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Adnexal serine/threonine kinase 11 (STK11) tumor is a newly recognized, histologically distinct neoplasm, typically arising in paratubal soft tissues, with unclear histogenesis. These tumors are characterized by alterations in the tumor suppressor gene STK11 that encodes serine/threonine kinase1, 2. Approximately 50% of STK11 tumors occur in patients with Peutz–Jeghers syndrome (PJS), an autosomal dominant condition caused by a germline mutation in the STK11 gene. PJS is characterized by mucocutaneous pigmentation and hamartomatous polyps throughout the gastrointestinal tract that may cause obstruction1-3. Individuals with PJS have an increased risk of many types of malignant tumor3. Macroscopically, adnexal STK11 tumors have been described as solid or cystic-solid masses, often containing areas of necrosis and hemorrhage2. Microscopically, these tumors are diverse, and there are many differential diagnoses to consider1, 2. Most STK11 tumors are aggressive and infiltrate locally, with the most common sites of metastasis at the time of diagnosis being the omentum and uterine serosa1. Lymph node metastases are rare in cases of STK11 tumor2. Due to the rarity of these tumors, with only 40 cases described in the literature, the optimal treatment is unknown. Most cases have been treated with chemotherapy, which is often platinum based, after cytoreductive surgery. Following suboptimal cytoreduction, the recurrence rate has been reported as almost 80% and the rate of disease-related death after surgical removal of recurrent disease (with or without adjuvant pharmacological treatment) as around 50%2. To the best of our knowledge, there are no published ultrasound, computed tomography (CT) or magnetic resonance images of adnexal STK11 tumors. In this Letter to the Editor, we present a case of paratubal STK11 tumor accompanied by ultrasound images and videoclips, with the aim of raising awareness of such tumors. The patient and her brother described herein provided informed consent for the publication of their data. A 30-year-old nulligravida with abdominal pain and bloating and a left-sided adnexal mass was referred to Glasbo Clinic, Center for Reproductive Health, Obstetrics and Gynecology, Yerevan, Armenia, in January 2025. Transabdominal and transvaginal ultrasound examinations were performed by an experienced ultrasound examiner (L.H.) using a Voluson E10 ultrasound machine (GE Healthcare, Zipf, Austria) with an abdominal convex 2–9-MHz probe and a vaginal 6–12-MHz probe, respectively. The examinations showed normal uterus and ovaries, with free fluid in the pouch of Douglas (Figure 1). A left-sided paraovarian multilocular-solid tumor was described, with smooth outer contouring, low-level echogenicity of the cystic fluid, a maximum diameter of 11.5 cm and moderate vascularization on color Doppler (color score of 3) (Figures 1 and 2, Videoclips S1–S3). The ultrasound diagnosis was a malignant extraovarian pelvic tumor. A CT scan confirmed the ultrasound findings. Serum CA 125 level was 269 U/mL (normal level, < 35 U/mL), human epididymis protein 4 level was 37.7 pmol/L (normal level, < 70 pmol/L) and CA 19-9 level was < 0.6 U/mL (normal level, < 27 U/mL). Laparoscopy revealed a normal abdominal cavity and normal uterus and ovaries, with 60 mL of clear, yellowish free fluid in the pouch of Douglas and a left-sided solid tumor adherent to the fallopian fimbriae. The tumor and left fallopian tube were removed laparoscopically and flush cytology was performed, revealing no malignant cells. The final histopathological diagnosis was a paratubal STK11 tumor (Figure S1), International Federation of Obstetrics and Gynaecology (FIGO) stage IC24 (TNM classification system5, pT1c2 pNx). Next-generation sequencing of tumor tissue is not available in Armenia and therefore was not performed. Postoperatively, management with close follow-up or adjuvant chemotherapy with carboplatin and paclitaxel were discussed. The rationale for considering chemotherapy was the growth of the STK11 tumor on the surface of the fallopian tube and the reported aggressiveness of the tumor, while acknowledging the absence of established treatment recommendations due to the extreme rarity of this type of tumor. However, the patient sought a second opinion in Germany, where she underwent left oophorectomy. Histological examination showed no evidence of tumor in the left ovary. At the time of writing, the patient is currently receiving chemotherapy with carboplatin and cisplatin in Germany. There was no family history of PJS diagnosis. After the diagnosis of a STK11 tumor was made in the patient, she and her brother were examined for clinical signs of the syndrome. Multiple pigmented spots on the skin and mucous membranes were observed in both siblings (Figure S2) and clinical diagnoses of PJS were made. Colonoscopy revealed histologically benign hamartomatous polyps in the patient. The other family members declined genetic consultation and colonoscopy. We trust that this work will help ultrasound examiners recognize the sonographic appearance of paratubal STK11 tumors and consider the possibility of PJS when presented with a mass with malignant ultrasound appearance that is adjacent to the ovary. If a normal ipsilateral ovary or normal ovarian tissue adjacent to the tumor cannot be seen on ultrasound, a STK11 tumor might be misdiagnosed as an ovarian malignant sex cord stromal tumor, a germ cell tumor (e.g. yolk sac tumor), an endometrioid or clear cell carcinoma, or a metastasis. The data that support the findings of this study are available from the corresponding author upon reasonable request. Videoclip S1 Transvaginal grayscale and color Doppler ultrasound showing normal uterus, normal right and left ovaries, fluid in the pouch of Douglas and paraovarian multilocular-solid tumor that was later confirmed histologically to be a paratubal STK11 tumor. Videoclip S2 Transvaginal grayscale and color Doppler ultrasound showing paratubal multilocular-solid STK11 tumor. Videoclip S3 Transabdominal grayscale and color Doppler ultrasound showing paratubal multilocular-solid STK11 tumor. Figure S1 Microscopic imaging of STK11 paratubal tumor (hematoxylin-eosin staining, ×1000 magnification). (a) Interanastomosing cords and trabeculae of tumor cells (black arrow) embedded in myxoid stroma (red arrow). Cuboidal-to-columnar tumor cells with eosinophilic or vacuolated cytoplasm and round to oval nuclei with nucleoli are visible. Immunohistochemistry revealed cytokeratin 7, AE1/3, Wilms tumor 1 antibody clone (WT49), inhibin–α and steroidogenic factor 1 positivity. (b) Intersecting fascicles of bland spindle cells (blue arrow) set in prominently myxoid stroma with rare, inconspicuous tubular structures. Figure S2 Photographs of patient (a,b) and patient's brother (c,d), showing hyperpigmented spots on lips (a,c) and fingers (b,d). Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.