Adult-Onset Pediatric-Type Follicular Lymphoma of the Parotid Gland.

Pediatric-type follicular lymphoma (PTFL) is an extremely rare B-cell lymphoma that primarily affects children and young adults, typically in individuals under 25 years old, with a median age of 15 years. Here, we report a rare case of PTFL in a 27-year-old adult male who presented with a slow-growing mass near his left ear. Initial CT scans of the neck revealed two oval-shaped, smooth, well-defined, homogeneously enhancing soft tissue density lesions in the superficial lobe of the left parotid gland. A complete surgical excision was performed, and a postoperative ultrasound of the neck confirmed complete removal, with only hypoechoic changes observed in the parotid gland. Comprehensive immunohistochemical analysis showed CD20-positive B cells co-expressing germinal center markers CD10 and BCL6 but negative for BCL2 and cyclin D1. The Ki-67 proliferation index was notably elevated, indicating high cellular activity. Additional findings included MEF2B positivity, confirming the lymphoma diagnosis, and an immunoglobulin gene rearrangement, which demonstrated a monoclonal B-cell population consistent with a neoplastic process. CD21 staining further revealed distorted follicular dendritic cell networks and attenuated IgD-positive mantle. Fluorescence in situ hybridization (FISH) analysis showed no rearrangements in BCL2, IRF4, or BCL6, and any deletion in the 1p36 region (TNFRSF14 gene), ruling out other lymphoma types. Histologically, the nodules showed distorted secondary follicles with obscured germinal centers, confirming PTFL. At the three-month follow-up, the patient demonstrated satisfactory healing with no signs of recurrence. This case underscores the importance for oncologists to perform a thorough differential diagnosis of head and neck masses, as PTFL can present with characteristics similar to classical follicular lymphoma, IRF4-rearranged large B-cell lymphoma, pediatric nodal marginal zone lymphoma, and reactive follicular hyperplasia. A comprehensive diagnostic approach, including clinical, pathological, and immunohistochemical analyses, is essential for developing an accurate diagnosis and management plan for PTFL, especially in atypical adult presentations.