Teratomas are non-seminomatous germ cell neoplasms derived from totipotent cells that differentiate into tissues representing all three embryonic germ layers: ectoderm, mesoderm, and endoderm [1]. While they predominantly occur in the gonads (ovaries and testes), primary extragonadal teratomas are rare, accounting for less than 5% of all teratomas [1][2]. The retroperitoneum is the third most common site, representing 1% to 11% of primary retroperitoneal tumors, primarily seen in children and rarely in adults older than 30 years [1][2].

Extramedullary hematopoiesis (EMH) refers to the production of blood cells outside the bone marrow, usually occurring secondary to severe hematological disorders like myelofibrosis, hemolytic anemias, or thalassemia [3]. However, its occurrence within a primary retroperitoneal mature cystic teratoma in an otherwise healthy patient is exceptionally rare [3]. We describe the clinical, surgical, and histopathological findings of this unusual composite entity.

Given the massive size and potential for local compressive complications, the patient underwent an elective surgical resection under general anesthesia. The patient was secured in the right lateral decubitus position, and a left supracostal flank incision was performed. The retroperitoneal space was entered sequentially by layers while maintaining meticulous intraoperative hemostasis.

Intraoperatively, a large, well-defined, encapsulated soft tissue mass was identified. The mass was successfully isolated and completely excised with negative macroscopic margins, without causing any intraoperative complications or measurable blood loss. Strict care was taken to maintain full capsular integrity to prevent the intraperitoneal spillage of highly irritative cystic contents. The total operative duration was approximately 180 minutes. Visual inspection during exploration confirmed that both ovaries and the surrounding viscera were entirely unaffected and free of disease. The patient experienced a smooth, uneventful postoperative recovery, was transferred to the regular inpatient ward, and was safely discharged home on postoperative day one.

The patient was placed on a regular surveillance protocol. At the 12-month postoperative follow-up interval, she remained completely asymptomatic. Recurrence was rigorously assessed via annual clinical examinations and follow-up contrast-enhanced abdominal/pelvic CT imaging, which demonstrated no evidence of local recurrence or residual disease.

Primary retroperitoneal teratomas are exceptionally rare germ cell tumors in adults, representing less than 11% of all primary retroperitoneal neoplasms [1][2]. Unlike their gonadal counterparts, primary extragonadal retroperitoneal teratomas originate from displaced primordial germ cells that fail to complete their migration along the urogenital ridge during early embryogenesis [1]. In adults, these tumors exhibit an indolent growth pattern and remain clinically silent for extended periods. They are frequently discovered incidentally or only after attaining a massive size, at which point they manifest with non-specific symptoms such as abdominal fullness, dull flank pain, or palpable abdominal masses caused by the compression of adjacent retroperitoneal viscera [1][2].

Radiographically, computed tomography (CT) and magnetic resonance imaging (MRI) serve as the cornerstones for pre-operative evaluation. The presence of a well-encapsulated, heterogeneous retroperitoneal mass demonstrating a clear coexistence of adipose tissue, fluid-attenuating cystic spaces, and coarse, dense calcifications or ossifications is highly pathognomonic for a mature cystic teratoma [4]. However, when retroperitoneal masses present with mixed fatty and hematopoietic components, they can easily mimic other distinct clinical entities. These include primary retroperitoneal liposarcomas, exophytic developmental duplication cysts, or retroperitoneal myelolipomas [4].

The most unique and physiologically remarkable feature of this case is the microscopic identification of active extramedullary hematopoiesis (EMH) interspersed within the teratoma’s mature adipose tissue matrix. Clinically, EMH functions as a systemic, compensatory physiological response to bone marrow insufficiency, typically triggered by underlying hematological disorders such as myelofibrosis, severe hemolytic anemias, hemoglobinopathies, or marrow-infiltrating neoplastic processes [3]. However, the development of localized EMH within a primary extragonadal teratoma in a patient with completely normal peripheral blood profiles and no systemic hematological diseases represents a distinct, autonomous localized phenomenon [3].

This localized hematopoietic proliferation can be explained by two prevailing scientific theories:

1)TheBoneMarrowNicheTheory: The close structural and spatial alignment of mature lamellar bone fragments and adjacent fatty adipose stroma within the teratoma essentially recapitulates a functional, microscopic bone marrow niche [3]. This unique osteo-adipose microenvironment possesses the precise architectural and biochemical properties required to home circulating pluripotent mesenchymal cells or migrating CD34+ hematopoietic stem cells, signaling them to settle, proliferate, and differentiate into mature erythroid, myeloid, and megakaryocytic lineages [3].

2)TotipotentGermCellDifferentiation: Because teratomas arise from totipotent germ cells capable of differentiating into any tissue of the three embryonic germ layers, the hematopoietic nests may simply reflect a direct, highly specialized mesodermal differentiation pathway where the tumor independently generates its own stromal and hematological architecture.

From a therapeutic perspective, complete surgical excision via exploratory laparotomy or a minimally invasive approach remains the definitive standard of care [1][5]. Total capsular resection minimizes the risk of local recurrence and prevents significant chemical peritonitis or acute inflammatory responses that can be triggered by the intra-abdominal spillage of highly irritative, keratinaceous, or mucinous cystic fluids [5]. Furthermore, securing the entire, intact surgical specimen is vital for a comprehensive histopathological evaluation. Pathologists must implement an exhaustive tissue sampling and sectioning protocol to meticulously scan for any focal micro-regions containing immature embryonic tissue (specifically neuroepithelium) or somatic malignant transformations, such as squamous cell carcinoma or adenocarcinoma arising within the benign components [6]. The identification of these elements entirely alters the staging, prognosis, and post-operative oncological management of the patient.

The institutional review board or local ethics committee provides a waiver of approval for single retrospective case reports that do not involve experimental interventions.

Written informed consent was obtained from the patient for the publication of this case report and any accompanying intraoperative and radiological images. A copy of the written consent form is available for review by the Editor-in-Chief of this journal.