Introduction. Struma ovarii (SO) is a monodermal teratoma in which thyroid tissue comprises more than 50% of the tumour. Papillary thyroid cancer (PTC) in SO is a rare finding, as only 5% of SO cases undergo malignant transformation. Malignant SO is usually asymptomatic and infrequently diagnosed preoperatively. Because of its rarity, there is no consensus about diagnosis and management, while treatment and follow-up procedures are not clearly established.

Material and methods. Herewith, we report two cases of PTC in SO. The first patient was a 25-year-old woman diagnosed with bilateral ovarian tumours. The second patient, 19-year-old woman, presented with unilateral ovarian mass. Both patients were qualified for surgical excision of the tumours. Histopathological specimens underwent both conventional histopathological assessment and immunohistochemical staining.

Results. In the first patient histopathology revealed SO with two foci of PTC. Immunohistochemically a positive expression of CK7, CK19, p63 and thyroglobulin (Tg) confirmed the diagnosis. She underwent total thyroidectomy in 2016 in order to enable ablative radioiodine therapy and facilitate further thyroglobulin monitoring. Unfortunately, the patient was lost from follow-up. In the second patient, histopathological diagnosis was follicular variant of PTC in SO. Postoperatively, a pelvic CT revealed osteolytic lesion 6 cm in size, being a metastatic change. The patient underwent unilateral ovariectomy, total thyroidectomy and multiple cycles of radioiodine therapy. Currently, 9 years following the diagnosis, the patient achieved disease remission.

Conclusions. PTC in SO still remains a diagnostic and therapeutic challenge. Immunostaining for CK7, CK19, p63 and Tg might be helpful in histopathological diagnosis. The decision on the need of total thyroidectomy and radioiodine therapy should be made individually. However, thyroid remnant ablation increases the sensitivity and specificity of follow-up testing using serum Tg level as a tumour marker.