Inflammatory myofibroblastic tumor (IMT) is a low-grade mesenchymal neoplasm characterized by myofibroblastic spindle cells and prominent inflammatory infiltrate (plasma cells, lymphocytes, eosinophils). Previously termed 'inflammatory pseudotumor' or 'plasma cell granuloma,' but the discovery of ALK gene rearrangement proved it to be a true neoplasm. It is one of the most common primary lung tumors in children and young adults. The lung is the most commonly involved organ (50%). Generally behaves benignly but may show local invasion and rarely metastasize.
Age Range
10-40
Peak Age
25
Gender
Equal
Prevalence
Rare
Inflammatory myofibroblastic tumor is a clonal proliferation of myofibroblastic spindle cells — not a reactive process but a true neoplasm. ALK (anaplastic lymphoma kinase) gene rearrangement is detected in approximately 50% of patients — this fusion protein drives cell proliferation through constitutive kinase activation. ALK-positive IMTs are more common in young patients and have better prognosis. Heterogeneous enhancement on CT reflects the mixed histological composition of the tumor: myofibroblastic compact areas enhance more intensely, while myxoid/edematous areas and inflammatory infiltrate regions enhance less. The delayed enhancement pattern (increased late-phase enhancement) reflects contrast retention in the tumor's fibrous and inflammatory components — the large extracellular space in fibrous tissue slowly accumulates and slowly releases contrast. Calcification presence (15-30%) results from dystrophic calcification.
Moderate enhancement in early arterial phase, increasing enhancement in delayed phase (3-5 minutes). This 'slow-in, slow-out' pattern reflects contrast retention in IMT's fibrous and inflammatory components. Differs from the early intense enhancement + washout pattern of malignant tumors. In a lung mass in a young patient, this enhancement pattern strongly supports IMT diagnosis.
Heterogeneous soft tissue mass: usually 3-10cm in diameter, well-defined or irregular contours. Heterogeneous appearance at soft tissue density (30-50 HU) on unenhanced CT. Low-density areas may represent myxoid/necrotic components, high-density foci may represent calcification.
Report Sentence
A heterogeneous soft tissue mass is identified in the lung parenchyma, and inflammatory myofibroblastic tumor should be considered in the differential diagnosis in a young patient.
Delayed phase enhancement: moderate enhancement in early arterial phase, increasing enhancement in delayed phase (3-5 minutes). This 'progressive enhancement' pattern reflects contrast retention in IMT's fibrous and inflammatory components and differs from the early intense enhancement + washout pattern of malignant tumors.
Report Sentence
The lung mass shows moderate early-phase and increasing delayed-phase enhancement (progressive enhancement pattern), suggesting a fibrous component-predominant lesion, particularly inflammatory myofibroblastic tumor.
Calcification foci within the mass: seen in 15-30% of IMT cases. May show punctate or coarse calcification pattern. Calcification presence in a lung mass in a young patient highlights IMT in the differential diagnosis because primary lung cancers rarely contain calcification.
Report Sentence
Calcification foci within the mass are identified, supporting the diagnosis of inflammatory myofibroblastic tumor in a young patient.
IMT shows moderate FDG uptake on PET-CT (SUVmax typically 3-8). FDG uptake originates from the tumor's inflammatory component (active macrophages, lymphocytes). This level of FDG uptake cannot reliably distinguish from malignant tumors — biopsy is required.
Report Sentence
Moderate FDG uptake is observed in the lung mass on PET-CT (SUVmax: ...); this level of uptake may be consistent with inflammatory myofibroblastic tumor but malignancy cannot be excluded.
Endobronchial extension: in some IMT cases, the mass extends toward the airway or shows an endobronchial component. This may be associated with obstructive atelectasis, post-obstructive pneumonia, or mucus plugging. Bronchial wall thickening and intraluminal soft tissue reflect the invasive character of IMT.
Report Sentence
Bronchial wall thickening and intraluminal soft tissue are observed in the bronchus adjacent to the mass, suggesting endobronchial extension.
Criteria
ALK gene rearrangement positive (detected by FISH or immunohistochemistry). Approximately 50% of patients.
Distinct Features
Younger patients (<40 years), better prognosis, potential for crizotinib (ALK inhibitor) therapy response. Crizotinib may be effective after recurrence or in surgically unresectable cases. ALK positivity does not correlate with local invasion risk.
Criteria
ALK gene rearrangement negative. Approximately 50% of patients. Alternative fusions: ROS1, PDGFR, RET may be detected.
Distinct Features
May occur in older patients, no response to crizotinib expected (no target). Surgery is primary treatment. In some cases, targeted therapy may be considered if ROS1 or PDGFR fusion is present. Recurrence risk similar to ALK-positive.
Criteria
Round-epithelioid cell morphology, nuclear membrane, aggressive clinical course. RANBP2-ALK fusion is characteristic.
Distinct Features
Aggressive variant of IMT — rapid growth, local invasion, higher metastatic potential. Intra-abdominal location more common. Poor prognosis — 5-year survival <50%. Crizotinib response variable.
Distinguishing Feature
Adenocarcinoma typically occurs in patients >50 years, shows spiculated margins and early intense enhancement + washout pattern. Unlike IMT, enhancement decreases in delayed phase. Calcification is very rare in primary adenocarcinoma, 15-30% in IMT. Age difference is an important clue — IMT is usually <40 years.
Distinguishing Feature
Pulmonary lymphoma usually shows diffuse infiltrative pattern or multiple nodules, with mediastinal lymphadenopathy. IMT appears as a solitary mass. Calcification is very rare in pre-treatment lymphoma, more common in IMT. PET-CT typically shows higher FDG uptake in lymphoma (SUVmax >10).
Distinguishing Feature
Hamartoma contains fat (-40 to -120 HU) and/or popcorn calcification — fat component is not seen in IMT. Hamartomas are generally smaller (<3cm), well-defined, and homogeneous. Enhancement in hamartoma is minimal or absent. Age distinction is less definitive — hamartoma may occur at any age.
Distinguishing Feature
Solitary fibrous tumor is pleural-based (IMT is usually parenchymal), shows very intense enhancement (hypervascular — staghorn vascular pattern), and may have pedunculated structure. Calcification is very rare in SFT, 15-30% in IMT. SFT is more common in adults (40-60 years), IMT in young patients. Delayed enhancement is more specific to IMT — early intense enhancement predominates in SFT.
Urgency
routineManagement
surgicalBiopsy
NeededFollow-up
6-monthPrimary treatment for inflammatory myofibroblastic tumor is complete surgical resection. Complete resection is curative in 90% of cases. After incomplete resection, recurrence rate is 15-25% — therefore clean surgical margins are targeted. In ALK-positive cases where surgery is not feasible or recurrence develops, crizotinib (ALK inhibitor) is an effective systemic treatment option — 60-70% response rate has been reported. Biopsy (CT-guided percutaneous or bronchoscopic) is required for preoperative histological diagnosis and ALK testing. Post-surgical CT controls at 6-month intervals are recommended for at least 5 years. Paraneoplastic symptoms (fever, anemia, elevated ESR) generally resolve after surgery.
IMT is a tumor with low-grade malignant potential. Surgical resection is curative. Recurrence may occur (25%). Targeted therapies such as crizotinib can be used in ALK-positive cases. May show partial response to steroid therapy.