Struma ovarii is a rare form of mature ovarian teratoma composed predominantly of thyroid tissue, often presenting as a unilateral adnexal mass with clinical hyperthyroidism.
Description
Struma ovarii is a rare specialised form of monodermal teratoma that contains predominantly thyroid tissue, accounting for more than 50% of the tumour. This ovarian neoplasm often presents as a unilateral adnexal mass and may be associated with clinical hyperthyroidism due to functional thyroid tissue.
Pathogenesis
The exact pathogenesis is unknown, but it is postulated that the ovarian germ cell line undergoes metaplasia to differentiate into thyroid tissue. The thyroid tissue within the tumour is functional and can produce thyroid hormones, leading to hyperthyroidism in some cases.
Subtypes
There are no specific subtypes of struma ovarii. However, the tumour can occasionally undergo malignant transformation into thyroid carcinoma, although this is exceedingly rare.
Epidemiology, Risk Factors & Associations
- Struma ovarii represents approximately 1% of all ovarian tumours.
- Typically presents in the 5th decade of life.
Clinical Features
- Patients may present with an adnexal mass or pelvic pain.
- In a minority of cases, patients may present with clinical hyperthyroidism if the thyroid tissue within the tumour is functional and producing excess thyroid hormone.
Complications
- Malignant transformation into thyroid carcinoma, though rare, is the most serious complication.
Pathological Features
Histopathology
- Macroscopic: The tumour is typically cystic and can contain a mixture of solid and cystic components.
- Microscopic: The tumour demonstrates thyroid follicles filled with colloid, mimicking normal thyroid tissue.
Serology
- Elevated levels of thyroid hormones (T3, T4) and suppressed thyroid-stimulating hormone (TSH) may be seen if the tumour is functional.
Biochemistry
- No specific biochemical markers.
Radiological Features
General Features
- Characteristically presents as a large multilocular cystic mass with a solid component that can have coarse calcification.
- An echogenic solid component on ultrasound is indicative of the presence of thyroid tissue.
US
- Transabdominal and transvaginal ultrasound: Typically reveals a complex, mostly cystic mass. The solid component may appear echogenic, indicating the presence of thyroid tissue. Peripheral calcifications may be present in some cases.
CT
- Non-contrast: Demonstrates a complex cystic and solid mass. Calcification may be seen within the solid component.
- Contrast-enhanced: The solid component demonstrates enhancement.
MRI
- T1WI: Solid components are isointense to muscle.
- T2WI: Solid components are isointense or slightly hyperintense to muscle.
- T1 C+: Solid components enhance.
- DWI/ADC: No specific characteristics.
NM
- Thyroid scintigraphy (I-123 or Tc-99m pertechnetate): Demonstrates uptake within the mass, reflective of functioning thyroid tissue, aiding in distinguishing struma ovarii from other ovarian tumours.
Grading and Staging
There is no specific grading and staging system for struma ovarii.
Diagnosis
Diagnosis is typically confirmed histologically following surgical resection of the tumour.
Differential Diagnosis
- Mature cystic teratoma: Can contain various tissue types. The presence of fat and calcification are common.
- Serous cystadenoma: Typically presents as a unilocular cystic mass, without a solid component.
- Mucinous cystadenoma: Typically multilocular with a larger size.
- Brenner tumour: Often contains calcification, but it’s typically small and solid.
Differential Diagnosis
- Mature cystic teratoma: Typically presents as a complex cystic mass with fat and calcifications. However does not show uptake on thyroid scintigraphy.
- Ovarian fibroma: Presents as a solid ovarian mass, often associated with Meigs’ syndrome. Typically hypoechoic on ultrasound compared to the echogenic thyroid tissue seen in struma ovarii.
- Ovarian serous cystadenoma: This benign epithelial ovarian tumour presents as a unilocular or multilocular cystic mass. It does not have the solid component of thyroid tissue seen in struma ovarii.
- Granulosa cell tumour: This sex cord-stromal tumour presents as a large, predominantly cystic mass, but can be mixed cystic-solid or solid. It can also cause elevated oestrogen levels leading to endometrial hyperplasia. No thyroid tissue or uptake is seen on thyroid scintigraphy.
- Metastases to the ovary (Krukenberg tumour): Metastases, particularly from gastric carcinoma, can mimic struma ovarii as a complex solid-cystic mass. However, these tumours typically affect both ovaries and do not contain thyroid tissue. They also lack radiotracer uptake on thyroid scintigraphy.
Management
Management typically involves surgical resection. In cases where the patient presents with hyperthyroidism, preoperative management with antithyroid medications may be necessary.