos exposure could be established. His purified protein derivative status was negative. His vital signs, the findings of physical and cardiopulmonary examinations, and results of basic laboratory investigations were unremarkable on presentation.
Radiologic Findings
A plain chest radiograph showed a solitary opacification on the left side of the chest. A CT scan of the chest revealed a 2 x 1 cm soft homogenous lesion in the anterior left hemithorax forming oblique angles with adjacent lung parenchyma consistent with either pleural or extrapleural origin, without evidence of apparent chest wall or rib involvement. No adjacent bony destruction or invasion was noted. A corresponding intense focal fluorodeoxyglucose uptake was noted on positron emission tomography (PET) scans of the chest (Fig 1, 2 ).
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Pathologic Findings
On gross inspection, a tan white homogenous 1.7 x 1.2 x 0.6 cm mass was noted. On cut section, small foci of yellow nodules (maximum dimension, 0.1 x 0.2 cm) were also noted.
Histology confirmed tumor cells of epithelioid proliferation, which formed diffuse sheets and showed focal infiltration into an associated dense fibrotic stroma but without infiltration of any of the margins of the specimen. The immunophenotype of the cells confirmed the diagnosis of mesothelioma as they stained strongly positive for CK 5/6, and calretinin (cytoplasmic staining with nuclear sparing), and were negative for other immunomarkers (eg, TTF1, WT1, BerEP4, B72.3, CD34, Bcl2, and CEA polyclonal) [Fig 3, 4 ].
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What is the diagnosis?
Diagnosis: Localized malignant mesothelioma
Discussion
Mesothelioma is a primary tumor of the serosal membranes that can involve the pleura, peritoneum, and pericardium.1 Diffuse malignant mesothelioma (DMM) is an aggressive variant that grows widely over the serosal membrane surfaces; patients have very poor outcomes, with death occurring within 24 months of its diagnosis in most cases. DMM presents with its characteristic extensive studding of the serosal surface by tumor nodules, which progresses to the encasement of organs in the later stages.23 A small number of cases of so-called localized malignant mesothelioma (LMM) have been described in the literature with immunohistochemical and ultrastructural features that are identical to those of DMM, but with improved clinical outcomes (Table 1
).4567891011121314151617
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Clinical and Radiologic Discussion
LMMs are rare, solitary, circumscribed, nodular tumors, which are attached to the surface of the pleura, peritoneum, or pericardium. Malignant mesotheliomas are predominately of three types (Table 1)
. Most LMMs are incidental findings or present with nonspecific symptoms. Unlike the situation with DMMs, in which most cases occur in men, the sex ratio of patients with LMMs is approximately equal in our review (Table 1).
While DMM is associated with asbestos exposure, in our review of 35 cases of LMM, only 5 cases had a definite history of exposure and another 3 cases had a history that was suspicious for exposure. Since information about asbestos exposure was not available in many of the cases, we were unable to ascertain the role of asbestos exposure nor were we able to establish any other common etiology in the causation of LMM (Table 1).
Patients in reported cases of LMM have had tumor varying in size from as small as 1.8 cm to as large as 10 cm. The pattern shown by CT scans in this case, consisting of a small localized subpleural nodule, is much more likely to be a solitary fibrous tumor (SFT) than a malignant mesothelioma. The diagnosis of localized mesothelioma can only be distinguished pathologically. The initial imaging of choice to define the nodule is a CT scan that has been followed by an MRI for better description in a few cases.141517 The role of PET scans in differentiating LMM from other entities (eg, fibroma) is still unclear. In the patient our case, an increased uptake was noted that might help differentiating an LMM from other entities; however, it must be emphasized that the role of PET scanning in diagnosing LMMs is yet to be defined. Similarly, only a handful of patients have been biopsied with imaging techniques, probably because of the small size of the tumor and occasionally because of their location. Again, the role of biopsy with improved imaging modalities is also yet to be determined.
LMM has better overall outcome than DMM. However, in LMM, according to our review of the literature, neither the histologic subtype nor tumor size was found to correlate with survival (Table 1). One of the important features of LMM is that many cases can apparently be cured with early surgical intervention. In contrast to other studies in our review, half of the patients in the study by Allen et al10 were alive, many after a significant period of follow-up. The authors of this study were unsure whether this was due to a slightly different case definition or to other factors such as better imaging modalities, improved staining procedures, earlier intervention, or advanced surgical techniques.
Those cases in the literature that were reported to recur tend to metastasize in the fashion of sarcomas, emphasizing their differences from DMM. While chemotherapy has a definitive role in DMM, very little if any information is available on the role of chemotherapy in patients with LMM and also in those patients where it reoccurs (Table 1).
Pathologic Discussion
Mesotheliomas are classified into localized and diffuse types, of mesothelial and submesothelial origin, respectively. Localized mesotheliomas of submesothelial origin are now called solitary fibrous tumors. Until the 1980s, the term fibrous mesothelioma was often used for what is now called SFT. Some of these older articles may have been referring to SFT when they discussed LMM. SFT and malignant mesothelioma are now recognized to be completely different neoplasms with important clinical, radiologic, and pathologic differences, and the term fibrous mesothelioma is no longer used for SFT, as SFTs are considered to be of mesenchymal stem cell origin. Mesothelioma cell origin is rare and may be of epithelioid, sarcomatoid (spindle), or biphasic subtypes.4567891011121314151617
Immunohistochemisty is useful in differentiating mesothelioma from adenocarcinoma, which can present similarly.123242526272829 Although not pathognomonic, an immunohistochemical profile that is positive for calretinin, keratin, thrombomodulin, and vimentin, and is negative for CEA, SP-A, and CD15, the markers for adenocarcinoma, is highly suggestive of the mesothelial origin of the cells.12426 Some authors have suggested that the proliferation index of 67Ki should be part of the pathology report, as it was found to have an important prognostic value.30313233
The characteristic appearance of mesothelial cells on electron microscopy (ie, numerous long, slender, and smooth microvilli) point to the diagnosis of mesothelioma.1329 By definition, LMMs are immunohistochemically, and ultrastructurally identical to DMMs. Epithelium-type LMMs predominate, and very few tumors are purely sarcomatous. DMMs show macroscopic and/or microscopic evidence of widespread tumors on the serosal surface, whereas LMMs are well circumscribed. Reported cases34353637 of LMM that recur as DMM should be carefully reviewed to ensure that a DMM was not present at the time of the original diagnosis. In conclusion, we would suggest that, although LMMs share similar microscopic and immunohistochemical features with DMMs, LMM should probably be considered a separate entity, distinct by its occult clinical presentation, its sharp morphologic demarcation, and its higher curability with surgical resection.
Footnotes
The authors have reported to the ACCP that no significant conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article.
Received for publication March 14, 2006. Accepted for publication September 8, 2006.
References
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