PT  - JOURNAL ARTICLE
AU  - Tine Nøhr Christensen
AU  - Seppo W. Langer
AU  - Gitte Persson
AU  - Klaus Richter Larsen
AU  - Annika Loft
AU  - Annemarie Gjelstrup Amtoft
AU  - Anne Kiil Berthelsen
AU  - Helle Hjorth Johannesen
AU  - Sune Høgild Keller
AU  - Andreas Kjaer
AU  - Barbara Malene Fischer
TI  - <sup>18</sup>F-FLT PET/CT Adds Value to <sup>18</sup>F-FDG PET/CT for Diagnosing Relapse After Definitive Radiotherapy in Patients with Lung Cancer: Results of a Prospective Clinical Trial
AID  - 10.2967/jnumed.120.247742
DP  - 2021 May 10
TA  - Journal of Nuclear Medicine
PG  - 628--635
VI  - 62
IP  - 5
4099  - http://jnm.snmjournals.org/content/62/5/628.short
4100  - http://jnm.snmjournals.org/content/62/5/628.full
SO  - J Nucl Med2021 May 10; 62
AB  - Diagnosing relapse after radiotherapy for lung cancer is challenging. The specificity of both CT and 18F-FDG PET/CT is low because of radiation-induced changes. 3′-deoxy-3′-18F-fluorothymidine (18F-FLT) PET has previously demonstrated higher specificity for malignancy than 18F-FDG PET. We investigated the value of 18F-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 18F-FLT PET/CT was higher than that of 18F-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 18F-FLT PET/CT and 18F-FDG PET/CT was higher after cRT than after SBRT. The sensitivity of 18F-FLT PET/CT was lower than that of 18F-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 18F-FLT PET/CT when 18F-FDG PET/CT was positive or inconclusive improved the diagnostic value compared with 18F-FDG PET/CT alone. In cRT HDVs, the probability of malignancy increased from 67% for 18F-FDG PET/CT alone to 100% when both tracers were positive. Conclusion: 18F-FLT PET/CT adds diagnostic value to 18F-FDG PET/CT in patients with suspected relapse. The diagnostic impact of 18F-FLT PET/CT was highest after cRT. We suggest adding 18F-FLT PET/CT when 18F-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.