Mucoepidermoid carcinoma (MEC) of the tracheal-bronchial tree is rare, comprising only 0.1% to 0.2% of primary lung malignancies [¹]. It is believed to originate from the minor salivary glands lining the tracheal-bronchial tree. Although considered an indolent tumor, local invasion and lymph node metastases may occur. Computed tomography (CT) usually reveals a solitary nodule or an endobronchial mass with or without post-obstructive pneumonia or atelectasis [²]. Recently, ¹⁸fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography (PET)-CT has been reported to be useful in lung cancers and carcinoids. However, the literature on PET-CT findings in MECs is scanty. We present three cases of this rare tumor with the FDG uptake patterns, and review the available literature.

This was an investigative protocol, which was approved by the ethics committee of our institution, and informed consent was gained for each patient.

All patients presenting with tracheal-bronchial tumors underwent ¹⁸F-FDG PET-CT and ⁶⁸Gallium 1,4,7,10-Tetraazacyclododecane-N^I,N^II,N^III,N^IIII,- tetra acetic acid (D) - Phel¹-Tyr³-octreotide (⁸Ga-DOTATOC) PET-CT using a dedicated PET-CT scanner (Biograph 64; Siemens Medical Solutions Inc, Mountain View, CA, USA) to assess the diagnostic value of these methods in evaluating bronchial tumors.

We present three such cases, for which the histologic diagnosis was MEC, and retrospectively analyze their PET-CT findings and histopathologic grading. Their clinical parameters and other details are given in Table 1.

MEC is an uncommon lesion accounting for under 1% of primary malignant bronchial tumors. Although generally indolent, local invasion and lymph node metastases may occur. The tumor generally affects patients aged over 30 years. Patients usually present with cough, haemoptysis, wheezing and recurrent pneumonia due to bronchial obstruction, but 9% to 28% of cases may be asymptomatic [²,³].

Histologically, MECs are composed of varying mixtures of mucus-secreting, columnar and goblet cells. They are classified as high-grade or low-grade based on histologic appearance (number of mitoses, nuclear pleomorphism and necrosis). The histopathologic grading also reflects the prognosis of these tumors [²].

On conventional radiology, MECs usually appear as oval or lobulated, slightly enhanced, endobronchial masses with occasional punctuate calcification. There may be post-obstructive pneumonia and/or peripheral atelectasis [³].

Functional imaging is emerging as a helpful tool for the evaluation of bronchopulmonary tumors. However, experience in MECs is limited, with 12 cases reported from six studies (summarized in Table 2) [^2-7]. The range of SUVmax values on ¹⁸F-FDG PET-CT scan varies from zero to 6.2 for low-grade MECS and from 2.86 to 23.4 for high-grade MECs. We found uniformly slight uptake (low SUV) on ¹⁸F-FDG PET-CT in all three cases, all of which had a histopathologic diagnosis of low-grade MEC (Figure 1). Thus, our study suggests that the SUVmax on ¹⁸F-FDG PET-CT scan may be a predictor for histopathologic differentiation of MEC.

Bronchial carcinoids constitute a common differential diagnosis for MECs, both by structural radiology and by ¹⁸F-FDG PET-CT. In doubtful cases, such as those with an inconclusive preoperative biopsy, a ⁶⁸Ga-DOTATOC PET-CT scan can be performed, results of which are usually positive for typical bronchial carcinoids and negative for MECs (Figure 2) [⁸].

Although the overall reported experience is very limited, ¹⁸F-FDG PET-CT scan might be a useful method for assessing MECs. ¹⁸F-FDG PET-CT may also have a prognostic relevance by predicting the histopathologic differentiation of the tumor.

¹⁸F-FDG PET-CT: fluorodeoxyglucose positron emission tomography-computed tomography; ⁶⁸Ga-DOTATOC PET-CT: ⁶⁸gallium 1,4,7,10-tetraazacyclododecane-N^I,N^II,N^III,N^IIII,- tetra acetic acid (D)-Phel¹-Tyr³-octreotide positron emission tomography-computed tomography; CECT: contrast enhanced computed tomography; MEC: mucoepidermoid carcinoma, SUVmax: standardized uptake value.

Written informed consent was obtained from patients two, three and from patient one's father for publication of this case report and accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal.

The authors declare that they have no competing interests.

TJ conceived the study and made a major contribution in the compilation, analysis, literature review and formatting of the manuscript, AK had a major contribution in the analysis and editing, RK helped in the study design and data acquisition, RD contributed in data analysis, MM helped in the review of the literature. All the authors have read the final manuscript and have approved it.