Case report: Concurrent primary thyroid MALT lymphoma and lymph node metastatic thyroid micropapillary carcinoma in Hashimoto's thyroiditis: a diagnostic and therapeutic challenge.

Background: Primary thyroid lymphoma (PTL) is an extremely rare malignancy, and its prognosis primarily depends on factors such as histological type and clinical stage. Papillary thyroid carcinoma(PTC) is the most common pathological type of thyroid cancer. Papillary thyroid microcarcinoma(PTMC) accounts for a significant proportion of PTCs and generally has a favorable prognosis;however, central lymph node metastasis (CLNM) occurs in approximately 10-30% of cases. Thecoexistence of PTL and metastatic PTMC is exceedingly rare, with only a few reports in the literature. Hashimoto thyroiditis (HT) is a common risk factor for both PTL and PTC. Methods: We present a case of HT diagnosed concurrently with PTL and PTMC, accompanied by level VI CLNM. We systematically reviewed the patient's clinical presentation, imaging findings, laboratory results, histopathological features, treatment, and follow-up. Results: A 40-year-old woman presented with neck discomfort. Ultrasonography revealed a solid hypoechoic nodule in the right thyroid lobe with irregular and slightly lobulated margins. The patientdeclined fine-needle aspiration cytology and opted for direct surgery. The patient underwent right thyroid lobectomy with isthmusectomy and biopsy of the right central compartment (level VI) lymph nodes. Postoperative pathology confirmed the coexistence of extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma), with pathological features suggestive of large B-cell transformation, and PTMC, with carcinoma identified in one of ten resected level VI lymph nodes (1/10). Immunohistochemistry showed that the lymphoma cells were positive for CD20, CD79a, Pax-5, and Bcl-2 and negative for CD5, CD10, and Cyclin D1. The patient received four cycles (six infusions) of a single-agent, obinutuzumab, as post-surgical therapy. The patient has now completed the full treatment course, remains in a stable condition, and shows no evidence of recurrence. Conclusion: The coexistence of PTL and PTMC is exceptionally rare and poses a significant risk of misdiagnosis. Comprehensive preoperative evaluation and precise histopathological examination are crucial for accurate diagnosis. Although PTMC generally has an excellent prognosis, central lymph node metastasis can occur in a subset of patients. Careful evaluation of suspicious cervical lymph nodes and individualized postoperative surveillance are therefore warranted. Therapeutic decision-making requires a balanced consideration of lymphoma stage and thyroid carcinoma risk stratification, underscoring the importance of a multidisciplinary approach.