In March of 2023, the International Mesothelioma Interest Group (iMig) published technical guidance for imaging of malignant pleural mesothelioma (MPM) in the Journal of Thoracic Oncology.1 The purpose of this publication was to outline best imaging practices through consensus of an international panel of MPM imaging experts.
MPM is a relatively rare but aggressive malignancy that presents unique challenges to radiologists tasked with interpreting exams for oncologic patient management; it also creates a barrier to the use of aggregated imaging data needed for research and development. This is because MPM typically presents as discontinuous nodules or rinds of tumor distributed across the multiplanar curvilinear surfaces of the pleural space, making it hard to measure by conventional RECIST. MPM also has a tendency for local invasion into adjacent structures such as chest wall, diaphragm, and mediastinum, which is often difficult to detect, and has implications for staging and for potential surgical resection. Finally, evaluation of a pleural tumor can be complicated by the presence of pleural plaques, complex pleural effusions, and atelectasis in adjacent lung, all of which are common in the setting of MPM.
These disease characteristics necessitate consistent radiological assessments to capture disease response to therapy and minimize technical variability across imaging sites. This harmonization of imaging enables optimization of individual patient care and potentiates development of advanced imaging techniques for MPM.
Keeping the limitations of MPM imaging in mind, there are steps that a clinical team can take to provide the radiologist with the best quality imaging.
First, it is ideal to have patients undergo imaging at a facility that has adequate experience evaluating MPM and where protocols are more likely to be tailored to this disease to ensure, for example, that the entirety of the pleural space and chest wall is optimally imaged. For CT imaging this means adequate anatomic coverage extending through the posterior costophrenic sulci, which are often distended caudally due to pleural fluid and tumor. When ordering CT exams for MPM assessment, the iMig panel recommends clinicians order CT of the chest and abdomen even when disease is confined only to the chest, since chest CT alone may miss critical sites of disease.
The iMig panel also recommends the use of intravenous contrast whenever possible because this optimizes tissue contrast between tumor and adjacent tissues of similar attenuation, making it easier to appreciate the true tumor extent. Acquisition of thin-section (1-2 mm) images and reconstruction in three planes of imaging (axial, sagittal, and coronal) is also necessary for a detailed pleural assessment to further increase the ability to detect subtle abnormalities and changes over time.
MRI plays a complementary role in MPM assessment in certain clinical scenarios, usually in the setting of suspected local structure invasion and equivocal CT findings. To optimize MRI acquisition, we recommend that the MRI coil be placed with sufficient coverage of the diaphragm and chest wall to avoid confounding artifacts, and the use of short bore magnets should be avoided if possible.
In general, obtaining imaging at the same facility over time allows for less variability derived from different scanner hardware, software, and imaging protocols; this is especially true of PET/CT imaging. FDG-PET measures glucose metabolism which is dynamic and can be affected by several variables, including hardware factors, imaging protocol, and patient factors. When ordering PET/CT imaging, it is recommended that the same exam protocol, with conformance to established PET/CT practices,2 be used over time. For example, if the PET/CT was performed with intravenous contrast at baseline, then all subsequent PET/CT exams should be performed with contrast.
Finally, once imaging is acquired, MPM growth patterns present unique challenges for accurate disease measurement. Armato et al. demonstrated that there can be a large degree of variability in tumor measurement, which is influenced by measurement practices3 that may affect the precision of disease response assessment. These observations resulted in the development of the modified Response Evaluation Criteria in Solid Tumors (mRECIST),4 and subsequent mRECIST 1.1,5 both of which employ radial measurements of disease targets.
The iMig panel recommends that the principles of mRECIST 1.1 should be used even when patients are not on a clinical trial since they were developed to ensure serially consistent measurement. These principles include selection of disease sites for measurement that are away from the curvature of the diaphragm and apex of the lung, a preference for the non-dependent lung (away from pleural fluid), and use of pleural measurements obtained on image sections at least 1 cm apart. As mentioned above, it is more likely that centers that assess substantial numbers of MPM patients will be employing these optimal practices in image acquisition and disease measurement.
The report of the iMig panel offers several considerations that can potentiate a worldwide effort to harmonize imaging protocols for MPM. In addition to clinical benefit for individual patients, the widespread use of optimized MPM imaging will ensure high-quality imaging data to fuel research including the development of novel imaging strategies for disease quantification and analysis. This is especially true for the field of radiomics and artificial intelligence, where large datasets of aggregated imaging and clinical data are needed to move the field forward.
In summary, the paper by Katz et al.1 recently published in JTO provides technical guidance for imaging of MPM and highlights several ways clinicians can aid in improving the quality of MPM images acquired for oncologic management. Having imaging performed at a center with experience in MPM and following the guidance provided by the iMig panel, making sure imaging exams are acquired at the same institution over time, ordering the appropriate type of exam with adequate use of intravenous contrast and incorporating complete spatial coverage of the entire pleural space, will ensure a more reliable assessment and improved outcomes for patients.
References
- 1. Katz SI, Straus CM, Roshkovan L, Blyth KG, Frauenfelder T, Gill RR, et al. Considerations for Imaging of Malignant Pleural Mesothelioma: A Consensus Statement from the International Mesothelioma Interest Group. J Thorac Oncol. 2023;18(3):278-98.
- 2. Boellaard R, Delgado-Bolton R, Oyen WJ, Giammarile F, Tatsch K, Eschner W, et al. FDG PET/CT: EANM procedure guidelines for tumour imaging: version 2.0. Eur J Nucl Med Mol Imaging. 2015;42(2):328-54.
- 3. Armato SG, 3rd, Nowak AK, Francis RJ, Kocherginsky M, Byrne MJ. Observer variability in mesothelioma tumor thickness measurements: defining minimally measurable lesions. J Thorac Oncol. 2014;9(8):1187-94.
- 4. Byrne MJ, Nowak AK. Modified RECIST criteria for assessment of response in malignant pleural mesothelioma. Ann Oncol. 2004;15(2):257-60.
- 5. Armato SG, 3rd, Nowak AK. Revised Modified Response Evaluation Criteria in Solid Tumors for Assessment of Response in Malignant Pleural Mesothelioma (Version 1.1). J Thorac Oncol. 2018;13(7):1012-21.
