Malignant Pleural Mesothelioma (MPM)
Malignant pleural mesothelioma (MPM) is a rare and aggressive form of cancer that originates in the pleura within the chest cavity. The pleura is thin cellular lining than envelops both the lung (termed the “visceral” pleura) and chest wall (the “parietal” pleura), as well as the diaphragm, and the heart and major blood vessels in the central portion of the chest. Transformation of the pleura into cancer, therefore, will form a layer of tumor that covers, and may in fact grow directly into any of these vital structures.
Types of malignant pleural mesothelioma (MPM)
There are three major subtypes of MPM, which are defined by their histology (appearance under the microscope).
- Epithelial MPM is the most common form and makes up about 60% of all cases.
- Sarcomatoid MPM comprises about 10% of all cases and these tumors display a somewhat more aggressive behavior.
- The remainder of cases are comprised of the Mixed, or Biphasic variant of MPM, that contains elements of both Epithelial and Sarcomatoid MPM.
Symptoms of Malignant Pleural Mesothelioma
The most common symptom of MPM is shortness of breath that is typically caused by a pleural effusion. This effusion is fluid that accumulates around the lung due to secretion of fluid by tumor cells, and the fluid does not allow the lung to fully expand. Chest pain, cough, weight loss, generalized malaise, and occasionally fevers may also be present.
Staging for Malignant Pleural Mesothelioma
Staging systems for cancers are meant to grade the spectrum of early to advanced disease, in order to guide appropriate therapy. The two most important factors in the staging of MPM are:
- The degree of invasion of surrounding structures, and
- The presence or absence of involvement to mediastinal lymph nodes. Mediastinal lymph nodes are lymph nodes of the central chest that thoracic tumors can spread to beyond the earliest stages.
Imaging studies such as CT, PET, and MRI are routinely used to assess tumor burden and invasion. Early stage tumors may present as focal areas of pleural thickening and later stages may present as large masses or tumors that completely encase the lung and invade major structures in the chest. To assess the involvement of mediastinal lymph nodes by MPM, PET scanning is useful, however the most accurate method of assessment is mediastinoscopy, a surgical biopsy technique to sample these nodes for microscopic evaluation.
Stages of MPM
- Stage I MPM represents early disease with low tumor burden.
- Stage II MPM represents more locally advanced disease with a greater tumor burden.
- Stage III MPM represents locally advanced disease with significant invasion of surrounding structures or spread to mediastinal lymph nodes.
- Stage IV MPM represents a technically unresectable tumor, or spread to the abdomen or other side of the chest.