A locally aggressive pelvic MEIS1::NCOA1 fusion sarcoma in a young adult female: a case report and review of the literature.

A 19-year-old nulliparous woman initially presented with acute right lower quadrant (RLQ) pain. A pelvic ultrasound revealed a complex fluid collection adjacent to the right ovary measuring 8.4 × 7.1 × 3.8 cm. Owing to a positive vaginal culture for Chlamydia, she was treated with antibiotics. Over the following six months, she remained under gynecologic surveillance for persistent RLQ pain. Serial imaging consistently revealed a complex cystic mass with internal septations in the right adnexal region, measuring up to 9 cm in the greatest dimension. Differential diagnoses included a tubo-ovarian abscess, pyosalpinx, or endometriotic cysts. However, she reported no prior history of pelvic pain or dysmenorrhea. Eleven months after she was lost to follow-up, she presented again with persistent pelvic pain. A repeat pelvic ultrasound revealed a complex cystic mass separate from the right ovary, measuring 8.3 × 3.9 × 2.8 cm (Figs.  1 A & B ). Diagnostic laparoscopy revealed extensive cystic material, organized clots, green-tinged fluid, and adhesive disease in the abdomen and pelvis (Figs.  1 C & D ). The uterus, fallopian tubes, and ovaries appeared normal. Pathologic examination of the pelvic cystic material, omental biopsy, and appendix was performed. Fig. 1 Radiologic and intraoperative findings. Transvaginal pelvic ultrasound demonstrated a complex mass ( A ) in the midline toward the right adnexa with vascularity ( B ) measuring 8.3 × 3.9 × 2.8 cm. Large volume of cystic material with both organized clot and fluid was present in the abdominopelvic cavity ( C & D ) Radiologic and intraoperative findings. Transvaginal pelvic ultrasound demonstrated a complex mass ( A ) in the midline toward the right adnexa with vascularity ( B ) measuring 8.3 × 3.9 × 2.8 cm. Large volume of cystic material with both organized clot and fluid was present in the abdominopelvic cavity ( C & D ) Grossly, the fragmented pelvic cystic lesion measured 14.0 × 11.5 × 5 cm, with areas of hemorrhage. Microscopically, a low-grade spindle cell neoplasm involving the omental biopsy and mesoappendix was identified, whereas the appendix itself remained uninvolved. The tumor was submesothelial with ill-defined borders (Fig.  2 A) and alternating hypercellular and hypocellular (edematous) areas (Fig.  2 B). The hypercellular areas exhibited sheets of plump spindle cells with moderate eosinophilic cytoplasm, round to oval nuclei, fine chromatin, and tiny nucleoli (Figs.  2 C& D ). The hypocellular zones featured randomly arranged plump, spindly, and stellate cells with prominent vessels in a myxoid background (Figs.  2 E & F ). Numerous capillary-sized vessels were observed with perivascular whorling of spindle cells (Fig.  2 G), prominent peri-arteriolar lymphocytes, and plasma cells (Fig.  2 H). The number of mitotic figures was < 1 per 10 high-power fields (HPFs), and necrosis was absent. Fig. 2 Histopathological findings. Submesothelial tumor ( A ) with alternating hyper- and hypocellular areas ( B ), sheets of plump spindle cells with moderate eosinophilic cytoplasm, round to oval nuclei, fine chromatin, and tiny nucleoli ( C & D ). Hypocellular areas show numerous thin-walled, ramifying blood vessels and randomly dispersed spindle cells ( E ). High-power magnification of hypocellular areas shows scattered spindle and stellate cells in a myxoid background (F), whorling of tumor cells around the capillaries ( G ), with prominent peri-vascular plasma cell aggregates ( H ). Tumor cells infiltrate the mesenteric adipose tissue ( I ) Histopathological findings. Submesothelial tumor ( A ) with alternating hyper- and hypocellular areas ( B ), sheets of plump spindle cells with moderate eosinophilic cytoplasm, round to oval nuclei, fine chromatin, and tiny nucleoli ( C & D ). Hypocellular areas show numerous thin-walled, ramifying blood vessels and randomly dispersed spindle cells ( E ). High-power magnification of hypocellular areas shows scattered spindle and stellate cells in a myxoid background (F), whorling of tumor cells around the capillaries ( G ), with prominent peri-vascular plasma cell aggregates ( H ). Tumor cells infiltrate the mesenteric adipose tissue ( I ) Immunohistochemistry (IHC) revealed strong and diffuse ER positivity (Fig.  3 A), weaker PR positivity (Fig.  3 B), focal and membranous CD10 (Fig.  3 C), and SMA positivity (Fig.  3 D). The Ki-67 proliferation index was approximately 10%. Table  1 lists the various positive and negative IHC stains and the potential differential diagnoses considered during the initial workup. Molecular testing with the “Mayo Complete Sarcoma Panel” identified a MEIS1::NCOA1 fusion involving exon 7 of the MEIS1 gene and exon 13 of the NCOA1 gene, with no additional pathogenic alterations. No pathogenic germline variants were identified. Fig. 3 Immunohistochemical findings. ( A ) ER shows diffuse strong expression. ( B ) PR expression is weaker and stains fewer tumor cells than ER. ( C ) CD10 shows focal membranous positivity in tumor cells. ( D ) SMA shows focal positivity in tumor cells Immunohistochemical findings. ( A ) ER shows diffuse strong expression. ( B ) PR expression is weaker and stains fewer tumor cells than ER. ( C ) CD10 shows focal membranous positivity in tumor cells. ( D ) SMA shows focal positivity in tumor cells Table 1 List of positive and negative immunohistochemistry stains and the potential differential diagnoses considered during initial work up Positive IHC stains Negative IHC stains • ER (strong and diffuse) • PR (moderate and patchy) • CD10 (focal and membranous) • SMA (focal and membranous) IHC stain Tumors with positive staining that were considered in the differential diagnosis AE1/AE3 • Carcinomas EMA • Carcinomas • Perineurioma CD99 • BCOR rearranged round cell sarcoma • Sex cord-stromal tumors Cyclin D1 • High-grade endometrial stromal sarcoma S-100 • NTRK fusion-positive uterine sarcoma HMB-45 • PEcoma MDM2/CDK4 • Dedifferentiated liposarcoma CD34 • Solitary fibrous tumors • NTRK fusion-positive uterine sarcoma CD117, DOG1 • Gastrointestinal stromal tumor Desmin • Smooth muscle tumors • Rhabdomyosarcoma WT-1 • Sex cord-stromal tumors Calretinin • Sex cord-stromal tumors SF1, Inhibin • Sex cord-stromal tumors ALK • Inflammatory myofibroblastic tumor List of positive and negative immunohistochemistry stains and the potential differential diagnoses considered during initial work up • ER (strong and diffuse) • PR (moderate and patchy) • CD10 (focal and membranous) • SMA (focal and membranous) • Carcinomas • Perineurioma • BCOR rearranged round cell sarcoma • Sex cord-stromal tumors • Solitary fibrous tumors • NTRK fusion-positive uterine sarcoma • Smooth muscle tumors • Rhabdomyosarcoma

This case highlights the diagnostic challenges and unique presentation of MEIS1::NCOA1/2 fusion sarcoma. Owing to a nonspecific histologic and immunoprofile, a broad fusion-based NGS panel is recommended to detect the characteristic gene fusion that defines this entity. This is the first case of MEIS1::NCOA1/2 sarcoma in a young female with diffuse ER/PR expression and extensive abdominopelvic involvement but no identifiable gynecologic or visceral origin. This strong ER/PR expression raises intriguing possibilities concerning the tumor’s lineage and potential therapeutic options, including fertility-sparing surgeries and hormone receptor-targeted treatments. Whether the MEIS1:NCOA1/2 fusion in this tumor makes it a distinct entity or merely represents the many faces of non-smooth muscle uterine mesenchymal tumors requires further investigation. Owing to the propensity for local recurrences, long-term follow-up and further research exploring the utility of adjuvant hormonal therapy are needed.

Based on the patient’s age, clinical presentation, and tumor morphology, our initial differential diagnoses primarily included uterine mesenchymal tumors of extra-uterine origin, such as LGESS (favored), leiomyosarcoma, NTRK fusion sarcoma, inflammatory myofibroblastic tumor, and PEcoma. We also considered sex cord-stromal tumors and soft tissue tumors outside the Müllerian system, including gastrointestinal stromal tumor, rhabdomyosarcoma, and dedifferentiated liposarcoma. However, the IHC pattern did not support these diagnoses (Table  1 ). Nevertheless, we believe that the diffuse ER (and, to a lesser degree, PR) positivity may be associated with the Müllerian system. Moreover, the monomorphic cytomorphology suggested a fusion-related mesenchymal neoplasm, prompting us to perform molecular profiling. The clinicopathologic features of previously reported MEIS1::NCOA1/2 fusion sarcomas, including the current case, are summarized in Table  2 [ 1 – 7 , 9 – 11 ]. Patient ages ranged from 19 to 76 years, with most cases occurring in the gynecologic tract and presenting with nonspecific symptoms mimicking benign conditions. The tumor sizes varied widely, from 6 mm to 21 cm [ 3 ] (Table  2 ). Table 2 Clinicopathologic features of MEIS1:NCOA1/2 fusion sarcomas Study Age/Sex Location Clinical presentation Macroscopic appearance Treatment Outcome Fattal et al. [ 10 ] 74/F Lung Incidentally discovered on surveillance CT in a long-standing smoker 1.7 cm, white bulging lung mass abutting the pleural surface Lobectomy with negative margins NED at 2 years follow-up Kommoss et al. [ 5 ] 26/F Uterus Acute renal colic and hydronephrosis due to ureteral compression by enlarged uterus 9 cm, irregular and infiltrative myometrial mass TAH with preservation of bilateral adnexa NED at 9 years follow-up Mejbel et al. [ 7 ] 62/F Uterus, right pelvic sidewall, and right round ligament Lower abdominal pain and constipation. Tumor was discovered during evaluation of a simple right ovarian cyst 3 cm, hemorrhagic mass TAH + BSO. Multiple resections, adjuvant hormonal (letrozole), and chemotherapy for multiple LRs Multiple diffuse LRs involving the peritoneum, omentum, mesentery, pelvic sidewall, and various pelvic organs Xing et al. [ 4 ] 47/F Uterus AUB and lower abdominal pain for 5 years 7.8 cm, friable, and hemorrhagic mass in endomyometrium with necrosis TAH + BSO. Six cycles of mesna, doxorubicin, and ifosfamide for DM Lung metastasis 12 mo’s after hysterectomy Niu et al. [ 6 ] 41/F Uterus Persistent AUB refractory to medical management 13 cm, solid mass with necrosis and hemorrhage with deep myometrial invasion TAH + BSO with omentectomy and pelvic peritoneal debulking. Multiple rounds of adjuvant chemotherapy LR in vaginal cuff and pelvis, metastasis to lung. Death 23 mo’s after diagnosis Ribeiro et al. [ 11 ] 55/F Left fallopian tube and omentum Abdominal pain, distension, and nausea Pelvic mass associated with peritoneal carcinomatosis and omental caking TAH + BSO NA Kao et al. [ 3 ] 58/F Uterus NA 20 cm TAH + BSO LR in mesentery, 5 mo’s after diagnosis 35/F Vagina Patient was on ovulation induction treatment and vaginal mass was detected incidentally 11 cm, hemorrhagic Intralesional excision Multiple LRs 76/M Scrotum NA 6 mm NA The current lesion was a recurrence of a primary scrotal lesion that was excised years ago 38/F Para-rectal Lesion was present for 2 years and enlarged during pregnancy 14 cm Excision Multiple intrapelvic recurrences, excised several times 20/F Kidney NA 21 cm NA NA 50/F Kidney and soft tissue implants NA 19 cm Total nephrectomy with removal of a portion of 10th rib LRs in retroperitoneum, retrocaval region, and chest wall/diaphragm 33/F Vulva NA 2.5 cm NA NA Argani et al. [ 1 ] 72/F Kidney Complex cyst of the left kidney 4 cm cystic renal tumor Partial nephrectomy NED 21/M Kidney NA 15 cm, pale-tan to white, solid, nodular, and friable mass with necrosis and hemorrhage Radical nephrectomy NA Agaram et al. [ 9 ] 22/M Iliac bone NA Associated with bone destruction and soft tissue involvement Four cycles of NAT followed by surgical resection and adjuvant chemotherapy NED at 8 mo’s follow-up 39/M Iliac bone NA NA NA NA NA: Not available; NED: No evidence of disease; LRs: Local recurrences; mo’s: months; AUB: Abnormal uterine bleeding; TAH + BSO: Total abdominal hysterectomy + bilateral salpingo-opherectomy; NAT: Neoadjuvant therapy; DM: Distant metastasis Clinicopathologic features of MEIS1:NCOA1/2 fusion sarcomas Kommoss et al. [ 5 ] Niu et al. [ 6 ] Agaram et al. [ 9 ] NA: Not available; NED: No evidence of disease; LRs: Local recurrences; mo’s: months; AUB: Abnormal uterine bleeding; TAH + BSO: Total abdominal hysterectomy + bilateral salpingo-opherectomy; NAT: Neoadjuvant therapy; DM: Distant metastasis The morphological and immunophenotypic spectra of MEIS1::NCOA1/2 fusion sarcomas are detailed in Table  3 [ 1 – 7 , 9 – 11 ]. Histologically, these tumors exhibit an infiltrative growth pattern comprising of monomorphic spindle cells arranged in short fascicles, with architectural configurations including storiform, whorling, and nodular patterns. A prominent capillary network can be seen. Similar to our case, alternating hypercellular and hypocellular zones with myxoid to hyalinized stroma have been reported in uterine and vaginal tumors [ 3 , 7 ] (Table  3 ). Mitotic figures can vary widely (0–40/HPF) [ 3 , 7 ], but tumors with distant metastasis show brisk mitoses, tumor necrosis, and lymphovascular invasion [ 4 , 6 ]. The immunoprofile for MEIS1::NCOA1/2 fusion sarcomas is variable and often exhibits focal ER, PR, and CD10 positivity, particularly in tumors from the gynecologic tract. Convincing positivity for WT1, β-catenin, CD99, CD117, BCOR, and MDM2/CDK4 has also been reported in individual cases. Interestingly, three cases of SpRMS (two pelvic osseous and one vaginal) also harbored MEIS1::NCOA1/2 fusions [ 9 ] (Table  3 ), suggesting that this fusion may not define the direction of lineage differentiation. Table 3 Morphological and immunohistochemical features of MESI1:NCOA1/2 fusion sarcomas Study Age/sex Location Original diagnosis Morphology Mitosis Tumor necrosis IHC Other molecular findings Fattal et al. [ 10 ] 74/F Lung LG spindle cell neoplasm Infiltrative pattern. Whorls and fascicles of primitive and monomorphic spindle cells. < 1/10 HPFs Absent SMA: Patchy + None Kommos et al. [ 5 ] 26/F Uterus LGESS Infiltrative pattern. Monomorphic spindle cells with focal fascicular pattern. 4/10 HPFs Absent Vimentin, CD10, CD56, CD99: Diffuse and strong + ER, WT1: Weak + None Mejbel et al. [ 7 ] 62/F Uterus, right pelvic sidewall, and right round ligament LGESS with adipose metaplasia Infiltrative pattern. Alternating hypo-and hypercellular areas with short fascicles of short spindle cells 0–40/10 HPFs Absent CD10: + in hypercellular zones ER, PR: Focal + None Xing et al. [ 4 ] 47/F Uterus HG uterine sarcoma Infiltrative pattern. Spindle cells in streaming and fascicular patterns. LVI present. Monotonous hypercellular round cell sarcoma-like pattern within lung metastasis 40/10 HPFs Present ER/PR: Focal + CD117: Diffuse + CD10 and Cyclin D1: Weak and focal+ CTNNB1 gene variant, HMGA2 , CDK4 , and MDM2 gene amplifications Niu et al. [ 6 ] 41/F Uterus Undifferentiated uterine sarcoma Infiltrative pattern. Hyperchromatic spindle cells in short fascicles in a myxoid b/g. Highly pleomorphic epithelioid cells 28/10 HPFs Present CD10, MDM2: Diffuse, strong + Cyclin D1: Patchy, weak + ER and PR: Negative CTNNB1 gene deletion, amplifications in MDM2 , MDM4 , CDK4 and FRS2 genes Ribeiro et al. [ 11 ] 58/F Left fallopian tube and omentum Mullerian adenosarcoma of fallopian tube primary Spindly and epithelioid mesenchymal cells infiltrating the fallopian tube wall 18/10 HPFs NA WT-1: Diffuse nuclear + CD10 and ER: Variably + None Kao et al. [ 3 ] 58/F Uterus Undifferentiated uterine sarcoma Unencapsulated and focally infiltrative to relatively well circumscribed. Primitive monomorphic spindle to ovoid cells in short fascicles, vague whorls, storiform, and/or solid patterns. Areas of alternating cellularity with myxoid stroma in hypocellular areas. < 1–33/10 HPFs Present in cases #5, 6 Nuclear betacatenin: + in case #1. ER, WT1, CD99, Cyclin D1, CD10, CD56, TLE: Variably + Pan-TRK: Diffuse + in 1 case Case #1: MDM2 amplification, CTNNB1 , (S33C) , ATRX and MPL gene mutations No NTRK1-3 gene rearrangement 35/F Vagina LGESS 76/M Scrotum LG sarcoma, NOS 38/F Para-rectal Sarcoma, NOS 20/F Kidney Undifferentiated spindle cell sarcoma 50/F Kidney and soft tissue implants Sarcoma, NOS 33/F Vulva SpRMS Argani et al. [ 1 ] 72/F (case #1) Kidney Unclassified primary renal sarcoma Nodular growth pattern formed by plump spindle cells in short fascicles and whorls NA Absent BCL2, CD56: + in both cases CD99: Diffuse and nonspecific cytoplasmic staining in both cases ER, nuclear WT1, synaptophysin: Focal + in both cases TLE1 and Cyclin D1 staining in case #1 (50–60%) greater than case #2 (5–10%) BCOR: + in 60% cells in case #2 NA 21/M (case #2) Kidney Unclassified primary renal sarcoma Similar to case #1 with a prominent epithelioid-round cell component NA Absent NA Agaram et al. [ 9 ] 22/M Iliac bone SpRMS Streaming pattern of monomorphic spindle cells Myogenin: Focal + Desmin: Diffuse + NA 39/M Iliac bone SpRMS Vaguely micronodular and whorling pattern of ovoid to short spindle cells Myogenin: Multifocal + MYOD1: Diffuse + NA LG: Low-grade, HG: High-grade, SpRMS: Spindle cell rhabdomyosarcoma, LGESS: Low grade endometrial stromal sarcoma, HPFs: High-power fields, NA: Not available, LVI: Lymphovascular invasion, b/g: Background, +: Positive Morphological and immunohistochemical features of MESI1:NCOA1/2 fusion sarcomas Fattal et al. [ 10 ] Vimentin, CD10, CD56, CD99: Diffuse and strong + ER, WT1: Weak + Mejbel et al. [ 7 ] CD10: + in hypercellular zones ER, PR: Focal + Xing et al. [ 4 ] ER/PR: Focal + CD117: Diffuse + CD10 and Cyclin D1: Weak and focal+ Niu et al. [ 6 ] CD10, MDM2: Diffuse, strong + Cyclin D1: Patchy, weak + ER and PR: Negative WT-1: Diffuse nuclear + CD10 and ER: Variably + Kao et al. [ 3 ] Nuclear betacatenin: + in case #1. ER, WT1, CD99, Cyclin D1, CD10, CD56, TLE: Variably + Pan-TRK: Diffuse + in 1 case Case #1: MDM2 amplification, CTNNB1 , (S33C) , ATRX and MPL gene mutations No NTRK1-3 gene rearrangement Argani et al. [ 1 ] BCL2, CD56: + in both cases CD99: Diffuse and nonspecific cytoplasmic staining in both cases ER, nuclear WT1, synaptophysin: Focal + in both cases TLE1 and Cyclin D1 staining in case #1 (50–60%) greater than case #2 (5–10%) BCOR: + in 60% cells in case #2 Myogenin: Multifocal + MYOD1: Diffuse + LG: Low-grade, HG: High-grade, SpRMS: Spindle cell rhabdomyosarcoma, LGESS: Low grade endometrial stromal sarcoma, HPFs: High-power fields, NA: Not available, LVI: Lymphovascular invasion, b/g: Background, +: Positive Our case represents an unusual presentation of a rare MEIS1::NCOA1/2 sarcoma with diffuse pelvic, omental, and mesenteric involvement but no visceral disease. Notably, this is the first reported case of diffuse and strong ER/PR expression, indicating the possibility of a gynecologic origin. Although ER and PR expression is not exclusive to gynecologic tissues [ 12 – 14 ], their diagnostic utility is well recognized for identifying extragynecologic tract Mullerian lesions/neoplasms in the appropriate clinical and histologic context [ 14 – 16 ]. Two possible explanations for this tumor’s origin are considered. First, it may represent an extrauterine mesenchymal tumor arising from endometriotic implants or Mullerian cell rests. Extrauterine LGESS has been described in the ovaries, omentum, pelvic cavity, rectovaginal septum, and colorectal serosa [ 17 – 20 ], with or without coexisting endometriosis. While our tumor lacked the classic morphology and gene fusions of LGESS, it may still share a similar tissue of origin. Given its potential to mimic benign chronic pelvic conditions, extrauterine mesenchymal tumors must remain on the differential, and early diagnostic laparoscopy should be considered when appropriate to avoid delays in care. The second possibility is that this tumor represents metastatic disease from an unidentified gynecologic primary. Ahn et al. described a case of pelvic LGESS with no uterine abnormality on clinical examination, but multiple endometrial nodules were found on hysterectomy [ 21 ]. Ribeiro et al. reported an extrauterine Mullerian adenosarcoma of fallopian tube origin, with strong WT1 and variable ER and CD10 positivity. The sarcomatous component was located outside the fallopian tube and mimicked a range of mesenchymal neoplasms. Meticulous sampling revealed endometriosis and glandular components within the fallopian tube wall, confirming its diagnosis and origin [ 11 ]. Taking all available evidence into account, our case is best categorized as a low-grade pelvic sarcoma of uncertain origin harboring the MEIS1::NCOA1/2 fusion. We acknowledge that our initial workup did not include BCOR and beta-catenin IHC stains, while their positivity has been reported in some case reports. However, the BCOR and CTNNB1 ( beta-catenin) genes were included in the “Mayo Complete Sarcoma Panel”, with no variants identified. Three MEIS1::NCOA1/2 sarcomas were reported to have additional molecular alterations, such as MDM2/CDK4 amplifications and CTNNB1 mutations (S33C) or deletions (H36–T41). They were morphologically undifferentiated or high-grade, and two of them had lung metastasis, seemingly imposing more aggressive behavior possibly due to increased genomic complexity (Tables  2 and 3 ). In contrast, the MEIS1::NCOA1/2 fusion is the only molecular change identified in our case, similar to some other case reports with low-grade sarcomas (Table  2 ). The pathogenic significance and clinical impact of these additional molecular alterations warrant further studies. The management strategies for uterine primary and recurrent MEIS1::NCOA1/2 sarcomas are largely extrapolated from those of LGESS and include total abdominal hysterectomy with bilateral salpingo-oophorectomy (TAH + BSO) along with complete resection of all visible macroscopic disease [ 21 ]. Given the tendency for multiple local recurrences, close long-term follow-up is essential. However, in young nulliparous women desiring future fertility, conservative approaches are increasingly being considered [ 21 ]. Endocrine therapy options include withdrawal of exogenous estrogen sources (e.g., hormone replacement therapy, tamoxifen) and pharmacologic suppression of endogenous estrogen production [ 22 ]. Several isolated case reports and small series have demonstrated successful pregnancies following fertility-sparing surgery and adjuvant endocrine therapy, which are generally withdrawn before attempting conception [ 21 , 23 – 27 ]. Recurrences are best managed with debulking surgery and interval endocrine therapy aimed at both preserving fertility and minimizing menopausal symptoms [ 22 ]. Definitive demolitive surgery (TAH + BSO) is recommended after childbearing is complete, following comprehensive counseling, regardless of the presence or absence of recurrence [ 23 ]. In our case, considering the patient’s young age and the normal intraoperative appearance of the gynecologic organs, TAH + BSO was deferred. This limited a complete evaluation of the gynecologic tract and precluded definitive staging. Management was therefore based on case reports, extrapolated treatment guidelines from uterine sarcomas, and shared decision-making. After extensive counseling, both ovaries were preserved, and antihormonal therapy was not initiated since all gross disease had been resected. Nevertheless, owing to the tumor’s ER/PR positivity and associated hormonal sensitivity, there remains a significant risk of future recurrence, potentially requiring antihormonal therapy, chemotherapy, or delayed TAH + BSO. The patient was referred to reproductive endocrinology for fertility planning and ongoing surveillance. This case highlights the diagnostic and management challenges of rare spindle cell tumors with a suspected gynecologic origin in young adults. These findings underscore the delicate balance between preserving fertility and achieving optimal oncologic outcomes.

Novel MEIS1::NCOA1/2 fusions have been reported in a subset of spindle cell tumors involving the genitourinary [ 1 – 3 ] and gynecologic tracts [ 3 – 7 ], as well as in two cases of intraosseous spindle cell rhabdomyosarcomas (SpRMS) [ 8 , 9 ]. Rare occurrences have also been documented in the para-rectal soft tissue [ 3 ] and lung [ 10 ]. These tumors are typically associated with low malignant potential but are prone to local recurrences. To date, only two cases of lung metastases have been reported, one originating from a renal sarcoma [ 2 ] and the other from a uterine sarcoma [ 4 ]. Histologically, these tumors feature primitive monomorphic spindle cells arranged in short fascicles with a nonspecific immunoprofile, necessitating molecular testing for accurate diagnosis [ 5 ]. Before the advent of molecular testing, they were frequently misclassified as low-grade endometrial stromal sarcomas (LGESSs) or undifferentiated uterine or renal sarcomas [ 1 , 3 , 7 ]. The identification of the MEIS1::NCOA1/2 fusion has since defined this tumor as a distinct molecular entity. We report the first case of MEIS1::NCOA1 fusion in a low-grade pelvic sarcoma with diffuse abdominal involvement in a young adult woman and discuss its diagnostic challenges and potential origin.