Case of the Week # 526

Case of the Week # 526

Javier Cortejoso
 on 10/31/2022
 on Monday, October 31st, 2022
I would like to make some considerations about the differential diagnosis of intra-abdominal masses found in routine ultrasounds performed in the third trimester. The images show a huge fundamentally solid mass with some cystic areas and tiny calcifications that occupies the lower abdomen, behind the bladder and indenting its posterior wall. Coronal sections show that the renal pelvis and calyces of both kidneys are dilated (moderate-severe urinary tract dilation, A2-A3). The upper kidney outline is clearly visible. In the video, the lower pole of the of the kidney located in the lower part is not clearly identified, perhaps due to the shadow created by the spine. Color Doppler ultrasound shows prominent blood flow in the mass, in addition to the two umbilical arteries surrounding the bladder and the tumor.

While ultrasound can be used to identify fetal tumors, these tumors are rare. The fetal abdomen is one of the most common sites for fetal tumors, besides the heart or the face and neck regions. The diagnosis of a fetal intra-abdominal mass presents a diagnostic and management dilemma because of the variety of possible differential diagnoses. In clinical practice, many tumors are not seen until relatively late during pregnancy and often as incidental findings in the late second or third trimester. Potential differential diagnoses are numerous, but the differential diagnosis typically can be restricted through a detailed assessment of the site and appearance of the lesion.

Most of the retroperitoneal masses detected antenatally are of renal or adrenal origin. Possible diagnosis of a fetal retroperitoneal mass includes neuroblastoma, adrenal hemorrhage, neurogenic cyst, retropleural pulmonary sequestration, mesoblastic nephroma, Wilms’ tumor, enteric duplication cyst, and teratoma.

Teratoma is the most common congenital neoplasm, with an incidence of 1 in 20,000 to 1 in 40,000 live births. It is an embryonal tumor derived from the primordial germ cells that may contain components arising from all three germ layers. Most congenital teratomas are histologically benign and typically located in the midline. Its most frequent location is the sacrococcygeal region (45% of cases), although they can also be located in head, neck, mediastinum, retroperitoneum, and the ovaries, among other locations.

The American Pediatric Surgery Association classifies sacrococcygeal teratomas into four categories depending on the extent of the tumor in relationship to the pelvis and abdomen (Altman, 1973): type I lesions are almost completely external; type II are predominantly external masses with pelvic extension; type III are visible externally but predominantly located within the pelvis and abdomen; and type IV are exclusively intra-abdominal lesions. In 80% of cases, the tumor is seen as an external lesion protruding from the perineal or sacral region. There have been few reports of prenatally detected type IV sacrococcygeal teratomas, and they appear on prenatal ultrasounds as either completely cystic intra-abdominal masses or mixed solid and cystic masses.

On fetal ultrasound, teratomas are large, heterogeneous masses due to their different tissue components. Mature teratomas are predominantly cystic lesions containing a mixture of fat, hair, debris and calcifications, while immature teratomas may be predominantly solid and containing areas of heterogeneously enhancing soft tissue. Another morphological classification uses ultrasound findings to distinguish three types of sacrococcygeal teratomas: predominantly cystic tumor (type A), cystic-solid tumors (type B), and solid tumor with fetal hydrops (type C). The prognosis progressively worsens from type A to C. The few cases of sacrococcygeal teratomas described in the literature that cause urinary retention are often fundamentally cystic.

Neuroblastoma is the second most common congenital tumor and the first among malignant ones. The tumor arises from primordial neural crest cells anywhere along the sympathetic chains, at the adrenal medulla (more than 90% of cases), and at the extra-adrenal retroperitoneum or the posterior mediastinum. The ultrasound findings are variable, ranging from purely cystic to homogeneously solid tumor, though the cystic pattern is more frequent in the fetal stage. Calcifications are present in 30% of all cases. Color Doppler ultrasonography can reveal prominent blood flow in the mass.

Fetal kidney tumors are rare; congenital mesoblastic nephroma and occasional cases of Wilm’s tumor have been reported. On prenatal sonography, congenital mesoblastic nephroma usually presents as a paravertebral unilateral solid homogeneous mass deforming the kidney, often with some small echo-free areas which represent hemorrhage and necrotic tissue. Mesoblastic nephromas are well circumscribed and tend to be large, so that the pyelic cavities are more or less dilated and modified in their topography. Doppler demonstrates diffuse vascularization with elevated diastolic frequencies and decreased pulsatility indices, suggesting reduced vascular resistance within the tumor. On imaging, the mesoblastic nephroma is indistinguishable from a Wilms’ tumor or nephroblastoma. One of the signs often leading to discovery of renal tumors is the presence of polyhydramnios.

Hemangiomas are ubiquitous vascular neoplasms that may be found in virtually every human organ. The most frequent location is the head and neck (60%). Although they may occur anywhere within the abdomen, including the solid organs, hollow viscera, ligaments, and abdominal wall, the liver is the most common site. On ultrasound, they appear as a well-defined homogenously echogenic mass (although they can be mixed cystic and solid lesions), with occasional calcifications. Color Doppler shows increased flow with decreased arterial resistance and increased venous drainage.

Within the connective tissue tumors are benign entities, such as fibromatosis, myofibromatosis, inflammatory myofibroblastic tumor, digital fibroma and fibrous hamartoma, as well as malignant entities such as fibrosarcoma and rhabdomyosarcoma. None of these tumors presents specific imaging characteristics, and usually present as soft tissue masses of variable echogenicity on ultrasound.

Rhabdomyosarcoma is the most common soft-tissue tumor in children, but very rare in fetuses. The most common locations are the head and neck (41%), the genitourinary tract and the abdominal cavity (34%), and the trunk or extremities (25%). Prenatal ultrasound may reveal a large echogenic soft-tissue mass replacing the normal tissues.

In this case there are several findings that are not usual in sacrocoxygeal teratomas. I have already commented that the majority are tumors that protrude from the perineal or sacral region, therefore externally visible. In the survey conducted by the Surgical Section of the American Academy of Pediatrics among its members, only 39 patients out of 398 had a totally intra-abdominal tumor (type IV) (Altman, 1974). On the other hand, pathologically and radiologically, teratomas are either both cystic and solid, predominantly cystic, or rarely solid (as in this case). The cystic areas may be small and distributed throughout the tumor, or they may be larger, isolated areas with or without septations. And it is also unusual for a predominantly solid teratoma to produce urinary tract obstruction, described in cystic tumors. However, the heterogeneous appearance with tiny calcifications, the vascularization shown with color Doppler and the location allow the diagnosis of an immature sacrococcygeal teratoma type IV, type C.

Suggested readings

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