Pediatric mediastinal tumor unveiled as T-cell prolymphocytic leukemia: diagnostic pitfalls-a case report

T-cell prolymphocytic leukemia (T-PLL) is a rare and highly aggressive mature T-cell malignancy that predominantly affects older adults. Its occurrence in childhood is exceptionally uncommon and may mimic other neoplasms, such as lymphoma or thymoma, particularly when associated with mediastinal masses or generalized lymphadenopathy.

A 13-year-old Turkish girl presented with severe respiratory distress. Thoracic imaging demonstrated a large anterior mediastinal mass. In the prevascular mediastinal compartment, the most common tumors include thymoma, germ cell neoplasms, and lymphoma. The initial needle biopsy was of limited diagnostic value due to crush artifacts and demonstrated a T-cell-predominant infiltrate with focal keratin positivity, initially suggestive of a thymic neoplasm. Based on this preliminary interpretation, empiric chemotherapy for presumed advanced thymoma was initiated. However, detailed systemic radiological assessment revealed that the mass was not confined to the mediastinum but also accompanied by widespread lymphadenopathy. Subsequent systemic radiological evaluation revealed widespread lymphadenopathy beyond the mediastinum, prompting an incisional lymph node biopsy. Immunohistochemical analysis revealed a neoplasm lacking an epithelial component (negative for P63, pancytokeratin, and cytokeratin) and composed entirely of lymphoid cells. The tumor cells showed diffuse positivity for CD45, CD3, CD5, CD8, and BCL-2, with a high Ki-67 proliferation index, and were negative for immaturity markers (CD34, TdT, CD33). Taken together, these findings supported a mature T-cell neoplasm most consistent with T-PLL, although the absence of molecular confirmation limited definitive diagnostic certainty. Bone marrow evaluation was non-diagnostic.

This case illustrates the diagnostic challenges of pediatric T-PLL and demonstrates that a multidisciplinary correlation of clinical, histological, immunophenotypic, and molecular features-together with repeated biopsy and comprehensive immunophenotyping-can be decisive for timely and accurate diagnosis.