IMT is historphologically diverse and exhibits the following three basic histological morphologies : ① the mucus-type pattern: the tumor stroma is mucoid and similar to nodular fasciitis; ② the cellular compact spindle cell pattern: a dense bundle of spindle fibroblasts and myofibroblasts were evident, with infiltration of histiocyte-like cells and inflammatory cells; and ③fiber-type pattern: a sparse arrangement of tumor cells and an interstitium with various degrees of collagen, calcification, gravel and ossification. Immunohistochemistry results revealed the following. The tumor cells express SMA and vimentin, partially express desmin, CD34, CD68, and ALK, and do not express CK or S100. Abnormal ALK expression and ALK gene rearrangement represent a pathological histological diagnosis and differential diagnosis, and ALK rearrangement was observed in approximately 50% of patients . Among them, aberrant ALK expression and ALK gene rearrangement have pathological histological diagnosis and differential diagnostic significance. In 1999, Nicholson et al first described PPMS as a novel low-grade malignant myxoid endobronchial neoplasm . Thway et al labeled this rare but unique myxoid tumor in 2011 as “primary pulmonary myxoid sarcoma (PPMS)” . Since then, 28 instances of PPMS have been reported. To date, this disease has not been reported in minors. PPMS is an exceptionally rare intrapulmonary tumor emerging from large bronchi in young adults averaging 46.3 years old with female predominance (female: male, 1.5:1) [6.11]. Although the tumor was considered to have a distinct prognosis, renal metastasis, brain metastasis, and contralateral lung metastasis were observed in a few cases [10.12.13]. Clinical and imaging of the disease are yet to detailed specificity of PPMS. Diagnosis primarily depends on tissue morphology and genetic characteristics. The tumor was 1.5 to 13.0 cm in size, with the median size of 3.5 cm and a well-circumscribed nodular mass with clear boundaries. The cut surface was grayish-white, grayish-yellow, and gelatinous .Microscopic oval, spindle, or stellate shaped tumor cells embedded in a prominent mucinous matrix [9.10] are  characterized by the oncogenic fusion gene EWSR1–CREB1, which has been incorporated into the latest WHO guideline as a feature of this tumor . However, in our case, histomorphology was very similar to PPMS, especially the tumor lobulated, microscopic nonrich mucus matrix with large astroand polygonal cells, small infiltration of inflammatory cells, and negative immunohistochemical desmin. Thus, accurate diagnosis becomes challenging. Second-generation sequencing of the cases revealed that the tumor was accompanied by ALK–TPM3 gene fusion, whereas EWSR1–CREB1 gene fusion was absent. The accuracy of the results was verified by the FISH test again, which revealed positive ALK gene rearrangement and negative EWSR1 rearrangement. Combining genetic testing and patient age, the diagnosis was IMT.
Differential diagnosis of the IMT of the lung includes gastrointestinal stromal tumor (GIST), leiomyoma, and fibromatosis . ① Primary myoepithelial tumors of the lung in which cells have a reticulate and trabecular growth pattern and a prominent mucoid matrix. Tumor cells differ from spindle to epithelioid, with pale stained nuclei, eosinophilic, and clear cytoplasm. Myooepitheliomas in the lung are rare and can distinguish IMT through positivity for cytokeratin, S100, and other myoepithelial markers. ② Fibromatosis (Fibromatosis): more myofibroblasts observed in the lesion; however, the cells were arranged in parallel wide bundles, slightly wavy bending. The peripheral infiltration surrounds the skeletal muscle and is closely related to the tendon and aponeurosis. The inflammatory cell infiltration was less diffuse than IMT, positive for CD34 and catenin, and did not express SMA and MSA. ③ External gastrointestinal stromal tumors in mesenteric, omentum, or retroperitoneal: nuclear vacuoles, rich in thick parenchyled vessels, positive for immunohistochemical markers CD117 and CD34, and negative for SMA. Shin et al reported a case of GISTs concurrent with gastritis myofibroblastoma and detailed the first copresence of these two tumor components.④ Leiomyoma: the boundary is clear, with rich eosinophilic fiber cytoplasm, smooth muscle tumor cells exhibiting regular arrangement, woven, and no obvious inflammatory cell infiltration; immunohistochemistry can assist differentiation.
Previously, IMT of lung was considered to have a prognosis in the past, and in most cases, patient may exhibit tumor-free survival. The disease may be caused by infection or chronic inflammatory stimuli, allergies, autoimmune disorders, and other factors . In the reported case, the child had a large tumor, and two histological forms, with nonrich mucoid matrix, exhibiting a close bundle arrangement of spindle fibroblasts and myofibroblasts and no obvious inflammatory cell infiltration. Immunohistochemical tumor cells of vimentin (+), EMA (+), ALK (+), and SMA (+), but no desmin, result in a poor prognosis. This phenomenon was seldom reported previously. Therefore, we speculate histologic morphology and tumor size may be related to various clinical outcomes. Furthermore, ALK is a tyrosine kinase receptor, and ALK expression and gene rearrangement plays a crucial role in the genesis of inflammatory myofibroblastoma . ALK gene rearrangements typically occur in children and young adults, and the ALK gene is fused to two related tropomyosin genes, namely TPM3 and TPM4. Epithelioid inflammatory myofibroblastic sarcoma is a subtype of IMT, which can appear as ALK nuclear membrane and perinuclear expression, aggressive biological behavior, rapid local recurrence and often death . In this case, ALK–TPM3 gene fusion has a very poor prognosis. We speculate if this gene fusion also a molecular marker indicating poor prognosis. Li et al reported two patients with ALK–RANBP2 inflammatory myofibroblastoma with rearrangement still died within a short time . However, because of the limited number of cases, future studies should focus on the confirmation of the phenomenon.
Currently, IMT is a less malignant mesenchymal tumor that typically occurs predominantly in children or adolescents. Tumor-free survival has been reported in most cases, and recurrence and distant metastasis have been reported in few cases. However, in the reported patient poor prognosis may be related to the tumor size, complex histological morphology, and ALK-TPM3 gene fusion. However, because of the small number of cases, conclusion are definite. To date, surgical resection remains the preferred treatment for IMT because of conventional chemotherapy or radiotherapy reporting poor efficacy. In previous reports, limited studies have focused on distinguishing PPMS from IMT. When a combination of IMT with non-mucus-rich matrix and limited inflammatory cells is reported, misdiagnosis and missed diagnosis of PPMS should be avoided. Genetic testing can improve diagnosis and differential diagnosis.