Evaluation of Response by FLT PET in Mesothelioma

Pleural mesothelioma (MPM) is an aggressive tumor that affects the pleura and originates from mesothelial cells. If untreated, median survival is 4-12 months following diagnosis. Asbestos exposure is a risk factor associated with 80% of cases. After the 1980s, regulations controlling the use of asbestos ensured that cases were limited. Approximately 3,000 new cases are diagnosed each year in the United States. In general, a minority of patients are candidates for surgery at the time of presentation, so the mainstay of treatment is systemic chemotherapy. For patients who are surgical candidates, surgery is usually part of a multimodal treatment process that also includes chemotherapy and/or radiotherapy. Early and accurate diagnosis has a critical impact on the management of the disease due to limited response to multimodal treatments. Patients are often diagnosed at an advanced stage, leading to poor overall survival. Thorax and upper abdomen CT imaging are standard initial imaging modalities for clinical staging of MPM. Although CT identifies the general extent of the primary tumor, it may not definitively identify some areas of tumor invasion. There may be difficulties especially in the evaluation of chest wall and diaphragm invasion.

18F-FDG PET/CT has been widely used for cancer diagnosis, staging, treatment response and prognostic information for many years with high accuracy rates. 18F-FDG PET/CT provides valuable information on differentiating benign and malignant pleural abnormalities, evaluating the possibility of malignant involvement of mediastinal and hilar lymph nodes, and detecting distant metastases. 18F-FDG PET/CT identifies metastatic disease undetected on CT in approximately 10% of patients. At the same time, the degree of involvement (SUV) in FDG PET plays a role in predicting disease prognosis. 18F-FDG PET/CT can also be used to evaluate the treatment response in patients receiving chemotherapy, but due to chemotherapy-related inflammatory changes, it is necessary to wait at least 2 weeks to evaluate the treatment response.

18F-Fluorothymidine (FLT) is a thymidine kinase 1-specific substrate that is increased in proliferating cells and is associated with the Ki-67 index, a proliferation marker. It allows noninvasive evaluation of cell proliferation, especially the early evaluation of the response to cytotoxic chemotherapy. 18F-FLT PET/CT imaging has shown success in early evaluation of response to systemic endocrine, chemotherapy, radiotherapy and combined chemotherapy in multiple tumor types. The prognostic value of a decrease in 18F-FLT uptake after initiation of treatment has also been reported.

In this study, it is aimed to evaluate the success of 18F-FLT PET/CT in the early evaluation of the response after the first cycle of chemotherapy in patients diagnosed with mesothelioma and receiving systemic chemotherapy. It is also aimed to evaluate the prognostic value of response evaluation made with this method.

It is planned to prospectively include 25 patients with MPM who scheduled for chemotherapy in the study. Included patients will undergo 18F-FDG PET/CT before chemotherapy followed by 18F-FLT PET/CT imaging within two weeks. 18F-FLT PET/CT will be performed on the 4th day after the 1st cycle of chemotherapy. After chemotherapy is completed, treatment response will be evaluated with 18F-FDG PET/CT. Patients will then be followed by their clinicians for relapse and progressive disease. Thus, the success of early 18F-FLT PET/CT in predicting end of treatment response will be evaluated.