Abstract: Background and purpose: Occult breast cancer is a difficult and hot spot in the field of breast cancer because of its occult onset, diagnostic difficulties, and unclear therapeutic strategies. In this study, we discussed the clinical pathological characteristics and the diagnosis and treatment strategy of occult breast cancer. Methods: Fiftysix cases of occult breast cancer received the preoperative examinations of ultrasonography, mammography, magnetic resonance imaging (MRI) and PET/CT. The different detection rates and pathological coincidence rates in suspicious primary lesions of occult breast cancer were compared. Treatment options included the neoadjuvant chemotherapy, modified radical mastectomy, breast-conserving surgery with axillary lymph node dissection and axillary lymph node dissection. All patients received postoperative chemotherapy with radiotherapy. Results: The detection rates of ultrasonography, mammography, MRI and PET/CT for the suspicious breast primary tumors were 7.14% (4/56), 29.41% (15/51), 37.50% (18/48) and 16.28% (7/43). Combined with the postoperative pathological examination, we found that the pathological coincidence rates were 66.67%, 50.00%, 50.00% and 50.00%. Twenty-six cases examined with ultrasonography, mammography and MRI did not have suspicious primary tumors, 21 patients underwent breast pathology examination with a positive rate of 14.29% (3/21). Thirty-nine patients received pathology examination, and the breast primary tumors were detected in 15 patients. The detection rate by pathology examination was 38.46%. According to the St.Gallen molecular classification standards, the ratio of Luminal A, Luminal B, HER-2 positive and triple-negative breast cancer was 7.14%, 46.43%, 12.50% and 33.93%, respectively. There were 52 cases with postoperative follow-up, the median follow-up period was 35 months (10-104 months), recurrence or metastasis occurred in 4 cases, and there was no death. Among the 15 patients with primary tumors, 2 cases eventually had recurrence or metastasis; 24 patients without the primary tumors did not have recurrence or metastasis; 17 patients received neoadjuvant chemotherapy, of whom 2 patients achieved pathologic complete response (PCR) and 2 patients had recurrence or metastasis. Modified radical mastectomy was performed on 39 cases, and recurrence or metastasis occurred in 2 cases. Breast-conserving surgery with axillary lymph node dissection was performed on 8 cases, and recurrence or metastasis occurred in 2 cases. Axillary lymph node dissection with radiotherapy was performed on 9 cases, and there was no recurrence or metastasis. Conclusion: MRI examination has important value in excluding the occult breast cancer. Breast pathology of primary tumors has lower detection rate in the patients without suspicious primary tumors on ultrasonography, mammography and MRI. The treatment options for occult breast cancer include the neoadjuvant chemotherapy, modified radical mastectomy, breast-conserving surgery with axillary lymph node dissection and axillary lymph node dissection. Patients who did not have primary tumors examined by breast pathology had a lower recurrence or metastasis rate than patients with the primary tumors.

Key words: Occult breast cancer, Axillary lymph node metastasis, Magnetic resonance imaging, Neoadjuvant chemotherapy