Introduction

Uterine smooth muscle tumours are mainly classified into benign leiomyomas and malignant leiomyosarcomas (LMS). They are distinguished using Stanford criteria, which evaluate cytological atypia, mitotic rate, and the presence or absence of tumour cell necrosis (TCN) [1]. Leiomyosarcoma is defined as a tumour exhibiting at least two of the following three features:

Conversely, leiomyomas are characterized by a bland appearance, no tumour cell necrosis, and ≤ 4 mitoses per 10 high-power fields. Variants of leiomyoma include:

A rare pathological variant is a smooth muscle tumour of uncertain malignant potential (STUMP) [3], which does not meet the previously described criteria [2].

According to the World Health Organization, STUMP is defined as a uterine smooth muscle tumour that cannot be definitively classified as benign or malignant [4]. Only about 2–5% of myomas are identified as STUMP [2]. It most commonly affects women aged 45–55 years [5]. Symptoms and signs reported by patients are similar to those of uterine fibroids, including hypermenorrhoea, menometrorrhagia, pelvic pain, and infertility [4]. Treatment primarily involves surgery, but a standardized procedure with clear surgical margins cannot be established due to uncertain carcinogenic potential.

Generally, most STUMPs have a good prognosis; however, some cases exhibit aggressive tumour behaviour [6]. Additionally, they can lead to recurrences after surgery. The relapse rate for this tumour is 8.7–11% [3]. Interestingly, STUMP can recur as leiomyosarcoma [2]. Differentiating STUMP from leiomyosarcoma is of critical clinical importance, as these entities differ substantially in prognosis, risk of recurrence, and recommended postoperative management. Uterine leiomyosarcoma is an aggressive malignancy with markedly poorer outcomes, with 5-year overall survival commonly reported as 25–76% and substantially lower survival in cases presenting with metastatic disease [7]. In comparison, the 5-year overall survival in STUMP is 92–100% [8].

Apart from that, it has been reported that smoking and tobacco use may decrease the risk of uterine fibroids, including STUMP and leiomyosarcoma [9, 10]. Nonetheless, the risk factors for STUMP occurrence and recurrence remain underinvestigated.

In our data, we discussed clinical characteristics of twelve patients diagnosed with STUMP, detailing their clinical features, surgical treatment, and follow-up. We also conducted a literature review to gain a deeper understanding of the issue, which should be particularly valuable to every gynaecologist who has encountered such a case.

Material and methods

This study was conducted with the participants’ written informed consent. Formal ethical approval from the Institutional Review Board was not required for a retrospective study, according to institutional policies. The retrospective analysis included 12 patients who underwent myomectomy or hysterectomy at the Department of Surgical, Endoscopic, and Gynaecological Oncology of the Polish Mother’s Memorial Hospital – Research Institute in Lodz, with final histopathological examination confirming STUMP. Our center is a leading reference institution for gynaecological pathology. In cases of diagnostic uncertainty, the histopathological diagnosis was confirmed by a regional consultant pathologist. The study used data from January 2019 to December 2024. All cases of uterine leiomyomas from this period were reviewed to identify eligible STUMP cases. Clinical, paraclinical, and treatment details were extracted from patient observation files and electronic health records maintained by the hospital. Before surgery, all patients underwent transvaginal ultrasonography as the primary imaging modality for the evaluation of uterine leiomyomas. No ultrasonographic features suggestive of malignancy were identified in any of the patients; therefore, additional preoperative evaluation with magnetic resonance imaging (MRI) was not required. After anonymizing personal data, descriptive statistics were used to characterize the patient population. Data on patients’ demographics, indications, tobacco use, surgical approach, tumour size, and follow-up were recorded. All patients were included in a surveillance program conducted at the gynaecological oncology clinic. Follow-up visits were scheduled every six months for five years, in accordance with recommendations in the literature. Each follow-up visit comprised a gynaecological examination, transvaginal ultrasonography, and recommended radiological imaging. Follow-up was also conducted through a telephone survey. Survival length, recurrence, and interview data were assessed. Survival time was measured from the date of surgery until the last follow-up, telephone survey, recurrence, or death from any cause. The choice of the surgical approach – laparoscopy or laparotomy – was determined by uterine size and mobility at the time of surgical qualification. Among patients who underwent myomectomy, one patient, aged 48, declined hysterectomy. The remaining four patients underwent this procedure due to their age and reported clinical symptoms. At the time of treatment, none of the patients were actively pursuing pregnancy; however, they did not rule out future reproductive plans. At present, only one patient is attempting to achieve pregnancy, with no success to date. Patients were followed up until July 2025.

Results

We analysed 12 patients who were diagnosed with STUMP after myomectomy/hysterectomy at the Department of Surgical, Endoscopic, and Gynaecological Oncology of the Polish Mother’s Health Center – Research Institute in Lodz. Patients’ characteristics are shown in Table 1.

The median age of the patients at the time of STUMP diagnosis was 44.5 years (range 33–57 years). The median parity was 1 (range 0–2). No one used tobacco. The most common symptoms reported by 50% of patients (n = 6) were prolonged, heavy menstrual bleeding. About 42% (n = 5) also complained of a pelvic mass, and one person (8%) reported chronic pelvic pain. In 50% of patients (n = 6), a hysterectomy was performed, with 50% (n = 3) done via laparotomy and 50% (n = 3) via transvaginal surgery. Myomectomy was performed in approximately 42% of cases, with 60% (n = 3) done by laparotomy and 40% (n = 2) by laparoscopy. In all cases of laparoscopy, morcellation was performed. One patient (8%) underwent a supracervical hysterectomy by laparotomy. The median size of the STUMP was 6.25 cm (range 4–20 cm). The median follow-up duration after surgery was 44 months (range 10–78 months). No recurrences were observed.

In all cases, histopathological features were evaluated, including necrosis, cellularity, cytologic atypia, and mitotic activity (mitoses per 10 HPF). Tumour cell necrosis was absent in most cases (7/12, 58%), whereas five tumours contained necrotic areas. Tumour cellularity varied from medium to high, with five cases displaying medium cellularity and seven demonstrating a higher cellular concentration. Cytologic atypia was predominantly medium (8/12, 67%), with three cases exhibiting mild atypia and only one showing high atypia. Mitotic activity was low across the cohort, with a range of 0–6 mitoses per 10 HPF.

Discussion

Smooth muscle tumour of uncertain malignant potential represents a rare and diagnostically challenging entity that continues to attract clinical and scientific interest. Studies reported in the literature have investigated, among other aspects, diagnostic and treatment approaches, malignant potential, and risk of recurrence. Below, we compare our findings with those reported by other authors.

The criteria for diagnosing STUMP are based on histopathological assessment. Smooth muscle tumour of uncertain malignant potential does not meet Stanford criteria for leiomyosarcoma or leiomyoma; it is very difficult to make an accurate diagnosis. Several variants meet the criteria for identifying a STUMP: some show moderate or severe atypia with 10 or fewer mitoses per 10 HPF and no TCN; others may be diffuse, with moderate to severe atypia, 10 or fewer mitoses per 10 HPF, and no TCN; some may show no atypia with 10 or fewer mitoses per 10 HPF and tumour cell necrosis; or no atypia with more than 20 mitoses per 10 HPF and no TCN may be observed. Ultimately, some may have no atypia, 10 or fewer mitoses per 10 HPF, and ambiguous necrosis [6]. Overall, the histopathological profile of our cohort demonstrates features characteristic of smooth muscle tumours of uncertain malignant potential, with variable cellularity and atypia, low mitotic activity, and minimal necrosis. These findings underscore the diagnostic challenges in distinguishing STUMP from benign leiomyomas and low-grade leiomyosarcomas.

Symptoms of STUMPs are similar to those of uterine leiomyomas. Patients usually experience abnormal uterine bleeding, symptoms of anemia, menorrhagia, dysmenorrhea, abdominal pain, a rapidly enlarging pelvic mass, pressure symptoms, pelvic pain, asthenia, infertility, or a combination of these [1, 4]. This is why research is ongoing to differentiate uterine changes using radiological imaging.

Regarding preoperative imaging, several studies worldwide show that MRI can distinguish STUMP/MMT (mixed mesenchymal tumours) from benign and malignant tumours. Bonneau et al. [11] preliminary data indicate that a high signal at b = 1000 s/mm² is significantly associated with STUMP/MMT, and a lower apparent diffusion coefficient was at the significance limit [12]. Additionally, fluorodeoxyglucose-positron emission tomography could help differentiate between LMS/STUMP and benign leiomyomas in patients with rapidly growing large uterine masses [13]. In ultrasound, STUMP cannot be distinguished from other uterine smooth muscle tumours; it provides only limited information about uterine changes [12]. Computed tomography (CT) is not helpful.

Due to challenges in preoperative diagnosis, STUMP is often an incidental finding after histological examination, hysteroscopy, or myomectomy specimen analysis. Once diagnosed, hysterectomy with or without bilateral adnexectomy remains the gold standard for women who do not wish to have more children. If the patient wishes to preserve fertility, a myomectomy (tumorectomy) combined with a “wait and watch” approach may be sufficient. However, a hysterectomy will be necessary once fertility hopes are fulfilled [14]. The most recommended approach is laparoscopic, but laparotomic or vaginal methods are also options. It is important to note that laparoscopic procedures require “in bag” morcellation to prevent the spread of potentially malignant cells into the abdominal cavity [14, 15]. The choice of the method depends on the tumour’s characteristics, the surgeon’s skills, and the patient’s overall health [14, 16]. Adjuvant therapy is relatively rarely used but remains a possible option, involving pelvic irradiation, chemotherapy (doxorubicin and cisplatin), and hormone therapy (medroxyprogesterone, aromatase inhibitors, and gonadotropin-releasing hormone analogues) [17]. However, surgical treatment alone is likely sufficient [1, 5].

Various studies suggest that patients should be followed up every 6 months for 5 years after treatment. Follow-up should include gynaecological examination, chest radiography, transvaginal ultrasound, abdominal transvaginal ultrasound, and abdominal CT for patients after hysterectomy or pelvic MRI following myomectomy [15, 18].

It has been reported that STUMP can recur as STUMP or even as a more malignant neoplasm. Shapiro et al. described the case of a patient who, after being diagnosed with STUMP, was under clinical and radiological monitoring, during which metastases of a more aggressive form of STUMP were detected. After 51 months of observation, she was diagnosed with metastasis of leiomyosarcoma to the humerus, consistent with uterine origin. After another year, metastatic changes were also found in the lungs [19]. Furthermore, Bicanin-Ilic et al. [15] presented the case of a patient who, after being diagnosed with STUMP, refused to undergo further surgery and a hysterectomy. A few months later, she reported to the hospital with a ruptured leiomyosarcoma at the site of the STUMP scar.

Generally, relapse rates are 8.7–11% [3, 5]. The recurrence and metastases depend on histopathological features and the completeness of surgery. Positive margins and higher-grade features, such as marked nuclear atypia, increased mitotic activity, and tumour necrosis, may contribute to relapse. Additionally, subserosal location significantly affects outcomes [17]. Worse prognosis and higher relapse risk also depend on immunohistochemical and serological markers, such as positivity for p16, p53, MIB-1, bcl-2, estrogen, and progesterone receptors, or elevated levels of CA 125 and HE-4 [1, 14, 17].

Some studies show that genomic profiling can be a tool to distinguish benign STUMPs from those with malignant potential. If the genomic index is ≥ 10, the STUMP has a recurrence risk and malignant potential and should be evaluated further. It should be tested using the transcriptomic Complexity Index in SARComa to assess the patient’s risk of death [18].

Smooth muscle tumour of uncertain malignant potential is a rare tumour that remains poorly understood. It presents a challenge for both clinicians and pathologists. Most research focuses on malignant potential, treatment, recurrence, and follow-up, but clear answers are lacking. As a result, there are no established guidelines for diagnosing STUMP. Consequently, more studies assessing its malignancy potential are necessary.

Conclusions

Smooth muscle tumours of uncertain malignant potential are quite rare and heterogeneous tumours. Smooth muscle tumour of uncertain malignant potential represents a significant diagnostic challenge for both pathologists and gynaecologists. Owing to the limitations of preoperative diagnostic modalities and imaging techniques, a definitive diagnosis can only be established through postoperative histopathological evaluation. Hysterectomy remains the standard therapeutic approach; however, myomectomy may be considered in selected cases where fertility preservation is a priority. Given the potential, albeit low, risk of recurrence or metastatic spread, prolonged clinical and radiological surveillance is warranted. Future investigations, particularly those exploring the molecular underpinnings of STUMP, are essential to improve diagnostic accuracy and to establish standardized management protocols.