Sir, A 24-year-old man was admitted to our hospital with complaints of gradually increasing abdominal girth and a big, palpable lower abdominal mass. His past medical or surgical history was not significant, and laboratory data and serum tumor markers were normal. An abdominal US showed a large pelvic solid mass posterior to the bladder with a central area of cystic degeneration, bilateral hydronephrosis, and minimal ascites. A contrastenhanced CT scan of the abdomen revealed a bulky (largest diameter was 23 cm), lobulated, retrovesical softtissue mass. A central area of low hypoattenuation, a finding suggestive of cystic or necrotic degeneration, was also present within the mass. The degree of tumor enhancement with intravenous contrast was modest, and the pattern was rather homogeneous. No foci of calcification, bowel obstruction, or diffuse peritoneal thickening were identified. However, retroperitoneal, bilateral external iliac, and right inguinal lymphadenopathies were seen. Based on the CT findings, a presumptive diagnosis of a softtissue sarcoma or high-grade lymphoma was made. MRI of the pelvis (Fig. 1) showed a large, lobulated mass without an apparent organ of origin, displacing the bladder and rectum. The solid component of the mass was hypointense on T1weighted images and, characteristically, was also relatively hypointense on T2-weighted images. The degree of enhancement after the administration of gadolinium was similar to that seen on CT, although the pattern was slightly more heterogeneous. Given the position of the lesion and the absence of an organ-based primary site, a mesenteric or peritoneal origin was highly suspected. A CT-guided fine needle aspiration cytology of the solid component of the mass suggested the diagnosis of a small blue round cell tumor. A transrectal biopsy (Fig. 2) yielded the specific diagnosis of a desmoplastic small round cell tumor (DSRCT). DSRCT is an aggressive malignancy found predominantly in male adolescents and young adults, and was first recognized as a distinct pathologic entity in 1991 [1]. The peritoneal cavity (omentum, small bowel mesentery, sigmoid mesentery, paravesical region) is, by far, the most common site of DSRCT, although it can also occur in the retroperitoneum, pleura, and tunica vaginalis of the testis [2]. The imaging findings of this tumor usually show bulky peritoneal softtissue masses without an apparent organ-based primary site, typically within the omentum, within the mesentery, or adjacent to the bladder. A small amount of ascites as well as hepatic metastases are other common associated features [2, 3]. Histologically, DSRCT is characterized by the presence of well-defined nests of uniform, closely packed malignant L. Gorospe (*) . T. Gomez Department of Radiology, Santa Barbara Hospital, C/ Malagon s/n, 13500 Puertollano, Spain e-mail: luisgorospe@yahoo.com Tel.: +34-926-421100 Fax: +34-926-431668