Imaging the Child with an Abdominal MassU.V.Willi

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General Considerations

The suspicion or diagnosis, even if it is nonspecific, of an abdominal mass in a child raises concern and anxiety. The pediatric radiologist may be the first person either to sus- pect or to diagnose a mass, quite commonly by ultrasonog- raphy (US). Therefore, adequate psychological interaction with the patient and the parent(s) is part of his or her pro- fessional competence. The findings should generally not be discussed while the examination is underway. Specific di- agnostic statements should be avoided, even by the experi- enced examiner. Caution should be exercised before giving preliminary histological and prognostic hypotheses. The in- volved clinician, possibly the oncologist, should be in- formed promptly about the morphological findings and the information exchanged between the examiner and the pa- tient and/or accompanying parent(s). This regards the situ- ation of a suspicious mass that has been demonstrated and where a malignant process might exist or be likely. If the mass turns out to be a hydronephrosis or a similar obvious- ly benign process, such concerns are less relevant.

Abdominal masses in children are often cystic and al- most always benign, especially in the younger child (i.e., newborn and infants). In a majority of these cases, the mass originates from the urinary tract, typically due to some obstructive malformation and/or dysfunction (Fig.

1). Beside neoplasia and malformation, the underlying causes of such masses may be pathological conditions such as inflammation (usually due to infection), trauma (including iatrogenesis) or metabolic or other systemic diseases, including malignancy.

Proper imaging technique includes the correct choice of the imaging tool(s) and, if there is a need for more than one modality, their use in the appropriate order. This will short- en the time to diagnosis, and avoid uncertainty and unnec- essary discussion, as well as help to limit costs. The role of the pediatric radiologist, therefore, is not only to make the diagnosis, but he or she also has a consulting duty toward the clinician who might think that, for example, all questions may be answered using magnetic resonance imaging (MRI).

It is crucial that the examiner be aware of the strengths and limits of the method(s) used and those available for use.

Imaging the Child with an Abdominal Mass

U.V. Willi

Department of Radiology, University of Zurich Children’s Hospital, Zurich, Switzerland

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Fig. 1. Bilateral pyeloureteric junction obstruction in a 6- week-old boy. a Plain radi- ograph of abdomen shows large mass on the left displacing descending colon. Mass was clinically impressive. b Film three hours post-intravenous contrast injection shows excessive dilatation of left renal pelvis and calyces with relative high contrast in- tensity; moderate right hydronephrosis. Scintigraphic estimation of renal function was 69% on the left, 31% on the right. Bilateral hydronephrosis had been discovered by fetal ultrasonography

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Clinical Aspects

When evaluating a child’s abdominal mass, the patient’s age, sex, history and clinical findings are the most im- portant parameters to be included in the process of es- tablishing a diagnosis. This renders pediatric radiology a distinct clinical specialty. Two different processes may look morphologically similar, yet have completely dif- ferent pathogenesis and significance. With the neces- sary clinical information and professional experience, one should be able to make a reasonable or even distinct diagnosis in a given case (Fig. 2, 3). On the other hand, the abdominal mass may be an incidental observation by the child or the parent and there may be no additional symptoms. A child with nephroblastoma, unless trau- matized, commonly presents with a large abdominal mass, without pain and apparently in good health.

Parents may report that the mass appeared quite sud- denly. Such a remark is not unusual, since nephroblas- toma is known for its rapid growth. Neuroblastoma, on the other hand, may lead to symptoms such as pallor, fa- tigue and failure to thrive. Morphologically, each of these two retroperitoneal tumors has specific character- istics that render the diagnosis in the majority of in- stances. However, each may present a differential diag- nostic challenge (Fig. 4, 5).

An inflammatory mass is commonly caused by an in- fectious process, such as appendicitis or infection of the urogenital tract. Therefore, the signs and symp- toms, including laboratory data, are those of an infec- tious disease and are often associated with abdominal pain. Local or diffuse pain, although quite nonspecific,

a b c

Fig. 2. Cystic teratoma in a 3-week-old girl with urinary retention. a Plain film of abdomen shows huge midline mass reaching from pelvic floor to displaced stomach. Midsagittal ultrasonographic scans through anterior lower abdomen show two partially superimposed cystic structures (b before, c after catheterization of bladder). Catherization helps in distinguishing antero-superior bladder with changing volume from infero-posterior cystic teratoma. Tiny linear structure in cranial aspect of teratoma is a hint for diagnosis

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Fig. 3. Hematocolpos in a 15-year-old adolescent with acute urinary retention. Midsagittal ultrasonography through anterior lower abdomen shows two partially superimposed cystic structures similar to Fig. 2 (a before and b after spontaneous micturition). With a less filled bladder, numerous tiny echoes due to menstruation and uter- ine cervix are noticeable

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a b

d e

Fig. 5. Neuro- blastoma in a 6-week-old boy causing ipsilat- eral urinary obstruction. Trans- verse ultrasonography with the child prone (a) and coronal scan (b) demonstrate solid echogenic mass next to right kidney; obstructive hydronephrosis from proximal extrinsic ureteral compression due to tumor.

Renal artery is engulfed by mass (a); arterial Doppler signal not illus- trated. Intravenously contrast-enhanced CT shows correlating fea- tures on three consecu- tive scans (c, d, e), in- cluding displacement and compression of in- ferior vena cava b

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Fig. 4. Nephroblastoma in a 2-year-old boy with painless left abdominal mass. Left coronal ultrasonographic scan shows apparently solid tumor mass in left flank suspect- ed to be a Wilms’ tumor (a). Intravenously en- hanced CT scans through mid part of well defined tumor (b, c) shows its dor- sally exophytic growth; ir- regular contrast enhancement suggests partial tumor necrosis

c

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is the most frequent symptom in abdominal disease, whether or not there is an appreciable anatomical cor- relate. Pain may also be a sequela of a noninflammato- ry mass that has been traumatized. Beside pain, func- tional abnormality of an organ system, fever, loss of weight and/or appetite, systemic disease, and even se- vere illness are all nonspecific in conjunction with an abdominal mass and do not, per se, indicate malignan- cy. Some may, however, be characteristic of a specific disease process such as lymphoma, leukemia or

metastatic disease.

Urinary retention may be a serious symptom. Therefore, a pre- or rather, retrorectal mass causing the symptom must be excluded or differentiated (Fig. 1, 2). On the oth- er hand, clinical uncertainty means that ‘urinary reten- tion’ is a notoriously claimed symptom leading to imag- ing procedures recommended because of inadequate evaluation of the patient’s history (e.g., no fluid intake for many hours with a subsequent lack of urine production and, thus, an empty bladder).

Approach to an Abdominal Mass

In attempting to diagnose and characterize an abdomi- nal mass, the anatomical site of origin should be sought.

Abdominal compartmentalization (as shown in Table 1) may help to localize a mass in one patient, but may be theoretical in another. Due to its large size, a mass may present as being intra-abdominal even though it may have originated in the retroperitoneum. It might be dif- ficult to decide whether a mass is located in the ab- domen or in the chest or whether it involves both com- partments (such as in cases of neuroblastoma). Nephro- blastoma and neuroblastoma are ‘typical’ retroperi- toneal masses of childhood, and their renal or extrarenal origin may be readily recognizable, although an ‘unusu- al case’ may unpredictably present. As for a pelvic mass, it is useful to differentiate whether its origin is prerec- tal, rectal (i.e., stool) or retrorectal. A hepatic mass is usually recognizable as such and can mostly be differ- entiated from an anterior abdominal mass of another origin. The final imaging diagnosis or differential diag- nosis will depend on the macroscopic tissue characteri- zation of a mass (i.e., solid, cystic, mixed), its architec- ture and definition, whether it is solitary or multiple (possibly metastatic) and involving one specific or more than one organ system.

Imaging an Abdominal Mass

US is probably the most popular imaging method in chil- dren. Its use is becoming more widespread, also among pediatricians who may not place it in any further imaging context. However, a plain radiography of the child’s ab-

domen in recumbent position is strongly recommended

when there is suspicion of an abdominal mass and in view of further exploration of the abdomen by imaging means. An initial appreciation of the mass process with regard to its location, size, density (e.g., calcifications) and effect upon adjacent structures (including the skele- ton) and its possible interference with the gastrointestinal and/or urinary tracts may be useful to guide the course of imaging evaluation. With this preliminary information, subsequent US is often more straightforward and reward- ing, taking into account also the child’s history, clinical findings and laboratory results. The finding from the ini- tial abdominal radiography (or US) may prompt an addi- tional radiography of the chest that may, in a positive or negative way, contribute to the diagnostic work-up.

US is the most versatile and effective method for tak- ing a first thorough look at the child’s abdominal content.

It is generally important to use US to initially explore the child’s abdomen systematically and carefully. In an ab- dominal mass, US is often sufficient to establish a rea- sonable diagnostic assumption based on the anatomical facts. Often, even a specific diagnosis becomes probable by US, so that a given hypothesis about a solid lesion then may need only biopsy for confirmation and possible initiation of treatment. For a nonspecific solid mass found by US, biopsy is the obvious next step (Fig. 6). The strength of US lies in its ability to allow the appreciation of anatomical structures; its limits are in the estimation of organ function, even if there are certain exceptions. On the other hand, Doppler US is used routinely for demon- strating the presence or absence of arterial and/or venous blood flow. It also helps, among the definition of numer- ous other anatomical structures, to distinguish the hepat- ic segments. US is the only imaging tool that can provide diagnostic information by making use of physiological organ movement from respiration. Free movement of the liver over a renal or suprarenal mass (due to nephroblas- toma or neuroblastoma) indicates that the peritoneum is likely not to be involved by the retroperitoneal mass.

Similarly, demonstration of some local change of rela- tionship between an adrenal neuroblastoma and the ipsi- lateral kidney indicates likely anatomical separation be- tween the two structures. In the follow-up evaluation of an abdominal mass by US, other criteria become useful parameters, such as persistence or change of size, shape, and echogenicity of a given mass. In the morphological- ly ambiguous situation of a hematoma, its echo charac- teristics typically change within days, while a solid tu- mor’s do not, unless it has been traumatized.

The improved availability of ‘high end’ imaging equipment such as MRI and multislice computed to-

Table 1. Differential site of origin of an abdominal mass

Peritoneum versus retroperitoneum Abdomen versus thorax versus both Kidney versus extrarenal location Pelvis versus upper anterior abdomen Liver versus extrahepatic location

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mography (MSCT), leads to their more frequent use for evaluating abdominal masses in children. Given the pro- fessional effort to protect children from unnecessary ra- diation and following the ‘ALARA’ concept (as low as reasonably achievable), there is a tendency to make use of diagnostic MRI. On the other hand, MSCT allows fast and efficient imaging, which is quite attractive for use in children, as it renders complicated anesthesia unneces- sary. In neuroblastoma, especially of the thoracic or tho- raco-abdominal type that has a tendency to involve the spinal canal, if available, MRI is the modality of choice.

One has to be aware of the fact that in spinal involve- ment by neuroblastoma, neurological abnormality or dysfunction is often rather mild or even absent. MRI is

recommended in a solid or cystic pelvic mass, especial- ly if it is due to some gynecologic malformation (e.g., hydro- or hemato-metrocolpos) or neoplasia. The need for immobility means that the pelvis and retroperi- toneum are better suited for evaluation by MRI than the anterior abdomen, which is more exposed to physiologi- cal motion from cardiac and respiratory dynamics.

While the imaging technique is considerably improved through the use of cardiac and respiratory gating and rapid sequences, there are often difficulties in aiming for high diagnostic precision, especially in infants and young children. MRI is, nevertheless, recommended in tumorous abnormalities of the liver and biliary tract for morphological analysis of the mass, as well as the in- volved canalicular hepatic structures.

MSCT and CT are excellent at defining an abdominal mass, either benign or malignant. CT characterizes the mass by measuring its density, it localizes the mass and demonstrates its extension, both with high precision. In most instances it is possible to locate the organ or site of origin of a mass. In a malignancy, tumor spread may be accurately recognized, as well as enlarged or unenlarged lymph nodes, as possible metastatic manifestations. The degree of uptake of systemically introduced contrast ma- terial by a solid mass allows estimation of its perfusion (i.e., vitality) and, conversely, the lack of perfusion may point toward necrosis of the mass. The same applies to MRI. Perfusion criteria may be valuable when evaluating a potentially metastatic lesion using either method.

MSCT and CT are most accurate and practical for diag- nosing nephroblastoma, its spread and further complica- tions within the regions of the abdomen, chest and skull.

Two-dimensional and Doppler US for the evaluation of venous tumor spread up to the right atrium are the com- plementary examinations of choice. MSCT and CT are excellent for diagnosing and staging abdominal lym- phoma and other infiltrative or complex abdominal mass- es (Fig. 7). However, MRI may be used with comparable accuracy.

In view of possible functional disturbance of the uro- genital and/or gastrointestinal tract by the mass, scintig- raphy and/or conventional digital radiographic/fluoro- scopic contrast studies may be necessary in the diagnos- tic work-up. These will yield functional as well as addi- tional or confirmatory anatomic information. Biliary scintigraphy may be considered for a cystic hepatic mass in order to demonstrate or exclude a direct or indirect re- lationship of the mass with the biliary system. Skeletal scintigraphy may be helpful for evaluating metastatic dis- ease, i.e., in staging of neuroblastoma or other bone metastasizing abdominal malignancies. However, MRI may be used instead of scintigraphy in the diagnostic work-up of metastatic bone disease. Meta-iodobenzyl- guanidine (MIBG) scintigraphy is valid for demonstrat- ing various forms of primary and/or metastatic neuroec- todermal tumors.

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Fig. 6. B-non-Hodgkin lymphoma in a seven and a half year-old boy who had shown pallor and fatigue for a few weeks. CT with intravenous contrast enhancement shows mid portion of right flank mass (a, b). Mass appears mostly solid and seems to grow intra- and perirenally. Despite the unusual morphological aspect (and re- duced health of the patient) the misdiagnosis of nephroblastoma was made, prompting suboptimal treatment and subsequent fatal outcome. Biopsy would have been indicated prior to therapy

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Biopsy

In a potentially malignant mass, histological characteri- zation or proof is required in almost all instances prior to deciding on specific therapy (immediate or secondary tu-

mor excision after initial chemotherapy). In lymphoma or leukemia, surgery is usually not indicated.

Transcutaneous biopsy under direct vision by US or con- trolled by CT is a well established procedure in children, and should be performed under general anesthetic. CT is recommended as a safe alternative if direct guidance by US is difficult to achieve or is unsafe because of the small size or problematic location of the target lesion (e.g., a paravertebral process).

Malignant Abdominal Mass

The most common malignant abdominal masses are of embryological origin, i.e., neuroblastoma, nephroblas- toma (Wilms’ tumor), and hepatoblastoma followed by lymphoma, germ cell tumor and soft tissue sarcoma (Table 2). Although it is usually possible to locate the site of origin of a retroperitoneal tumor as renal or ex- trarenal, there may be occasional morphological diffi- culties in the differentiation between neuroblastoma and nephroblastoma. As stated before, the clinical condition may be helpful in distinguishing the two entities, al- though biopsy will yield the diagnostic basis for further management and therapeutic decisions. The differential diagnoses of nephroblastoma are listed in Table 3.

Lymphoma and leukemia have to be considered as dif- ferential diagnostic alternatives in some unusual mor- phological presentations (Fig. 6). Both may also be con- sidered in the differential diagnosis of rhabdomyosarco- ma or a primitive neuroectodermal tumor (PNET).

Depending on the differentiation, germ cell tumors are often benign. With regard to the ovary, the differential diagnoses of malignant germ cell tumors are listed in

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Fig. 7a, b. Gastric wall teratoma in a 7-month-old boy with painless abdominal mass. a Plain abdominal radiograph shows large mass in left upper abdomen displacing stomach and intestinal structures and containing calcifications. b Axial CT scan with intravenous contrast enhacement through upper abdominal area demonstrates large intra-abdominal, well-defined heterogeneous mass of generally low density in close relationship with thin left gastric wall and ventrally to pancreatic tail and spleen

Table 3. Differential diagnosis of nephroblastoma Nephroblastoma (favorable and unfavorable histology)

Nephroblastomatosis (precursor of nephroblastoma, often bilateral, synchronous or not synchronous)

Clear cell sarcoma (formerly bone metastasizing tumor of kidney)*

Rhabdoid renal tumor (sarcoma)*

Renal cell carcinoma Transitional cell carcinoma Malignant renal lymphoma Intrarenal growth of neuroblastoma Congenital mesoblastic nephroma (benign) Multilocular cystic nephroma (benign)

* high grade malignancy

Table 2. Common malignant abdominal tumors in the child Nephroblastoma (Wilms’ tumor)

Neuroblastoma, including hepatic metastases in stage IVs Hepatoblastoma

Lymphoma Germ cell tumor

Soft tissue sarcoma (rhabdomyosarcoma, PNET, Ewing)

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Table 4. The soft tissue sarcomas comprise the rhab- domyosarcoma, the primitive neuroectodermal tumor (PNET) and the Ewing sarcoma.

Conclusions

Prompt diagnosis and characterization of an abdominal mass in a child is an ever new challenge of diagnostic imaging. Awareness of the specific pediatric conditions (pathophysiology, changing anatomy of the growing or- ganism, age-related and characteristic pathology) is a prerequisite with regard to a correct interpretation of the findings, as well as to the prompt and successful pre- ceeding acquisition of imaging data. The experience and bias of the radiologist performing the evaluation, in terms of the choice of diagnostic instruments, will and should influence the choice of the technical proce- dure(s). However, there are procedural recommendations that have general validity. Professional expertise com- bined with state-of-the-art technology allows highly ac- curate diagnostic imaging, sometimes even by the sim- ple means of US and X-ray. Technically more powerful and sophisticated equipment such as MRI and MSCT may be indispensable and the method(s) of choice in nu- merous cases. Yet, one should remember that there are always challenges to differential diagnosis and the very unexpected case may be waiting even for the skilled and experienced diagnostician.