Although SO has elicited considerable interest since it was first described, many diagnostic aspects are still unknown. SO, defined as containing 50% or more thyroid tissue, accounts for approximately 5% of all ovarian teratoma, with an incidence of malignant transformation reported in 5% to 37% of SO [1
]. Most cases are found incidentally and for this reason the only clinical data are obtained from retrospective reports. Previous studies have demonstrated that the majority of patients with SO are asymptomatic or accompanied by non-specific symptoms similar to other ovarian neoplasms. At the time of diagnosis, the most common symptoms were lower abdominal pain, palpable abdominal mass, ascites, and abnormal vaginal bleeding [5
]. Unusual clinical presentations such as hyperthyroidism and Meigs' syndrome have been also documented [2
]. The incidence of hyperthyroidism was reported to be 5% to 8% and 17% to 33% of the cases had ascites at diagnosis [4
]. The pathophysiology of hyperthyroidism in SO is still unknown. Matsuda and colleagues have reported that malignant SO can be diagnosed before operation by the evaluation of free T3, and T4, Thyroglobulin, and TSH. The extremely high levels of Thyroglobulin in local ovarian venous blood compared with that in peripheral blood provide evidence of the production of Thyroglobulin in the ovarian tumor and the normalization of it’s serum levels is related to surgical resection of the tumor [13
]. In our case the patient did not show any specific symptoms and the diagnosis of SO was incidental and only established after surgical removal.
Thyroid tissue in SO is morphologically, biochemically identical to that of cervical thyroid gland. For this reason the diagnosis of malignant SO is in conformity to the same criteria used for thyroid carcinoma, such as the presence of ground glass nuclei, vascular invasion, and mitotic activity [3
]. Unfortunately, the concept of malignant SO in the literature is confusing due to lack of standardized prognostic parameters. The diagnosis of a well-differentiated variant is particularly difficult due to lack of a well-defined tumor capsule in the ovarian tissue. In these cases, the presence of infiltration by tumor cells into the surrounding ovarian tissue, and involvement of vascular system, or metastasis, highly supported the diagnosis of malignancy [2
]. Although the histological aspects of malignancy are often documented, the majority of patients that underwent surgical treatment for thyroid type carcinoma arising in SO did not show a clinically aggressive outcome with high rates of recurrences and metastatic spread. The biological behavior remains often enigmatic and seems not to be correlated with the observed long-term clinical outcomes.
With the efforts of understanding the prognostic parameters and defining more appropriate treatment care, a molecular and morphological consensus is advocated to standardize diagnostic criteria of malignancy in cases of thyroid type carcinoma in SO.
Distant metastasis has been reported to be a rare feature of SO in approximately 5% of cases [12
]. Other clinical studies demonstrated a higher intra-abdominal metastatic rate of 23% [5
]. Common metastatic sites are the omentum, peritoneum, lymph nodes, fallopian tubes, and the contralateral ovary [12
]. Some patients presented with distant metastases to the lungs, bone, brain, liver, and mesenteric surfaces of the spleen and diaphragm [8
]. In clinical diagnosis, thyroid type carcinoma in SO has to be differentiated from cystadenoma and other primary ovarian cancers or metastatic tumors. Generally, SO appeared as a smooth-margin multicystic mass with a high attenuation signals during pre-contrast on CT scan. Signal intensities on T1-weighted images were usually intermediate to high, and those on T2-weighted images were low as recently reported by Shen and colleagues [14
]. The majority of tumors showed a mixed cystic and solid mass, with capsule wall thickness of 3 mm on average, and contained transparent, green, or brown fluids [14
]. To our knowledge five lethal cases have been reported in literature with recurrences and metastatic disease [8
]. In our case no metastasis have been documented during 1-year follow-up after surgery.
The treatment of malignant SO remains controversial and no consensus exists on the surgical and non-surgical modalities. Surgical treatment consists in total abdominal hysterectomy plus bilateral salpingo-oophorectomy with omentectomy and lymph nodes sampling, a reasonable therapy for postmenopausal women with diagnosis of carcinoma. In other conditions, especially when fertility has to be preserved, conservative surgery concerning unilateral oophorectomy might be proposed together with a strict follow-up [5
]. This hypothesis should be carefully discussed with the patients. Laparoscopic surgery may offer some advantages in surgical staging and its mini-invasive removal but this technique is not standardized for these uncommon conditions [6
]. After the initial surgery, some authors have advocated near total thyroidectomy and radioactive iodine ablation to detect and treat recurrent disease [11
]. Total thyroidectomy is mandatory to exclude a primary thyroid neoplasm in the differential diagnosis of SO. Total thyroidectomy followed by ¹³¹ I radio ablation therapy should be reserved for patients with recurrence or residual disease [5
]. Thyroglobulin is a well established marker for monitoring the recurrence of malignancy. An increased serum levels of Thyroglobulin represent the early detection of recurrence as reported in many studies [6
SO containing thyroid type carcinoma must be distinguished from papillary or follicular thyroid carcinoma metastatic to the ovary [4
]. It is mandatory to study the thyroid gland for the differential diagnosis of primary or secondary tumor of the ovary. De Simone and colleagues have proposed thyroidectomy to confirm normal thyroid gland, by excluding a primary thyroid carcinoma, and potentiate radioactive iodine therapy [3
]. No consensus has been reached in performing prophylactic total thyroidectomy after the diagnosis of thyroid type carcinoma in SO. In this situation, the combination of surgical removal with subsequent thyroidectomy and radiotherapy do not represent a standardized therapy but it is a comprehensive therapeutic modality, still not supported by scientific data in reducing the risk of recurrence and increase prognostic indices of selected cases.