A 20-year-old Korean man was transferred to our hospital for the evaluation and treatment of a pelvic mass. One month earlier, he had been admitted to an outside hospital for right lower quadrant pain and a fever of 3 weeks' duration. The clinical diagnosis was acute appendicitis, and an appendectomy was performed. A postoperative fever developed 2 days before admission to our hospital. By ultrasonography, a 9-cm lobulated heterogeneous low echogenic masslike lesion was detected in the pelvic cavity. His other medical and family histories were unremarkable.
On admission, a 9-cm cavitary mass with an irregular wall in the pelvic cavity was detected on a computed tomographic scan that was associated with the anterior deviation of the urinary bladder and the haziness of the adjacent pelvic soft tissue (arrows, Figure, A). By cystography, the urinary bladder was deviated to the left side with extrinsic compression by the mass. Under the clinical diagnosis of periappendiceal abscess, a resection of the mass was planned. On diagnosis of a malignant neoplasm on the pelvic mass from a frozen section, severely adherent organs were resected, including segments of the large and small intestines, soft tissues of the abdominal wall, and the urinary bladder as well as the pelvic mass.
The mass in the pelvic cavity was fragmented (20 × 20 × 8 cm in aggregate). Grossly, the mass was variegated with grayish white, pinkish tan, hemorrhagic, and necrotic cut surfaces. The mass was partly solid and partly cystic. The resected soft tissues of the abdominal walls (4 × 2.5 × 2 cm in aggregate) were involved by the tumor and showed similar gross findings. The wall of the large intestine was focally thickened (2.0 cm in maximum thickness) with irregular serosal surfaces, and the cut surface of the thickened wall was pinkish gray, hemorrhagic, and necrotic (Figure, B). The urinary bladder showed a diffusely thickened wall (1.8 cm in maximum thickness) with hemorrhagic serosa and mucosa. The cut surface of the thickened wall was involved by the tumor and showed a gross appearance similar to that of the large intestine. No perforation was observed in the large and small intestines or the urinary bladder. There was no gross or microscopic tumor in the pelvic or intestinal lymph nodes.
On examination by light microscopy, there were diffusely arranged round cells with alveolar patterns and rosette formations (Figure, C) involving the pelvic soft tissue, urinary bladder, large intestine, and abdominal wall. Extensive necrosis and high mitotic activity (more than 14 mitoses/1 high-power field) were seen.
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Pathologic Diagnosis: Diffuse Large B-Cell Lymphoma With Homer-Wright–Type Rosette Formation
Morphologically, the mass was suggestive of Ewing sarcoma/peripheral neuroectodermal tumor (PNET), and immunohistochemical (IHC) stains for desmin (1:200; Dako Corporation, Carpinteria, Calif), cytokeratin (1:100; Zymed, San Francisco, Calif), CD99 (1:50; Dako), and leukocyte common antigen (1:200; Dako) were performed to confirm the diagnosis of Ewing sarcoma/PNET and to exclude the possibility of other tumors. On IHC staining, the tumor cells were positive for leukocyte common antigen (Figure, C) and negative for the other immunomarkers, thus favoring the diagnosis of malignant lymphoma. For the determination of the lineage and the nature of the tumor cells, additional IHC stains, including CD79a (1:100; Dako) and CD3 (1:100; Dako), and an electron microscopic study of the rosette-forming area were performed, as well as polymerase chain reaction testing to determine the arrangement of the immunoglobulin heavy-chain (IgH) gene locus. This tumor was diagnosed as a diffuse large B-cell lymphoma, which was confirmed by CD79a positivity as well as by a B-cell IgH gene rearrangement that was detected by polymerase chain reaction. On electron microscopic examination, the rosettes showed a central aggregate of narrow cytoplasmic processes surrounded by neoplastic lymphocytes (Figure, D). In the review of the previous appendectomy slides, from which a diagnosis of acute appendicitis had been made at the outside hospital, lymphoma cells forming rosettes were seen in the attached periappendiceal adipose tissue. The appendix itself was free of tumors.
In the diagnosis of soft tissue tumors, as with other tumors, pathologists consider the age of the patient, the location of the tumor, and the growth characteristics of the tumor. Growth patterns of soft tissue tumors vary considerably, but some characteristic findings are associated with the types of tumors in which they are found most frequently. One of them, the Homer-Wright rosette,1 which forms a rosette with a central solid area of fibrillary material, is known as the prominent feature of Ewing sarcoma/PNET, neuroblastoma, neuroepithelioma, and malignant peripheral nerve sheath tumor.2 However, because rosette formation may be seen in other tumors, clinical information, IHC findings, and/or cytogenetic studies are required to accurately diagnose these tumors.
In the current case of a young man (20 years old), the clinical presentation of a pelvic soft tissue mass, in conjunction with histologic findings that included a prominent Homer-Wright rosette formation, was strongly suggestive of Ewing sarcoma/PNET. However, IHC staining and additional tests that used polymerase chain reaction for detecting the IgH gene arrangement showed that this tumor was a malignant lymphoma of B-cell lineage. Ultrastructurally, the rosettes were composed of a central aggregate of cytoplasmic processes formed from neoplastic lymphocytes. In the literature, 4 cases of lymphomas with Homer-Wright rosettes in the lymph node and bone marrow have previously been reported.3,4 However, to our knowledge, no case with extranodal lymphoma has been documented among them. In one of the 4 reported cases, the patient had histories of a right hemicolectomy for adenocarcinoma 16 years earlier and a biopsy-proven nodular lymphoma in the epigastric lymph node 3 years earlier.3 She underwent chemotherapy for a malignant lymphoma. During the follow-up, she developed a recurrent adenocarcinoma at the previous ileocolic anastomosis site, with metastases to the terminal ileum and hepatic flexure, and a recurrent malignant lymphoma in 1 right iliac lymph node and 2 left inguinal lymph nodes. Rosette-forming areas were detected in this recurrent lymphoma (follicular lymphoma) in all 3 of these lymph nodes. Two other cases showed small lymphocytic lymphomas of T-cell and B-cell lineages detected in the lymph nodes and bone marrow, respectively, and the last case had a large cell lymphoma of B-cell lineage in the lymph nodes.4 All 4 of the patients of these cases were elderly women (65–81 years with a mean age of 74.5 years) with rosette-forming lymphomas observed either in the lymph node or bone marrow. In the present case, however, there was no lymphoma involvement in the lymph nodes. To our knowledge, the current case is the fifth reported case of rosette-forming lymphoma that extensively involved the solid organs and soft tissues of the abdominal wall. Although nodal structure was not detected, this tumor most likely originated from the lymphoid tissue in the omentum adjacent to the appendix or in the pelvic cavity. Ultrastructurally, the rosette formation appeared to be an extension of cytoplasmic processes formed from neoplastic lymphocytes. However, the mechanism of the formation was not clear in our case or in the other reported cases, and more cases are required to study the mechanism of rosette formation in malignant lymphomas.
In conclusion, this case illustrates that a rosette structure, which is an important morphologic finding in the diagnosis of neuroblastoma and Ewing sarcoma/PNET, can be found in other neoplasms. Therefore, in round cell tumors with prominent rosettes, a malignant lymphoma should be included in the differential diagnosis.