1 CASE PRESENTATION

A 68-year-old female presented with no significant past medical history presented with lower back pain localized to the left side. MRI revealed a 4.0 cm presacral mass centred on the midline and directly abuts the anterior sacrum without infiltrative features. The patient underwent fine-needle aspiration (FNA) for further evaluation. Diff-Quik and Papanicolaou stained smear showed abundant haematopoietic elements intermixed with mature fat droplets. The haematopoietic cells were composed of immature myeloid and erythroid precursor cells admixed with neutrophils, lymphocytes and eosinophils. Few megakaryocytes were seen. Figure 1A,B. Cellblock of FNA showed mature adipocytes with no significant atypia and trilineage haematopoiesis with markedly increased megakaryocytes. Figure 2A,B.

2 QUIZ

3 EXPLANATION

Myelolipoma is a rare benign neoplasm composed of a variable proportion of a mixture of mature adipocytes and haematopoietic elements. While myelolipoma is predominantly encountered in the adrenal gland, the occurrence in extra-adrenal sites such as the presacral region, retroperitoneum, liver and mediastinum has been reported in the literature.¹ The pathogenesis of myelolipoma is not well understood and is currently undergoing ongoing research to further characterize the unique cytogenetic and molecular alterations that contribute to the neoplastic process in myelolipoma.

Presacral myelolipoma commonly presents as an incidental finding on radiologic imaging; however, large lesions may be symptomatic and present as encapsulated, heterotopic fat-containing masses supported by a network of reticulum fibres. Fine-needle aspiration (FNA) cytology offers a minimally invasive and effective means of obtaining cytological samples from deep-located lesions.²

In the illustrated case, the differential diagnosis included myelolipoma and extramedullary haematopoiesis. Surgical resection and microscopic examination revealed a combination of fully mature adipocytes and extramedullary trilineage haematopoietic cells, resembling hypercellular bone marrow, accompanied by a significant elevation in megakaryocyte count. The overall findings with a correlation with histopathologic examination revealed a definitive diagnosis of presacral myelolipoma. The cellular composition of presacral myelolipoma in the cytological sample is usually heterogeneous, reflecting the coexistence of mature adipocytes and haematopoietic elements. The relative proportions of adipocytes and haematopoietic cells may vary. The cytological appearance of adipocytes is typically bland and unremarkable without significant nuclear atypia or pleomorphism. The adipocytes are large with abundant clear or vacuolated cytoplasm. The nuclei of adipocytes are small, round to oval and eccentrically located due to the abundant cytoplasm. The trilineage haematopoietic elements include myeloid and erythroid cell precursors. The myeloid elements reveal the presence of individual cells or clusters with varying degrees of maturation of granulocytes and monocytes. Multidisciplinary diagnostic approaches combining radiologic imaging features, FNA and core needle biopsy of a presacral myelolipoma aid in accurately diagnosing this unique entity.

Fine-needle aspiration cytology of several presacral lesions reveals mature adipocytes that may overlap with the cytopathologic features of myelolipoma; however, the trilineage haematopoietic elements are key features of presacral myelolipoma and are not typically seen in other entities such as lipoma, angiomyolipoma and liposarcoma. Table 1 summarized cases of presacral myelolipoma diagnosed via FNA cytology.^{3, 4}

Presacral myelolipoma and presacral extramedullary haematopoiesis are two benign mature adipocyte lesions that can present as discrete, encapsulated, lipomatous masses with variable amounts of non-lipomatous trilineage haematopoietic components and should be included in the differential diagnosis of a well-circumscribed fat-containing presacral lesions.

Extra-adrenal presacral myelolipoma often raises a concern for liposarcoma due to the frequent resemblance of radiologic features. Aspirate cytology shows hypercellular cell groups with atypical spindle cells. Lipoblasts can be seen with vesicular mature adipocytes, while lipoma is usually composed of mature adipocytes demonstrating nuclear vacuolization with no significant atypia. The lack of abundant trilineage haematopoiesis helps in differentiating this entity.

Angiomyolipoma should be considered in aspirates cytology of extrarenal masses when an admixture of three components, including smooth muscle spindle cells, mature adipocytes and thick-walled hyalinized blood vessels is observed. Notably, trilineage haematopoietic elements are not typically seen.

Myeloid sarcoma, commonly associated with acute myeloid leukaemia and encountered in the skin, gastrointestinal tract, bone and lymph node, presents with predominant myeloid blasts composed of single cells with large, round eccentrically located nuclei, coarse chromatin occasional binucleation and prominent nucleoli.

In many cases, surgical excision is pursued followed by histopathologic examination to confirm the diagnosis. The specimen typically exhibits a mix of fully mature adipocytes and extramedullary trilineage haematopoietic cells, simulating hypercellular bone marrow, often associated with a notable increase in megakaryocyte count.

4 AUTHOR CONTRIBUTIONS

NS was involved in conceptualization, data curation, writing—original draft, and writing—review and editing; PV was involved in conceptualization, writing—review and editing; ZL was involved in conceptualization and writing—review and editing; OS was involved in conceptualization, writing—original draft, and writing—review and editing. AA was involved in conceptualization, data curation, writing—original draft, and writing—review and editing

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.