Background and purpose: Malignant transformation of mature cystic teratoma (MCT) of the ovary is a rare event

and its diagnosis is rarely made preoperatively. Therefore

in many cases

frozen section (FS) assessment of ovarian tumors is used to provide intraoperative diagnosis and guide further surgical management. The present study aimed to find the causes of inconsistency between intraoperative frozen section diagnosis and final diagnosis. Methods: We performed a retrospective review of patients who were treated for MCT of the ovary from 2008 to 2017 at West China Second University Hospital. The analyses of clinicopathologic features of the patients with malignant transformation were made with an emphasis on the factors which were associated with misdiagnosis during FS evaluation

and the related literature was reviewed. Results: Intraoperative consultation was performed on 5 790 of 9 213 MCTs

of which 18 cases were malignant on final diagnosis. The final diagnoses of those cases included 8 cases of squamous cell carcinoma

6 cases of adenocarcinoma

1 case of adenosquamous carcinoma

1 case of malignant fibrohistiocytoma

1 case of malignant melanoma

and 1 case of carcinoid. Fourteen of 18 cases exhibited full concordance between FS and final diagnosis in terms of discriminating benign from malignant lesions. The malignancy arising from MCT of the ovary were confirmed in 4 cases. Conclusion: Slides review of discrepant cases revealed that major pathological causes of under-diagnosis were sampling errors. Although preoperative diagnosis is difficult

radiologic imaging and elevated tumor markers in addition to essential risk factors (patient’s age and tumor size) should guide the necessary FS assessment to make a definitive diagnosis.