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Diagnosing relapse after radiotherapy for lung cancer is challenging. The specificity of both CT and 18 F-FDG PET/CT is low because of radiation-induced changes. 3'-deoxy-3'- 18 F-fluorothymidine ( 18 F-FLT) PET has previously demonstrated higher specificity for malignancy than 18 F-FDG PET. We investigated the value of 18 F-FLT PET/CT for diagnosing relapse in irradiated lung cancer. Methods: Patients suspected of relapse of lung cancer after definitive radiotherapy (conventional fractionated radiotherapy [cRT] or stereotactic body radiotherapy [SBRT]) were included. Sensitivity and specificity were analyzed both within the irradiated high-dose volume (HDV) and on a patient basis. Marginal differences and interobserver agreement were assessed. Results: Sixty-three patients who had received radiotherapy in 70 HDVs (34 cRT; 36 SBRT) were included. The specificity of 18 F-FLT PET/CT was higher than that of 18 F-FDG PET/CT (HDV, 96% [95% CI, 87-100] vs. 71% [95% CI, 57-83] [ P = 0.0039]; patient-based, 90% [95% CI, 73-98] vs. 55% [95% CI, 36-74] [ P = 0.0020]). The difference in specificity between 18 F-FLT PET/CT and 18 F-FDG PET/CT was higher after cRT than after SBRT. The sensitivity of 18 F-FLT PET/CT was lower than that of 18 F-FDG PET/CT (HDV, 69% [95% CI, 41-89] vs. 94% [95% CI, 70-100] [ P = 0.1250]; patient-based, 70% [95% CI, 51-84] vs. 94% [95% CI, 80-99] [ P = 0.0078]). Adding 18 F-FLT PET/CT when 18 F-FDG PET/CT was positive or inconclusive improved the diagnostic value compared with 18 F-FDG PET/CT alone. In cRT HDVs, the probability of malignancy increased from 67% for 18 F-FDG PET/CT alone to 100% when both tracers were positive. Conclusion: 18 F-FLT PET/CT adds diagnostic value to 18 F-FDG PET/CT in patients with suspected relapse. The diagnostic impact of 18 F-FLT PET/CT was highest after cRT. We suggest adding 18 F-FLT PET/CT when 18 F-FDG PET/CT is inconclusive or positive within the previously irradiated volume to improve diagnostic value in patients for whom histologic confirmation is not easily obtained., (© 2021 by the Society of Nuclear Medicine and Molecular Imaging.)

Purpose: There is currently no single modality for accurate characterization of enlarged mediastinal lymph nodes into benign or malignant. Recently (18)F-fluorothymidine (FLT) has been used as a proliferation marker. In this prospective study, we examined the role of (18)F-fluorodeoxyglucose ((18)F-FDG) positron emission tomography/computed tomography (PET/CT) and (18)F-FLT PET/CT in categorizing mediastinal lymph nodes as benign or malignant., Materials and Methods: A total of 70 consecutive patients with mediastinal lymphadenopathy detected on computed tomography (CT) or chest radiograph underwent whole body (18)F-FLT PET/CT and (18)F-FDG PET/CT (within 1 week of each other). Lymph nodal tracer uptake was determined by calculation of standardized uptake value (SUV) with both the tracers. Results of PET/CT were compared with histopathology of the lymph nodes., Results: Histopathology results showed thirty-seven patients with sarcoidosis, seven patients with tuberculosis, nine patients with non-small cell lung cancer, five patients with Hodgkin's lymphoma and twelve patients with non-Hodgkin's lymphoma. The mean FDG SUVmax of sarcoidosis, tuberculosis, Hodgkin's and non-Hodgkin's lymphoma was 12.7, 13.4, 8.2, and 8.8, respectively, and the mean FLT SUVmax was 6.0, 5.4, 4.4, and 3.8, respectively. It was not possible to characterize mediastinal lymphadenopathy as benign or malignant solely based on FDG SUVmax values (p > 0.05) or FLT SUVmax values (p > 0.05). There was no significant difference in FDG uptake (p > 0.9) or FLT uptake (p > 0.9) between sarcoidosis and tuberculosis. In lung cancer patients, the FDG SUVmax and FLT SUVmax of those lymph nodes with tumor infiltration on biopsy was 6.7 and 3.9, respectively, and those without nodal infiltration was 6.4 and 3.7, respectively, and both the tracers were not able to characterize the nodal status as malignant or benign (p > 0.05)., Conclusion: Though (18)F-FLT PET/CT and (18)F-FDG PET/CT reflect different aspects of biology, i.e., proliferation and metabolism, respectively, neither tracer could provide satisfactory categorization of benign and malignant lymph nodes. The results of this study clearly suggest that differentiation of mediastinal nodes into benign and malignant solely based on SUVmax values cannot be relied upon, especially in settings where tuberculosis and sarcoidosis are common.

Although 2-deoxy-2-[18F]-fluoro-D-glucose positron emission tomography/computed tomography (18F-FDG PET/CT) is a sensitive nuclear medicine modality, specificity for characterizing lung cancer is limited. Tumor proliferation and early response to molecularly targeted therapy could be visualized using 3′-deoxy-3′[(18)F]-fluorothymidine (18F-FLT) PET/CT. The superiority of 18F-FLT PET/CT over 18F-FDG PET/CT in early therapeutic monitoring has been well described in patients after chemotherapy, radiotherapy, and/or chemo/radiotherapy. In thispilot study, we explorethe use of 18F-FLT PET/CT as an early response evaluation modality in patients with lung cancerand provide specific case studies of patients with small cell lung cancer and non-small cell lung cancer who received novel targeted therapies. Early response for c-MET inhibitor was observed in four weeks and for MDM2 inhibitor in nine days. [ABSTRACT FROM AUTHOR]