Intramural leiomyoma (fibroid) is the most common benign tumor of the uterus, located within the myometrium. It consists of monoclonal proliferation of smooth muscle cells and is characterized by a pseudocapsule surrounded by fibrous tissue. It occurs in 20-40% of reproductive-age women; prevalence reaches 70% by age 50. On imaging, it appears as a well-defined, round/oval mass — marked T2 hypointensity on MRI is characteristic of smooth muscle and fibrous tissue. On US, it is hypoechoic with potentially irregular borders; posterior shadowing is common. Degeneration types (hyaline, cystic, red/hemorrhagic, myxoid, calcific) alter imaging features and may cause diagnostic difficulty. It is hormone-dependent — expresses estrogen and progesterone receptors, grows during pregnancy, shrinks after menopause. FIGO subclassification (Type 3-4-5) is based on the lesion's relationship with the endometrial cavity and serosa: Type 3 contacts the endometrium and is 100% intramural; Type 4 is entirely intramural, contacting neither endometrium nor serosa; Type 5 is subserosal ≤50%. Treatment options for symptomatic cases include medical (GnRH agonists, ulipristal acetate), interventional (uterine artery embolization — UAE, MR-guided focused ultrasound — MRgFUS), or surgical (myomectomy, hysterectomy).
Age Range
30-55
Peak Age
40
Gender
Female predominant
Prevalence
Very Common
Intramural leiomyoma develops from monoclonal proliferation of smooth muscle cells in the myometrium. MED12 mutation is the most common genetic anomaly (70%). The tumor is separated from surrounding normal myometrium by a pseudocapsule — this capsule consists of compressed myometrium and connective tissue, providing the surgical enucleation plane. Smooth muscle cells produce abundant extracellular matrix (collagen Type I and III) — this fibrous matrix is the primary determinant of imaging features. Tight packing of collagen fibers and low water content shortens T2 relaxation time → marked T2 hypointensity on MRI. On US, alternating layering of fibrous and muscle cells creates a 'whorled' pattern — this histological structure manifests as heterogeneous echo pattern and posterior shadowing on US. Leiomyoma vascularity differs from surrounding myometrium — peripheral vascularity is prominent while central vascularity is relatively low, determining the enhancement pattern. Growth is estrogen and progesterone dependent: hormonal stimulation increases cell proliferation and extracellular matrix accumulation. Degeneration develops when tumor growth exceeds blood supply: hyaline degeneration is most common (60%), cystic degeneration causes fluid accumulation → T2 hyperintense areas, red (hemorrhagic) degeneration is common in pregnancy → T1 hyperintensity (methemoglobin), calcific degeneration occurs post-menopause → dense calcification on CT, myxoid degeneration → very high T2 signal (may mimic leiomyosarcoma). These degeneration types create diagnostic pitfalls and MRI characterization is critical.
Markedly hypointense, round/oval, well-defined mass within the myometrium on T2W — whorled internal structure with surrounding hyperintense pseudocapsule. This appearance reflects concentric layering of smooth muscle and collagen fibers and is pathognomonic for leiomyoma. Homogeneous hypointensity in the absence of degeneration confirms the diagnosis.
Well-defined or partially defined, hypoechoic or heterogeneous solid mass within the myometrium. Whorled internal echo pattern is characteristic. Posterior shadowing (edge or diffuse) is common and serves as a diagnostic clue. Heterogeneity increases with size. Cystic degeneration areas create anechoic foci. Calcific degeneration appears as dense echogenic foci + shadowing. Pseudocapsule may be seen as a hypoechoic halo. Multiple fibroids are common and deform the uterus.
Report Sentence
A ___ x ___ x ___ mm heterogeneous hypoechoic, well-defined solid mass is seen within the myometrium of the uterine corpus, consistent with intramural leiomyoma.
Characteristic peripheral/circumferential vascularity pattern on Doppler — feeding vessels surrounding the myoma are traced along the pseudocapsule. Central vascularity is generally low or absent. Central flow may be increased in large myomas. RI (resistive index) is generally high (>0.5), different from low RI values of malignant lesions. Vascularity disappears in degeneration areas.
Report Sentence
Peripheral circumferential vascularity pattern is observed in the solid lesion on color Doppler, consistent with leiomyoma.
Markedly hypointense, well-defined, round/oval mass within the myometrium on T2W. Signal intensity is similar to or lower than skeletal muscle — distinctly darker than normal myometrium. Pseudocapsule appears as a thin hyperintense ring on T2 (compressed edematous myometrium). Whorled internal structure manifests as homogeneous hypointensity or mild heterogeneity. Homogeneous hypointensity without degeneration is pathognomonic for leiomyoma. Multiple lesions are common and differentiation from junctional zone is made on T2.
Report Sentence
A markedly hypointense, well-defined, round mass measuring ___ x ___ x ___ mm is seen in the myometrium of the uterine corpus on T2W with a thin hyperintense pseudocapsule; consistent with intramural leiomyoma.
Mass isointense or slightly hypointense to myometrium on T1W. Homogeneous signal. No hyperintensity (unless hemorrhagic degeneration). T1 hyperintense areas in red degeneration (common in pregnancy) — methemoglobin accumulation. Fatty degeneration is rare but can create high T1 signal. T1 sequence is valuable in combination with T2 for differentiating degeneration types.
Report Sentence
The lesion shows signal isointense to the myometrium on T1W; no evidence of hemorrhagic degeneration is detected.
Leiomyoma generally does not show significant diffusion restriction on DWI — ADC values are normal or mildly reduced. This finding is critical for differentiation from leiomyosarcoma: marked diffusion restriction and low ADC values (<1.0 × 10⁻³ mm²/s) are expected in leiomyosarcoma. Cellular leiomyomas may show mild diffusion restriction. Degenerative leiomyomas show variable ADC values. DWI is an important sequence for evaluating leiomyosarcoma suspicion in rapidly growing fibroids and contributes to early diagnosis of sarcomatous transformation.
Report Sentence
No significant diffusion restriction is detected in the lesion on DWI/ADC map; no findings consistent with leiomyosarcoma.
Homogeneous or heterogeneous enhancement on contrast MRI. Typical leiomyomas enhance less than myometrium. Pseudocapsule is clearly seen after contrast — thin, smoothly enhancing ring. Enhancement decreases or disappears in degeneration areas. Prominent contrast uptake may occur in myxoid degeneration — may mimic leiomyosarcoma. Enhancement homogeneity supports benign lesion; heterogeneous enhancement combined with diffusion restriction → leiomyosarcoma suspicion.
Report Sentence
The lesion shows homogeneous/heterogeneous enhancement on contrast-enhanced series with a thin enhancing pseudocapsule.
Well-defined mass of soft tissue density within the myometrium on CT. Calcification is common and characteristic — peripheral coarse calcification or central amorphous calcification. On non-contrast CT, leiomyoma may be isodense or slightly hypodense relative to normal myometrium. Cystic degeneration areas show low density. Less enhancement than myometrium on contrast phase. CT is limited compared to MRI for leiomyoma characterization but superior for calcification detection.
Report Sentence
A well-defined mass of soft tissue density measuring ___ x ___ mm with peripheral coarse calcifications is seen in the uterine myometrium on CT; consistent with leiomyoma.
Criteria
Homogeneously T2 hypointense, homogeneous enhancement, no degeneration features
Distinct Features
Most common type. Homogeneously hypoechoic on US with posterior shadowing. Uniform hypointensity on MR T2. No DWI restriction. Causes no diagnostic difficulty.
Criteria
Homogeneous eosinophilic hyaline material accumulation, cell loss, even lower T2 signal
Distinct Features
Most common degeneration type (60%). Increased collagen accumulation → more prominent T2 hypointensity. Echogenicity may change on US. Clinically usually asymptomatic.
Criteria
Fluid-filled cavities from liquefaction necrosis, markedly hyperintense areas on T2
Distinct Features
Heterogeneous signal on T2 — hyperintense cystic areas on hypointense background. Anechoic areas within solid mass on US. Low signal on T1. No enhancement in cystic areas. May mimic ovarian cystic tumors.
Criteria
Venous thrombosis → hemorrhagic infarction, common in pregnancy, T1 hyperintense
Distinct Features
Characteristic in pregnancy. High signal on T1 (methemoglobin), variable signal on T2. Presents with acute pelvic pain and fever. Peripheral high signal on DWI. Peripheral T1 hyperintense rim is pathognomonic.
Criteria
Dystrophic calcification accumulation, common post-menopause, dense calcification on CT
Distinct Features
Most common post-menopause. Coarse calcification on CT — peripheral 'eggshell' or central. Echogenic foci + shadowing on US, may obscure the mass. Signal loss on all MR sequences (susceptibility artifact).
Distinguishing Feature
Leiomyosarcoma shows heterogeneous high T2 signal (necrosis, hemorrhage), marked diffusion restriction (low ADC <1.0), irregular borders, history of rapid growth and new postmenopausal mass. Leiomyoma shows homogeneous T2 hypointensity, no DWI restriction, smooth borders.
Distinguishing Feature
Adenomyosis shows diffuse thickening of junctional zone (>12mm), small hyperintense foci within low T2 signal (ectopic endometrial glands), ill-defined borders, no pseudocapsule. Leiomyoma shows well-defined round mass, pseudocapsule present, normal junctional zone.
Distinguishing Feature
Endometrial carcinoma shows mass originating from endometrial cavity, intermediate-high T2 signal, marked diffusion restriction, invading endometrial-myometrial junction (junctional zone disruption). Leiomyoma originates from myometrium, T2 hypointense, endometrial cavity intact.
Distinguishing Feature
Adenomyoma shows focal adenomyosis area — well-defined but no pseudocapsule, small hyperintense dots within low T2 signal (ectopic endometrial glands), originates from junctional zone. Leiomyoma shows homogeneous T2 hypointensity, no endometrial gland foci, distinct pseudocapsule present.
Urgency
routineManagement
surveillanceBiopsy
Not NeededFollow-up
6-monthIntramural leiomyoma is the most common benign uterine tumor; risk of malignant transformation is negligible (<0.5%). No treatment needed for asymptomatic cases, annual US follow-up is sufficient. Treatment options for symptomatic cases (menorrhagia, pelvic pressure, infertility): medical (GnRH agonist, ulipristal acetate), interventional (UAE, MRgFUS), surgical (myomectomy — fertility preserving; hysterectomy — definitive). Rapid growth (>25-50% volume increase in 6 months), new postmenopausal mass or postmenopausal growth raises leiomyosarcoma suspicion → biopsy/surgery needed. FIGO subclassification (Type 3-4-5) is determinative in treatment planning: presence of submucosal component (Type 2-5) affects hysteroscopic resection suitability. Pre-op MRI mapping is critical for surgical planning in multiple fibroids.
Intramural leiomyomas are usually asymptomatic and incidentally detected. Symptomatic cases may present with menorrhagia, pelvic pressure, and infertility. Treatment depends on size, symptoms, and fertility plans. Small asymptomatic leiomyomas can be followed up. For large or symptomatic cases, myomectomy, UAE, or hysterectomy is considered.