Radiological imaging plays an important role in the diagnosis, staging and management of malignant meothelioma. To make a diagnosis, various types of diagnostics tests are used such as X-ray, Magnetic Resonance Imaging (MRI), CT, and Positron Emission Tomography (PET).
CT
If pleural malignant disease is suspected, the primary imaging modality that is used is intravenous contrast-enhanced CT. With CT diagnostic tests, doctors can visualize the entire pleural surface and diaphragm. A scan delay of 45-60 seconds enables pleural surfaces to be analyzed while still allowing examination of the mediastinal nodes. A standard diagnostic protocol should focus on the liver and adrenal glands. However, if the patient has a history of pelvic or abdominal malignancy, the CT scan should include the pelvis and the lower abdomen as well.
It is a challenge to differentiate between malignant and benign pleural disease. Some of the CT findings that help indicate presence of pleural disease include the following –
1) A circumferential pleural rind
2) Nodular pleural thickening
3) Pleural thickening of less than 1 cm
4) Mediastinal pleural involvement
These findings had specificities of 100%, 94%, 94% and 88% respectively. Sensitivities recorded were 41%, 51%, 36% and 56% respectively. It is rare that a CT scan test reveals the existence of bilateral pleural calcification in malignant mesothelioma cases. However, a substantial reduction in thoracic volume is commonly visible on a CT scan. Data available from some series indicates that this occurs in up to 73% of mesothelioma cases. Although the positive predictive value of these findings is high, their absence does not necessarily negate the diagnosis of pleural malignancy.
MRI
MRI screening is not generally used in the evaluation of malignant mesothelioma. However, in case of patients with suspected resectable disease, a MRI scan test can reveal more staging information, in addition to that provided by CT scan tests. With the use of gadolinium enhancement, MRI allows improved identification of the tumor’s spread into the chest wall or the diaphragm. It allows doctors to select the most appropriate surgical treatment for a patient. An MRI scan test is also preferred when intravenous iodinated contrast is contraindicated in patients.
PET
The standardized uptake value (SUV), as derived from a PET scan, provides a semi-quantitative measure of a lesion’s metabolic activity. The SUV is substantially higher in mesothelioma in comparison to other benign pleural conditions, for example, inflammatory pleuritis and pleural plaques. A research study has found that PET scans have a sensitivity and specificity of 96.8% and 88.5% respectively when it comes to making a differentiation between benign and malignant pleural disease. Since PET scanning is also more accurate in diagnosing mediastinal nodal metastases, both metabolic and anatomical information, as gained through PET scans, helps in the staging and preoperative assessment of mesothelioma. PET scans are also useful in identifying the optimal site in case of CT guided pleural biopsy, and it has been verified that response to treatment may be assessed by examining the changes in the fluorodeoxyglucose (FDG) uptake occurring within the tumor. This is indicative of its role in the evaluation of response to treatments such as chemotherapy and chemo-radiotherapy.
Staging and evaluation of disease response
Around six different staging systems are currently available for malignant mesothelioma, but none of these have shown to accurately predict survival. As of now, the International Mesothelioma Interest Group (IMIG) has proposed a TNM staging system, which is similar to what is used in case of non-small cell lung carcinoma.
Evaluating tumor response to treatment is vitally necessary in terms of patient benefit. Criteria for tumor response, as described by the World Health Organization (WHO), were most useful in the measurement of bi-dimensional lesions. However, these criteria work poorly when it comes to measuring the irregular growth pattern which is evident as a rind around the chest wall of mesothelioma patients. More recently, a uni-dimensional measurement of tumor size has been used to evaluate response under the Response Evaluation Criteria in Solid Tumors (RECIST) system. However, since this technique is based on the supposition that tumors are mostly spherical, it has limited use when it comes to evaluating malignant mesothelioma.
A revised RECIST criteria is now available with specific focus on malignant
mesothelioma. Now, the evaluation of response to treatment is achieved
by measuring the tumor’s uni-dimensional thickness as perpendicular to
the chest wall. Measurements are taken at two sites at three specific
levels on CT.