Malignant Mesothelioma

Cyclin-Dependent Kinase Inhibitor 2A/Alternative Reading Frame

CDKN2A/ARF is the most frequently inactivated tumor suppressor gene in malignant mesothelioma and encodes two important cell cycle regulatory proteins, p16 (INK4A) and, in an alternative reading frame, p14 (ARF) . p16, a cyclin-dependent kinase (CDK) inhibitor, blocks phosphorylation of retinoblastoma protein, and p14 (ARF) blocks murine double minute 2 (MDM2) , thus resulting in a positive regulation of p53. Homozygous deletion of CDKN2A/ARF thus results in inactivation of two major tumor suppressing pathways, retinoblastoma protein and p53.

CDKN2A deletion is found in about 70% of primary tumors and nearly all mesothelioma cell lines.

Neurofibromatosis Type 2

The NF2 gene encodes a tumor suppressor protein Merlin, a member of the band 4.1 family of cytoskeletal linker proteins. Inactivating NF2 mutations are found in 35% to 40% of MPMs. The mechanisms of Merlin-mediated tumor suppression are not well defined. Merlin mediates contact-dependent inhibition of cell proliferation in normal cells, primarily through inhibition of mammalian target of rapamycin (mTOR) in an AKT-independent manner. mTOR activity is aberrantly upregulated in the absence of Merlin, leading to increased cell proliferation.

Laparoscopy and Thoracoscopy

Extension of the tumor through the diaphragm into the peritoneal cavity or dissemination to the contralateral side can occur, which will influence the staging and treatment options. Rice et al. reported on 109 patients who had routine laparoscopy before planned EPP and found nine (8.3%) patients with transdiaphragmatic extension of tumor and one (0.9%) with diffuse peritoneal carcinomatosis. Alvarez et al. performed thoracoscopic examination of the contralateral side and found contralateral chest involvement in three (10%) of 30 patients from a selected group.

Mediastinal Evaluation

Lymph node metastases occur in approximately 50% of patients with MPM undergoing multimodality therapy and portends a poor prognosis. Current imaging modalities are inaccurate for defining N stage, and, therefore, cervical mediastinoscopy has been advocated for pretreatment staging of MPM. The sensitivity, specificity, and accuracy for cervical mediastinoscopy have been reported by two groups and vary from 60% to 80%, 71% to 100%, and 67% to 93%, respectively.

Endobronchial ultrasound (EBUS)- and endoscopic ultrasound (EUS)-guided fine-needle aspiration of mediastinal lymph nodes have been highly effective for staging nonsmall cell lung cancer. Rice et al. compared 50 consecutive patients with mesothelioma who had staging cervical mediastinoscopy with 38 patients who had staging EBUS. Sensitivity and negative-predictive value were 28% and 49%, respectively, for cervical mediastinoscopy, and 59% and 57%, respectively, for EBUS. Furthermore, 11 patients had EUS preoperatively, and metastases were found in the infradiaphragmatic nodes in five patients. Tournoy et al. performed EUS and fine-needle aspiration in 32 patients with presumed early-stage mesothelioma and identified N2 metastases in four patients (12.5%). In 17 patients who subsequently had extrapleural pneumonectomy, one false-negative result (4.7%) was found. Some centers now prefer to stage the mediastinum by combined EBUC and EUS before entering a patient into a multimodality trial.

Diagnostic Biomarkers

The development of robust diagnostic serum biomarkers for mesothelioma is important, as exposure to the etiologic agent is often known. A population with heavy asbestos exposure would be rational participants in a screening program. The availability of a blood-based biomarker would facilitate early detection and treatment. The most important candidate diagnostic serum biomarkers are mesothelin (serum mesothelin-related protein), osteopontin, and fibulin-3. Elevated levels of mesothelin are highly specific, unless patients have concurrent renal failure, and add to the diagnostic certainty or direct additional investigations when a diagnosis of mesothelioma is suspected. However, mesothelin lacks sensitivity at the time of diagnosis, thus limiting its use in screening. Osteopontin does not perform as well as mesothelin. In 2012, Pass et al. reported on fibulin-3 as a blood and effusion biomarker. Plasma levels of fibulin-3 were significantly higher for patients with mesothelioma than in people exposed to asbestos without mesothelioma, with a sensitivity of 100% and specificity of 94% reported. Effusion levels of fibulin-3 were also significantly higher in effusions from mesothelioma than from other etiologies; nevertheless, these findings should be further validated before translation to practice. Carcinoembryogenic antigen is a well-known marker that will not be elevated in case of a MPM. It can be used for quick screening for other tumor types with pleural dissemination.

Prognostic Biomarkers

A number of simple prognostic biomarkers have been well established in the literature for MPM. At pathologic diagnosis, a diagnosis of sarcomatoid or nonepithelioid subtypes of mesothelioma is uniformly associated with poor prognosis. The same large retrospective series, mostly based on collections of clinical trial data, has also validated readily available laboratory parameters, including low hemoglobin, thrombocytosis, high white blood cell count, and elevated serum lactate dehydrogenase level as poor prognostic indicators. In 2010, Kao et al. proposed and independently validated an elevated neutrophil-to-lymphocyte ratio as a poor prognostic indicator; however, the patient groups used were heterogeneous and relatively small, and others have been unable to confirm these findings. High serum vascular endothelial growth factor (VEGF) levels have been found to correlate with poor prognosis and advanced stage of disease. A recent publication used a Classification and Regression Tree analysis to group patients into prognostic categories at diagnosis, deriving four prognostic groupings with median survivals ranging from 7.5 months (risk group 4) to 34 months (risk group 1) using readily available clinical indicators including weight loss, hemoglobin level, performance status, histology, and albumin level.

Considering prognostic serum biomarkers more specific to mesothelioma, mesothelin levels at diagnosis may also be prognostic; however, mesothelin levels appear to reflect tumor bulk, as the addition of tumor bulk metrics to the model eliminates the significance of serum mesothelin. No strong evidence exists that serum osteopontin is useful in prognostication, although lower tissue expression of osteopontin may be associated with longer survival and plasma osteopontin as well as mesothelin were found to increase prognostic accuracy in combination with the EORTC and CALGB mesothleioma prognostic indices.

Many candidate tumor molecular and histologic prognostic markers have been reported (phosphatase and tensin homolog, VEGF expression, fibroblast growth factor 2, cyclooxygenase-2, platelet-derived growth factor, epidermal growth factor receptor, epithelial-to-mesenchymal transition, osteopontin, and c-MET expression); however, at the time of publication, none has been sufficiently validated to be entered into routine clinical use worldwide. The anticipation is that the extensive molecular profiling and characterization efforts proceeding worldwide will identify new molecular prognostic biomarkers that will translate to routine clinical practice in addition to providing potential molecular targets for therapy.

Prognostic Imaging Biomarkers

In addition to providing anatomic staging information regarding the sites of disease involvement, a substantial body of evidence indicates that the bulk of tumor as demonstrated on imaging and metabolic characteristics of tumor may be prognostic biomarkers. Tumor volume as measured by CT is a prognostic indicator, but it can be challenging to implement automated volumetric measurements due to the difficulty of distinguishing tumor from pleural fluid and atelectasis. Quantitative parameters from FDG-PET may be simpler to implement reproducibly and with less manual input. FDG-PET has consistently shown prognostic value in MPM, although the appropriate metric for quantitative assessment remains the subject of debate. A higher maximum standardized uptake value (SUV) is a poor prognostic indicator. However, the inclusion of volumetric parameters may also be important, with the concepts of total lesion glycolysis or total glycolytic volume incorporating both SUV and a measure of lesion volume and performing better than SUV alone in studies incorporating both measures.