A collision tumor of buccal squamous cell carcinoma and diffuse large B-cell lymphoma: a case report and literature review.

BACKGROUND: Collision tumors are rare entities in which two histologically distinct malignancies coexist in close proximity without histological admixture or transition. The coexistence of buccal squamous cell carcinoma (SCC) and diffuse large B-cell lymphoma (DLBCL) in the oral region is exceptionally uncommon. Because the clinical and radiological findings may overlap, such cases can pose substantial diagnostic and therapeutic challenges. CASE PRESENTATION: We report a case of a 66-year-old man with a long history of smoking and betel quid chewing who presented with a rapidly enlarging mass in the left buccal region. Biopsy at a local hospital showed moderately to well-differentiated SCC. However, subsequent imaging revealed widespread, discontinuously distributed hypermetabolic lesions involving the thyroid gland, cervical and mediastinal lymph nodes, and terminal ileum, a pattern inconsistent with the usual lymphatic spread of buccal SCC. Repeated minimally invasive biopsies of cervical and thyroid lesions failed to establish a definitive diagnosis of lymphoma, partly because of prominent reactive lymphoid infiltration in the setting of Hashimoto's thyroiditis. After multidisciplinary discussion, radical resection of the locally advanced buccal tumor was prioritized because of the risk of local progression and loss of resectability. Final histopathological examination of the surgical specimen demonstrated a collision tumor composed of independent SCC and DLBCL components. Immunohistochemistry and fluorescence in situ hybridization identified the lymphoma as non-germinal center B-cell-like DLBCL with isolated MYC rearrangement. The patient subsequently received Pola-R-CHP immunochemotherapy and achieved a favorable early response. CONCLUSION: This case highlights the need to consider a second primary malignancy when the distribution of lesions does not follow the expected metastatic pathway of head and neck SCC. It also illustrates the limitations of needle biopsy in lymphoid lesions arising in a background of chronic inflammation. Adequate tissue sampling, careful pathological review, PET-CT-based systemic assessment, and multidisciplinary decision-making are essential for accurate diagnosis and individualized treatment of complex collision tumors.