STUMP (Smooth Muscle Tumor of Uncertain Malignant Potential) is a diagnostic category describing uterine smooth muscle tumors that cannot be definitively classified histologically as either leiomyoma or leiomyosarcoma. Included in the WHO 2014 classification among tumors of uncertain biologic behavior, STUMPs exhibit borderline positivity in two of the three histopathologic criteria — mitotic activity, cellular atypia, and coagulative tumor cell necrosis — without fully meeting leiomyosarcoma diagnostic criteria. Typically occurring in women aged 40-60, incidence is less than 1% of all uterine smooth muscle tumors. Clinical behavior is generally benign, but 7-11% recurrence and rare metastasis risk exist. Imaging cannot definitively distinguish STUMP from leiomyoma or leiomyosarcoma; definitive diagnosis relies on surgical pathology. MRI may show suspicious findings such as diffusion restriction and heterogeneous enhancement, but no imaging finding is pathognomonic.
Age Range
35-65
Peak Age
48
Gender
Female predominant
Prevalence
Rare
STUMP originates from smooth muscle cells of the myometrium and shares the same clonal origin as leiomyoma. Pathophysiologically, it occupies an intermediate position on the leiomyoma-leiomyosarcoma spectrum. Histologically, mitotic figure count (typically 5-10/10 HPF — versus >10/10 HPF in leiomyosarcoma), degree of cellular atypia, and presence of coagulative necrosis are evaluated; when only one or two of these three criteria are borderline positive, STUMP is diagnosed. At the molecular level, p53 mutations, MED12 mutations, and HMGA2 rearrangements may be found at variable rates — reflecting the transitional genetic profile between leiomyoma and leiomyosarcoma. From an imaging perspective, intratumoral necrosis manifests as hyperintense foci on T1-weighted images and heterogeneous signal on T2-weighted images. Increased cellularity and high nuclear-to-cytoplasmic ratio restrict free water molecule movement on diffusion-weighted imaging (DWI), resulting in low ADC values. Enhancement heterogeneity results from the mixed distribution of necrotic and viable tumor areas, producing an enhancement pattern that overlaps with leiomyosarcoma.
STUMP has no pathognomonic imaging finding; diagnosis relies on combined evaluation of overlapping features between leiomyoma and leiomyosarcoma. The triad of T2 heterogeneity + DWI restriction + necrotic areas on contrast-enhanced MRI is the most alerting combination. Surgical pathology correlation is mandatory when this triad is present.
On T2-weighted images, STUMP demonstrates heterogeneous signal; unlike the homogeneous hypointense T2 signal of typical leiomyoma, a mixture of high and low signal areas is observed. High signal areas reflect cellular atypia, edema, or early necrosis zones, while low signal areas represent fibrous or smooth muscle components. This heterogeneity may overlap with leiomyosarcoma, but leiomyosarcoma generally exhibits more pronounced T2 hyperintensity.
Report Sentence
Solid mass in the myometrium demonstrating heterogeneous signal on T2-weighted sequences, deviating from the homogeneous hypointense pattern of typical leiomyoma; STUMP or leiomyosarcoma should be considered in the differential diagnosis.
On DWI, STUMP generally shows intermediate-to-high signal with partially low values on ADC maps. Diffusion restriction is more prominent than in typical leiomyoma but generally less pronounced than in leiomyosarcoma. ADC values range 0.9-1.2 × 10⁻³ mm²/s, positioned between leiomyoma (>1.2) and leiomyosarcoma (<0.9). Due to this overlap, DWI alone is insufficient for differential diagnosis but should be evaluated in conjunction with T2 and enhancement findings.
Report Sentence
The mass demonstrates intermediate diffusion restriction on DWI with partially low signal on ADC maps; this finding indicates higher cellularity than typical leiomyoma, raising the possibility of STUMP or leiomyosarcoma.
On T1-weighted images, intratumoral hyperintense foci may be observed in STUMP. These foci correspond to methemoglobin accumulation or proteinaceous content in coagulative necrosis areas. While T1 hyperintense foci are rarely seen in typical leiomyoma, they are more frequently detected in STUMP and leiomyosarcoma due to necrotic components. The presence of these foci on precontrast T1 images is a warning sign requiring surgical pathology correlation.
Report Sentence
Hyperintense foci observed within the mass on precontrast T1-weighted images, consistent with coagulative necrosis or hemorrhagic component; this finding represents deviation from benign leiomyoma.
On contrast-enhanced dynamic MRI, STUMP demonstrates heterogeneous enhancement. Viable tumor areas enhance while necrotic or hyalinized areas remain non-enhancing. This pattern differs from the homogeneous or minimal enhancement of typical leiomyoma. Enhancement heterogeneity overlaps with leiomyosarcoma and is not distinguishing by itself. Areas of prominent enhancement in early arterial phase may indicate increased tumoral neovascularity.
Report Sentence
Heterogeneous enhancement observed in the mass on contrast-enhanced MRI with non-enhancing areas consistent with necrosis; this finding deviates from typical leiomyoma and suggests the possibility of STUMP or leiomyosarcoma.
On B-mode ultrasonography, STUMP is generally indistinguishable from leiomyoma and appears as a well-defined, heterogeneous echogenicity solid mass in the myometrium. In some cases, cystic areas (necrosis) or calcifications may accompany. Heterogeneous echo pattern and irregular cystic changes may raise suspicion from typical leiomyoma, but these findings are non-specific. US has limited role in primary diagnosis of STUMP and further evaluation with MRI is required for suspicious findings.
Report Sentence
Solid mass with heterogeneous echogenicity in the myometrium accompanied by cystic necrosis areas; MRI evaluation is recommended for these findings deviating from typical leiomyoma.
On Doppler ultrasonography, STUMP may demonstrate increased and irregular vascularity. While typical leiomyoma shows peripheral regular feeding vessels, STUMP may display mixed central and peripheral increased flow signals with irregular distribution. Increased flow with low resistance index (RI <0.4) is more frequently seen in malignant smooth muscle tumors but is not specific for STUMP. Doppler findings alone are not diagnostic but support the indication for MRI.
Report Sentence
Increased irregular central and peripheral vascularity with low resistance index detected on Doppler examination of the mass; these findings deviate from typical leiomyoma and further MRI evaluation is recommended.
On contrast-enhanced CT, STUMP appears as a heterogeneously enhancing solid mass in the myometrium. Necrotic or degenerative areas may be detected as low-density regions. CT provides less information than MRI for STUMP diagnosis but may be incidentally detected on pelvic examinations. Peritumoral edema or invasion findings increase malignancy suspicion. CT's role is primarily distant metastasis screening and preoperative staging.
Report Sentence
Heterogeneously enhancing solid mass in the myometrium on contrast-enhanced CT with accompanying low-density necrotic areas; further characterization with MRI is recommended.
Criteria
Prominent cellular atypia present but mitotic activity low (<5/10 HPF) and no coagulative necrosis. Generally the STUMP subtype with best prognosis.
Distinct Features
T2 heterogeneity on MRI may be milder, DWI restriction generally less pronounced than leiomyosarcoma. Recurrence risk is below 5%. Close follow-up after surgery is sufficient.
Criteria
Increased mitotic activity (5-10/10 HPF) present but no significant atypia or coagulative necrosis. More common in younger women.
Distinct Features
May show more prominent diffusion restriction on DWI (increased proliferation). T2 signal generally more homogeneous. Recurrence risk is 7-10%. Fertility-sparing surgery may be discussed.
Criteria
Coagulative tumor cell necrosis present but atypia and mitotic activity at borderline levels. STUMP subtype carrying highest recurrence risk.
Distinct Features
Most prominent T1 hyperintense foci on MRI and widest non-enhancing necrotic areas on contrast-enhanced images are observed in this subtype. T2 heterogeneity is prominent. Recurrence risk is 11-15%. Closer follow-up and leiomyosarcoma-like surveillance protocol is recommended.
Distinguishing Feature
Typical leiomyoma shows homogeneous hypointense signal on T2, no significant restriction on DWI, and homogeneous mild enhancement on contrast-enhanced MRI. T1 hyperintense necrosis foci and low ADC values are not expected.
Distinguishing Feature
Leiomyosarcoma generally shows more pronounced T2 hyperintensity, lower ADC values (<0.9 × 10⁻³ mm²/s), wider necrosis areas, and more aggressive enhancement pattern than STUMP. However, definitive distinction by imaging is unreliable — surgical pathology is required.
Distinguishing Feature
Endometrial stromal sarcoma originates from endometrium and shows intravascular extension (worm-like vascular invasion); STUMP originates from myometrium and does not show vascular invasion. Endometrial stromal sarcoma generally shows higher signal on T2.
Distinguishing Feature
Adenomyosis shows junctional zone thickening (>12 mm) and T2 hyperintense foci (ectopic endometrial glands) within T2 hypointense myometrial mass. Unlike STUMP, it does not form a well-defined mass, originates from junctional zone, and DWI restriction is less prominent.
Urgency
urgentManagement
surgicalBiopsy
NeededFollow-up
6-monthSTUMP diagnosis relies on surgical pathology and cannot be definitively diagnosed by imaging. For suspicious findings (rapid growth, T2 heterogeneity, DWI restriction, necrosis), hysterectomy or wide myomectomy is recommended. Close postoperative follow-up is required — pelvic MRI and clinical evaluation every 6 months for the first 5 years. Recurrence usually occurs in the pelvis, but rare lung metastasis has been reported. Morcellation is contraindicated — fragmentation increases the risk of intraperitoneal spread.
STUMP diagnosis is histological — imaging alone cannot make the diagnosis. Surgery (hysterectomy or myomectomy) is the primary treatment. Recurrence risk is 10-15%. Long-term follow-up is required (late recurrence has been reported). Close follow-up is mandatory after fertility-preserving surgery.