This video addresses the common diagnostic pitfalls surrounding Inflammatory Myofibroblastic Tumor (IMT), emphasizing its distinction from the historically confusing umbrella term "inflammatory pseudotumor" and various reactive processes. IMT is defined as a distinct neoplasm characterized by a prominent proliferation of plump, myofibroblastic spindle cells interspersed with a rich inflammatory infiltrate (lymphocytes, plasma cells, and occasionally eosinophils/neutrophils). Molecularly, over half of these tumors feature rearrangements of the ALK gene, resulting in positive cytoplasmic ALK immunostaining. The pathologist illustrates classic IMT examples in the lung, a myxoid variant in the liver, and a rare, genuine case in the skin. The video contrasts these true tumors with a reactive inflammatory polyp from the prostatic urethra, highlighting that the critical diagnostic feature of IMT is the underlying neoplastic spindle cell population, not just the presence of inflammation and a mass ("pseudotumor"). IMT is classified as having intermediate malignant potential due to a small risk of aggressive behavior and metastasis.
Inflammatory Myofibroblastic Tumor (IMT): Diagnosis and Pitfalls
0:05 Historical Confusion: The term "inflammatory pseudotumor" previously applied broadly to any mass with spindle cells and inflammation, leading to confusion. Over time, distinct entities emerged, leading to the classification of Inflammatory Myofibroblastic Tumor (IMT).
0:42 Molecular Basis: IMT is a distinct neoplasm. Over 50% of cases exhibit rearrangements of the ALK gene, which can be detected by molecular testing and positive ALK cytoplasmic immunostaining.
1:36 Diagnostic Principle: The primary takeaway is that "inflammatory pseudotumor" does not always equal IMT. The term inflammatory pseudotumor should be avoided due to historical confusion.
2:02 Essential Microscopic Features: The tumor requires two components:
Inflammatory Component: Prominent infiltrate, often including lymphocytes and plasma cells (sometimes eosinophils and neutrophils) scattered throughout the tumor.
Myofibroblastic Component: If the inflammatory cells are mentally subtracted, a prominent population of plump spindle cells (myofibroblasts) must remain. These cells typically have open, vesicular chromatin and prominent nucleoli.
3:05 Immunostaining and Markers: IMT cells generally stain positive for Smooth Muscle Actin (SMA) and are usually negative for Desmin. ALK positivity is highly suggestive but not required for diagnosis, as other molecular events are being discovered.
4:28 Clinical Presentation and Prognosis: IMT is most commonly found in the abdominal or pelvic region, particularly in children and young adults. It is classified as a tumor of intermediate malignant potential; while most cases are indolent/benign-behaving, a small subset (<5%) can metastasize.
5:06 Related Entities: A more aggressive differential diagnosis is Epithelioid Inflammatory Myofibroblastic Sarcoma (EIMS), which features larger, round epithelioid cells and behaves very aggressively (different from typical IMT).
6:31 Liver Example (Myxoid Variant): A case in the liver demonstrates a looser, more myxoid appearance. A rich plasma cell infiltrate may necessitate checking the differential for IgG4-related sclerosing/fibrotic diseases.
8:35 Distinguishing IMT from Reactive Change (Granulation Tissue): If the inflammation is removed and only scattered reactive myofibroblasts, vessels, and granulation tissue remain, it is likely not a true IMT, which is a key pitfall to avoid.
9:19 Skin IMT (Rarity): True IMT in the skin or subcutis is extremely rare. Most reported cases are reactive changes (e.g., granulation tissue) or other inflamed entities (e.g., Rosai-Dorfman disease). A true, ALK-positive case in the skin is presented as a rare, bonafide example.
12:40 Pitfall Example (Reactive Polyp): A polyp in the prostatic urethra is shown as an example of a reactive pseudotumor with inflammation and vessels, but it lacks the critical neoplastic proliferation of plump myofibroblasts seen in true IMT. ALK would be negative in such reactive processes.