11-year-old castrated male Labrador

Referred for an abdominal mass and right anal sac nodule.

Study performed:

  • CT angiography was acquired in the following phases: pre-contrast, arterial, venous and delayed phases using Omnipaque (iohexol 350 mg/ml, 600 mg/kg IV via pressure injector @ 5ml/sec). The delayed phase was acquired 2 minutes and 30 seconds following the contrast administration. The series were reconstructed in 2.0mm slices in soft tissue and bone algorithms. Only the pre-contrast and delayed phases in soft tissue algorithm are provided herein.

To download DICOM click here

  • Arising from the head of the spleen, there is a well-demarcated and hypoattenuating (13-18HU) round mass (7.7cm x 9.8cm) admixed with multiple areas of fat attenuation. The mass has moderate internal septation, is smoothly marginated, and has a fine rim which is isoattenuating to normal spleen in all phases. There is mild and homogeneous contrast enhancement of the internal septae and peripheral margins. There is no evidence of peritoneal effusion.
  • In the right anal sac, there is an ill-defined soft tissue nodule (approximately 6.8 mm), which avidly enhances and effaces with the adjacent wall. The left anal sac is filled with uniform fluid.
  • The adrenal glands are mildly, diffusely enlarged (R: 15.0mm, L: 12.0mm) with punctate mineralization which is more pronounced on the left. The caudal pole of the left adrenal gland is hypoattenuating in all phases.
  • Incidentally, both kidneys have numerous triangular non-enhancing defects, consistent with renal infarcts, the pancreas has mildly nodular margins and there are moderate degenerative lumbosacral changes and severe coxarthrosis with vacuum phenomenon.
  • There is no evidence of abdominal lymphadenopathy.
  • Large splenic mass with fatty components. Given these characteristics, mixed lipomatous neoplasia of mesenchymal origin (e.g. myelolipoma, liposarcoma, lipofibrosarcoma) or angiomatous origin (e.g. angiolipoma, or with hemangioma/hemangiosarcoma elements) were prioritized.
  • Right anal sac nodule. The primary differential diagnosis is apocrine gland adenocarcinoma or adenoma.
  • Mild bilateral adrenomegaly with punctate mineralization. Pituitary-dependent hyperplasia was prioritized, with neoplasia (metastatic or primary) considered less likely. The hypoattenuating caudal pole of the left adrenal gland is most consistent with myelolipoma. Stippled adrenal mineralization was considered most compatible with benign dystrophic mineralization than associated with primary malignancy such as carcinoma or pheochromocytoma.
  • Splenectomy and right anal sacculectomy were performed on the following day. Intraoperative inspection of the pancreas was unremarkable.
  • Histopathology revealed:
    • Splenic myelolipoma with necrosis. Samples were not immunoreactive for Factor VIII related antigen, and therefore not endothelial in origin.
    • Right apocrine gland adenocarcinoma.
  • The patient was treated with carboplatin for the apocrine gland adenocarcinoma. Hypercalcemia (ionized calcium: 1.6 mmol/L) and inappropriately elevated parathyroid hormone (2.5 pmol/L) were detected a month after treatment, suggesting primary hyperparathyroidism. Cervical ultrasonography detected a right parathyroid nodule (6.8 mm), for which parathyroid adenoma was prioritized. The owner elected to treat the dog with zoledronic acid intravenously, and the patient remained normocalcemic, 11 months later at time of writing.
  • The caudal pole of the left adrenal gland was sonographically homogeneously hyperechoic with a fine rim of hypoechoic tissue, supporting adrenal myelolipoma. Adrenal glands remained sonographically static in size and appearance over the following 9 months.
  • Myelolipomas are benign neoplasms composed of various quantities of mature adipose cells and hematopoietic elements resembling bone marrow, and are said to be rare. Splenic myelolipomas are commonly detected in older cats and dogs by ultrasonography as strongly hyperechoic nodules along the mesenteric border of the spleen, with variable acoustic shadowing. These benign lesions can also appear deeper in the splenic parenchyma, commonly along the vessels. Their size and number vary greatly, and in rare instances, these lesions can reach considerable size. In one study, primary mesenchymal tumors of the spleen (non-angiomatous/non-lymphomatous) represented 13% of splenic lesions resulting in splenectomy, and 2% of these were lipoma-myelolipoma. Lipoma-myelolipoma show a benign biological behavior and surgical resection is considered curative.
  • The only CT description of splenic myelolipoma in a dog known to the authors is very similar to the case described herein. In people, where splenic myelolipoma have been more extensively reported, CT demonstrates masses with a predominantly fatty composition and a soft tissue-attenuating rim and inconsistent intralesional soft tissue-attenuating areas due to hemorrhage or extramedullary hematopoiesis. Whether the splenic and presumed adrenal myelolipoma merely coexist or represent multifocal disease, as suggested from a handful of human and one canine case reports, is undetermined.
  • In people, a diagnosis of Multiple Endocrine Tumor type 1 (MEN1) may be established on the basis of the occurrence of two or more primary MEN1-associated endocrine tumors (i.e. parathyroid adenoma, pituitary adenoma, pancreatic islet cell tumor). Lipomas are one of the tumor types associated with this syndrome. While MEN1 has been rarely described in veterinary medicine, a genetic basis remains to be proposed. At time of writing, the patient is well and further imaging or diagnostics are not planned.
  • Schwarz LA, Penninck DG, Gliatto J. Ultrasound Corner Canine Splenic Myelolipomas. Vet Radiol Ultrasound 2001; 42: 347-348
  • Spangler WL, Culbertson MR, Kass PH. Primary Mesenchymal (Nonangiomatous/Nonlymphomatous) Neoplasms Occurring in the Canine Spleen: Anatomic Classification, Immunohistochemistry, and Mitotic Activity Correlated with Patient Survival. Vet Pathol 1994; 31: 37-47.
  • Wisner E, Zwingenberger A. Atlas of Small Animal CT and MRI. Ames, Iowa: John Wiley & Sons; 2015. 693 p.
  • Wood WG, Restivo TE, Axelsson KL, Svahn JD. Myelolipoma in the spleen: a rare discovery of extra-adrenal hematopoietic tissue. J Surg Case Rep. 2013;(3)
  • Zeng Y, Ma Q, BS, Lin L, Fu P, Shen Y, Luo QY, Zhao LH, Mou JH, Xiao HL. Giant Myelolipoma in the Spleen: A Rare Case Report and Literature Review. Int J Surg Pathol 2016; 24(2):177– 180
  • Aguilera NS, Auerbach A. Extra-adrenal myelolipoma presenting in the spleen: A report of two cases. Hum Pathol (N Y). 2016; 6: 8–12