Functional Imaging to Differentiate Pulmonary Carcinoids

TO THE EDITOR: We read with great interest the article by Kayani et al. (1). They evaluated 11 cases of typical carcinoids, 2 of atypical carcinoids, 1 of small cell neuroendocrine tumor, 1 of large cell neuroendocrine tumor, 2 of diffuse idiopathic pulmonary neuroendocrine hyperplasia, and 1 of adenocarcinoma with neuroendocrine differentiation using ¹⁸F-FDG PET/CT and ⁶⁸Ga-DOTATATE PET/CT. They reported that, compared with typical carcinoids, atypical and other less differentiated carcinoids revealed significantly higher uptake on ¹⁸F-FDG PET/CT scans (P = 0.005) and significantly lower uptake on ⁶⁸Ga-DOTATATE PET/CT scans (P = 0.002).

We observed an interesting finding by evaluating the ratios of maximum standardized uptake value (SUVmax) on ⁶⁸Ga-DOTATATE PET/CT to SUVmax on ¹⁸F-FDG PET/CT scans. The ratios range from 1.2 to 19.5 in typical carcinoids (median SUVmax, 7.3) but from 0.1 to 1.1 in other less differentiated carcinoids (median SUVmax, 0.16). This difference in the ratios of typical versus atypical and other less differentiated carcinoids is statistically significant. The P value is 0.002 (Mann–Whitney test for nonparametric data, 2-tailed). Thus, the ratio of SUVmax on ⁶⁸Ga-DOTATOC PET/CT and ¹⁸F-FDG PET/CT may also be of help in predicting the histopathologic variety of the carcinoid tumor and may have an equally high accuracy. A larger study is indicated to validate this observation and to objectively determine a cutoff value for the possible differentiation.

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