Minute pulmonary meningothelial-like nodules (MPMN) was first described by Korn in 19602. Since this kind of lesion was composed of nests of epithelioid, similar to “carotid body tumors” and associated with pulmonary vessels, Korn named them “pulmonary chemodectomas”. However, subsequent studies showed a series of different opinion about MPMNs3-7. Warnock and Gaffey reported “pulmonary chemodectomas” had similar structural and immunohistochemical features of meningothelial cells3,4. And a new item “minute meningothelioid nodule” was accepted. Mukai reported some MPMN immunostaining positive for myosin and vimentin, suggested its origin from muscle cells6. Weissferdt reported that MPMN was associated with CNS meningiomas in common genetic pathways7. However, Ionescu reported MPMN lacked mutational damage, while meningiomas showed high frequency of loss of heterozygosity, suggested its reactive origin5. Not a common sense about mechanism of MPMN was reached, on the other hand, radiologic reports about MPMN were sparse.
Our study reported clinical, pathological and radiologic features of 59 cases of MPMN, and discussed management of these patients. Frequency of MPMN was 0.77%, lower than previous studies based on operation (7.0%-13.8%)6,8,9. Actual number of frequency might be larger because some MPMNs were discovered incidentally. Female was in the majority in our study (49/59, 83.1%), which was in accordance with previous reports8,9. 79 MPMNs were identified in pathology specimens. Of them, 36 nodules had no definite location on CT scan because they were discovered in the peripheral tissue of other resected nodules incidentally. The remaining 43 nodules were visible on chest CT scan, with an average size of 4.5mm. CT appearance included pure ground-glass opacity in 36, mix in 2 and solid nodules in 5. No specific imaging features were observed on chest CT scan. Though most MPMNs presented as pure ground-glass opacity (36/43, 83.7%), some MPMNs could still simulate malignant nodule (Figure1, Figure2). Considering little was known about mechanism of MPMN, there was no means to use CT imaging to identify benign meningothelial-like lesions which can be ignored rather than resected. It was still difficult to differentiate MPMN from malignant nodules based on CT appearance.
In fact, most patients with MPMN underwent surgery because of other malignant nodule on chest CT scan (52/59, 88.1%) instead of MPMN, that means the resection of MPMNs were additional. Those malignant nodule on chest CT scan were confirmed as lung cancer pathologically, which explained the majority of patients’ main diagnosis was malignant cancer (43/59, 72.9%). Long-term CT-scan follow-up for two patients revealed persistent ground-glass opacity, and no change in size on chest CT scan, finally underwent resection of MPMNs (Figure 3, Figure 4). Moreover, five patients underwent resection in consequence of malignant finding of MPMN.
That arouse a requirement about the management of MPMN or suspected MPMN. Surgical decision partly based on continuous monitoring of nodules. For those 29 patients with visible MPMN on chest CT scan, nearly 50% patients had a pre-operative follow-up more than 6 months (13/29, 44.8%). Part of other benign lesions presented as ground-glass opacity such as inflammatory lesion may disappear on chest CT scan over time, so to avoid unnecessary pulmonary resection, CT surveillance would be useful for management of MPMN. Considering smaller GGN after 5 years of stability still had possibility of growth10, management of MPMN may include long-term CT surveillance. What’s more, finding a means of CT imaging to differentiate MPMN from other kind of nodule would play a big role in establishing the standard management of MPMN.
This study has several limitations. Due to the occasionality of some MPMNs, some patients may miss diagnosis. Furthermore, we selected patients underwent pulmonary surgery, so patients with MPMN choose CT follow-up instead of surgical treatment were not included.
In summary, most minute pulmonary meningothelial-like nodules tend to present as ground-glass opacity, especially pure ground-glass opacity. Continuous CT monitoring revealed no radiologic change over time. Continuous CT monitoring was necessary part of management of MPMN.