However, a more recent retrospective study of 92 patients accumulated by the Surveillance, Epidemiology, and End Results (SEER) program of the National Cancer Institute found the mean age of disease onset was 47.4 years with peak occurrences at 0 to 4, 36 to 40, and >50 years old; middle-aged individuals (41 to 64 years) represented 1/3 of all cases. In this study, the commonest sites of tumor occurrence were the lower limb and hip (22% of cases), upper limb and shoulder (12% of cases), and head, face, and neck (9% of cases). Another recent study of 25 patients found the commonest sites of IMT were the abdomen (40% of cases) and lung/thoracic wall (32% of cases). Individual IMT cases are also reported to present in the urinary bladder, anal canal, and parameningeal spaces (i.e. sites adjacent to the meninges such as the nasopharynx, middle ear, paranasal sinuses, infratemporal fossa and pterygopalatine fossa). Apparently, the age and organ/tissue distribution of IMT various with the patient population examined: in general it can present in individuals of almost any age and in almost any organ or tissue site. IMT most commonly presents as a tumor localized to a single site but may be associated with distal metastases in up to 5% of all cases or up to 10% of cases in which the tumor cells express an ALK fusion protein. The tumors range in size from 1–25 cm (average 6.5 cm) with two-thirds being 1.5–6.5 cm.
ALK is a serine/threonine-specific protein kinase that directly or indirectly stimulates PI3K/AKT/mTOR, Ras GTPase, ERKs, Janus kinase, STAT proteins, and other cell signaling elements. Activation of these elements stimulates cell growth, proliferation, survival, and other tumor-promoting behaviors. As an example of this chromosomal translocation, the ALK gene located on the short or "p" arm of chromosome 2 at position 23 (notated as 2p23) merges with the CLTC gene on the long, i.e. "q" arm of chromosome 17 at position 13 (notated 17q23) to form a chimeric gene notated as t(2;17)(p23;q23). This chimeric gene makes a CLTC-ALK fusion protein with uncontrolled ALK serine/threonine-specific protein kinase activity. Other genes that fuse with AKT found in IMT include: TFG, DCTN1, EML4, TPM3, TPM4, ATIC RANBP2 (most if not all RAMB2-ALK chimeric genes occur in the EMIS form of IMT), CARS1, and SEC31L1. IMT cases may express other chimeric genes in which the active parts of ROS1 (found in 10% of IFT cases and coding for a tyrosine kinase which promotes cell growth), PDGFRB (coding for a protein that may promote the development of cancer), and NTRK (coding for a receptor tyrosine kinase that may promote the development of cancer) merge with other genes. The fusion protein products of these chimeric genes, like those of ALK fusion proteins, are overproduced, overactive, and thereby may contribute to the development of IMT. Diagnosis
Histopathologic examination of the tumors in IMT generally reveals myofibroblastic spindle cell sheets in a myxoid background (i.e.