Applied Radiology

Diagnosis

Findings

Contrast-enhanced CT scan of the abdomen revealed a diffuse process evidenced by multiple soft-tissue masses and minimal ascites. Heterogeneously enhancing masses are demonstrated within the porta hepatis and right hepatic lobe (figure 1). Large mesenteric and peritoneal-based masses are present within the mid-abdomen (figure 2). A large (7-cm) enhancing mass was seen in the retrovesicle space, displacing the bladder and rectum (figure 3).

Discussion

Intra-abdominal desmoplastic small round cell tumor is a rare malignancy with a predilection for adolescent and young adult males. First described in 1989, there are fewer than 50 cases reported in the literature. Intra-abdominal desmoplastic small round cell tumor is a unique malignancy, which differs from other small round cell tumors of childhood by its histologic and immunocytochemical features.

Desmoplastic small round cell tumor is an undifferentiated malignancy associated with serosal surfaces, especially the peritoneum. Patients typically present with abdominal pain and a palpable mass or abdominal distension. The radiographic appearance is one of a diffuse peritoneal carcinomatosis often without obvious organ involvement. The hallmark imaging feature reveals large, lobulated peritoneal and mesenteric masses affecting the omentum and serosal surfaces. Acites is often scant and, although metastasis to organs such as the liver and kidney may occur, the solid abdominal organs are typically spared. Large retroperitoneal masses and enlarged lymph nodes may be seen throughout the peritoneal cavity. Involvement of the pelvic serosal reflections is also typical, with masses often seen displacing structures in the paravesical region. Common associated findings include: serosal hepatic metastasis, nodular peritoneal thickening, punctate peritoneal calcifications, and resultant hydronephrosis and bowel obstruction.¹

On CT, masses may show variable enhancement due to the degree of vascularity and necrosis. Heterogeneity due to tumor hemorrhage may also be a feature of some lesions. As with CT, the MRI appearance of DSRCT is nonspecific. Relatively low signal intensity on T2-weighted imaging, due to the densely packed cellularity of small cell neoplasms in addition to a variable desmoplastic response within lesions, may suggest the diagnosis of a small cell histology based on imaging. On ultrasound, lesions are usually well defined and hypo- or anechoic mimicking lymphoma, again as a result of the underlying small cell tumor histology.

Desmoplastic small round cell tumor is characterized by discrete nests of uniform, closely packed malignant cells, with hyperchromatic nuclei, scanty cytoplasm, and patchy epithelial differentiation, which are distributed in a background of focally desmoplastic stroma. Tissue biopsy is often suggestive of the diagnosis; however, confirmation is required using immunohistochemistry or electromicroscopy. These tumors exhibit a typical immunophenotype, namely multidirectional expression of epithelial, neural, and muscular markers, differentiating them from other round blue cell tumors.

The differential diagnosis of DSRCT is somewhat dependent upon age of presentation. It is characteristically a disease of adolescent males; however, patients may present up to the 6th decade. In infants and adolescents, diagnostic considerations include other small round cell tumors such as rhabdomyosarcoma, neuroblastoma, and mesenteric carcinoid. Reports exist of DSRCT misdiagnosed as intra-abdominal rhabdomyosarcoma and carcinoid, even after review of initial biopsy specimens.^2,3 The nonendemic form of Burkitt's lymphoma is also very similar in appearance and presentation, occurring in the peritoneal cavity and mesentery with a predilection for the ileocecal region. Metastatic Wilms' tumor, hepatoblastoma, and pancreatoblastoma are differential considerations, but should be differentiated given the preponderance for primary solid organ involvement in these conditions.

In older patients presenting with this disease, the diagnosis becomes even more difficult. Primary differential considerations such as lymphoma, peritoneal mesothelioma, and peritoneal carcinomatosis from an intra- or extra-abdominal primary neoplasm should all be entertained. In immunocompromised patients, intra-abdominal infections such as tuberculosis, mycobacterium avium complex, and even malignancies such as Kaposi's sarcoma may present with similar diffuse peritoneal disease. Almost uniformly, the diagnosis of DSRCT will come after extensive histologic and immunohistochemical evaluation.

Desmoplastic small round cell tumor is a highly aggressive neoplasm with a propensity for widespread peritoneal and, occasionally, extra-peritoneal spread. This aggressive growth, relative insensitivity to chemotherapy, and general incomplete resectability result in an almost uniformly fatal prognosis. Although some response to chemotherapy may be possible,⁴ complete surgical resection is rare, and efforts are generally palliative.

CONCLUSION

Desmoplastic small round cell tumor of the peritoneum is a rare, aggressive small-cell neoplasm typically presenting with multiple large serosal and mesenteric-based soft-tissue masses. Although imaging of this tumor is somewhat nonspecific, this rare entity should be considered in the differential diagnosis of a young male patient presenting with features of widespread peritoneal malignant disease without obvious solid organ involvement.

1. Pickhardt PJ, Fisher AJ, Balfe DM. Desmoplastic small round cell tumor of the abdomen: Radiologic-histologic correlation. Radiology. 1999;210:633-638.

2. Frappaz D, Bouffet E, Dolbeau D, et al. Desmoplastic small round cell tumors of the abdomen. Cancer. 1994;73:1753-1756.

3. Liang WY, Chen WY, Tsay SH, Chiang H. Desmoplastic small round cell tumor: Report of 3 cases and review of the literature. Kaohsiung J Med Sci. 2000;16:261-265.

4. De Lena M, Caruso ML, Marzullo F, et al. Complete response to chemotherapy in intra-abdominal desmoplastic small round cell tumor: A case report. Tumori. 1998;84:412-416.

Prepared by Brian J. Fortman, MD, Department of Radiology, The Johns Hopkins Hospital, Baltimore, MD, and Douglas P. Beall, MD, Department of Radiology, The Mayo Clinic, Rochester, MN.