Ann Nucl Med DOI 10.1007/s12149-014-0848-3

LETTER TO THE EDITOR

Reply to ‘Is it time to validate the prognostic role of F-18-FDG PET/CT scan in thymic epithelial tumors?’ Masayuki Chida • Setsu Sakamoto

Received: 4 March 2014 / Accepted: 20 March 2014 Ó The Japanese Society of Nuclear Medicine 2014

Thank you very much for your thought-provoking letter regarding our article. As you indicated, thymic epithelial tumors are typically indolent and slow-growing neoplasms, representing a limited recurrence rate and a very long mean disease-free survival [1]. The figure below is the one omitted based on the instruction of the reviewer. As demonstrated there, all subjects in the low-risk group showed no recurrence, while some in the high risk and thymic cancer groups had a relapse. The period of observation until recurrence in our study was 1319 ± 790 days (about 43 months) for all cases, which ranged from 247 to 2587 days for nonrecurrent and 54 to 1043 days for recurrent cases. During these follow-up period, three of 9 patients in the high-risk group developed recurrence, of whom two were diagnosed with stage IVa and one with stage III invasive thymoma. Thymic cancer recurred during the observation period in four of 6 subjects. Of the thymoma cases, only four of 31 patients were diagnosed as stage III or higher and three of those had a relapse. All of the recurrent thymomas were clinically highly malignant and recurred within a relatively short period of time. As shown in Table 1, the maximum of standardized uptake value (SUVmax) determined by FDG-PET varied

M. Chida (&) Department of General Thoracic Surgery, Dokkyo Medical University, Mibu, Tochigi, Japan e-mail: [email protected] S. Sakamoto Positron Emission Tomography Center, Dokkyo Medical University Hospital, Mibu, Tochigi, Japan

even in cases with the same WHO classification. SUVmax determined by FDG-PET was not concluded to be useful for stage I and II cases, or the low-risk group, as no significant difference was demonstrated during the observation period (1319 ± 790 days for all cases) in the present study, due to lack of relapsed case in these subjects. However, considering the nature of the thymic epithelial tumors, some subjects in the low-risk group might develop recurrence after the very long time-span of follow-up. A longer observation period may be required for detecting relapse in such cases. Controversy might exist over how much long of the follow-up period is appropriate to validate the prognostic role of FDG-PET/CT scan in thymic epithelial tumors. Lococo et al. [2] did not investigate on the prognosis of thymic epithelial tumors, because of short mean follow-up time (about 24 months). We consider that proper period of time might depend on the standpoint of the observers. In other words, an appropriate follow-up period might vary by the way of thinking as the medical doctors taking care of patients, or the way of thinking as a researcher in the field of oncology examining the prognostic role of whole thymic epithelial tumors. If someone, as a researcher in the field of oncology, eager to examine prognostic role of whole thymic epithelial tumors, it might require much longer followup period, up to decades. Needless to say, as medical doctors, we think that it is important to be aware of possibilities to recur even in the patients in low-risk group after the long-term follow-up. A longer observation period may be required for detecting relapse in such cases. FDG-PET is considered to be superior for detecting thymic epithelial tumors that recur relatively early. Patients with low SUVmax values (1.22, 1.86, 2.92, 3.9) in the high-risk group did not have relapse during the observation period. As a matter of course, regardless of

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the initial FDG-PET results, we will follow every patient after a therapeutic procedure keeping our concentration. We think that FDG-PET provides useful information for management of patients with thymic epithelial tumors, and do not think that our observation period was short. Moreover, our study was performed in a single center, so our SUVs are more reliable than multicenter study. Because our data are free from many underlying factors affecting SUV, when comparing SUVs between centers in multicenter PET study, even using the same PET/CT scanners (Reconstruction parameter, calibration error between scanner and dose calibrator, timing mismatch between scanner and dose calibrator, etc.) [3].

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References 1. Seki N, Sakamoto S, Karube Y, Oyaizu T, Ishihama H, Chida M. 18 F-fluorodeoxyglucose positron emission tomography for evaluation of thymic epithelial tumors: utility for World Health Organization classification and predicting recurrence-free survival. Ann Nucl Med. 2014. doi:10.1007/s12149-014-0804-2. 2. Lococo F, Cesario A, Okami J, Cardillo G, Cavuto S, Tokunaga T, Apolone G, Margaritora S, Granone P. Role of combined 18FFDG-PET/CT for predicting the WHO malignancy grade of thymic epithelial tumors: a multicenter analysis. Lung Cancer. 2013;82:245–51. 3. Adams MC, Turkington TG, Wilson JM, Wong TZ. A systematic review of the factors affecting accuracy of SUV measurements. Am J Roentgenol. 2010;195:310–20.