Displaced plaque in retroperitoneal adenopathy.

ABSTRACT

Background. This study was designed to determine when to consider incidental retroperitoneal masses on the basis of a displaced calcified atheromatous abdominal aorta on lateral radiographs.

Methods. We did a retrospective review of 143 normal abdominal helical computed tomography scans of individuals aged 50 years and older to measure the distance between the posterior aortic wall and anterior cortex of vertebral bodies from T12 through L3.

Results. The normal abdominal aorta maintains a close relationship to the vertebral column. The distance should not be more than 10 mm in men and 7.3 mm in women.

Conclusion. Displacement of aortic calcified atheroma greater than these distances should prompt a search for a retroperitoneal mass.

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CLINICAL DIAGNOSIS of a retroperitoneal lesion is difficult. Although modern diagnostic techniques such as computed tomography (CT) permit accurate assessment of the retroperitoneum, many patients with retroperitoneal disease present with nonspecific symptoms, including abdominal and back pain, and often have abdominal and spinal radiographs for workup rather than a cross-sectional study. We recently encountered a patient in whom initial workup for low back pain included a lateral lumbar radiograph that showed a calcified atheromatous abdominal aorta displaced anteriorly (Fig 1). Both CT (Fig 2) and ultrasonography (Fig 3) revealed retroperitoneal lymph node enlargement. This case prompted our study to establish the normal location of the abdominal aorta in relation to the vertebral column and its implication on diagnosis of retro peritoneal disease.

MATERIALS AND METHODS

To establish the normal anteroposterior distance between the abdominal aorta and the vertebral body, we retrospectively reviewed 143 consecutive helical CT scans with 5.0 to 8.8 mm slice thickness images of the entire abdominal aorta in individuals aged 50 years and older. Enhancement was used in 121 scans. All scans were analyzed on a 1k*1k AGFA cathode ray monitor. Measurements were made with electronic calipers on maximally magnified images. Four anatomic levels were measured at the midvertebral bodies of T12, L1, L2, and L3. The shortest distance between the anterior cortex of the vertebral body and adventitia of the aortic posterior wall was measured. A statistical analysis system (SAS) program was used to analyze the data gathered for the whole group, for each sex, and for three age groups--50 to 59, 60 to 69, and 70 and above.

In the second part of the study, this concept was applied to a group of patients with lymphoma. The tumor registry data base provided a list of 216 patients aged 50 or older, found to have enlarged lymph nodes between 1995 and 1998. Those who had nonabdominal forms of lymphoma were excluded. The remaining 143 patients had 44 positive scans for retroperitoneal lymphadenopathy by report. The records of only 24 of these patients included a lateral plain radiograph of the chest and thoracic and/or lumbar spine. These films were reviewed for detectable vascular calcification and displacement.

RESULTS

Measurements were obtained from all four spinal levels except for two scans that stopped at L2. The Table summarizes results, showing mean, standard deviation, range, and 95th percentile of the distance between the aorta and the vertebral column. Applying the Student's t test showed no significant difference by age group or sex except at L2 and L3, where measurements for men were greater than those for women (P<.0001).

We were able to identify 24 patients who had lymphoma with retroperitoneal lymphadenopathy and for whom a lateral plain radiograph was available for study, including 20 lateral chest films, 11 lateral lumbar spine films, and 3 lateral thoracic spine films. Five patients had calcification detectable on films of the lumbar spine (3 patients), thoracic spine (1 patient), and chest (1 patient). Three patients had abnormal anterior displacement (2 at L3 by 2.7 cm and 2.5 cm, and 1 at Tl2 by 1.6 cm).

DISCUSSION

Aortic displacement from the vertebral column (the floating aorta sign) has been described as a CT sign of retroperitoneal masses, (1) but the degree of displacement has not been quantitated. We found that the posterior aortic wall is expected to be within 10 mm from the spine in men and 7.3 mm in women in at least 95% of the normal population. Using these figures when assessing lateral plain radiographs may lead to an occasional early and potentially lifesaving detection of an unexpected retroperitoneal lesion. Also, after allowing for the normal aortic diameter, (2) the anterior aortic wall should be within 40 mm from the spine for men and 37.3 mm for women. Anterior wall calcification located more anteriorly suggests a retroperitoneal mass or an aortic aneurysm. Patients aged 50 and older were studied, since vascular calcification is much less common in younger patients. (3)

Applying these numbers on plain radiographs brings up the issue of magnification. Correction for magnification can be achieved by deriving the correction factor (CF), which represents the x-ray source to film distance (D) less the object to film distance (d) divided by the x-ray source to film distance (CF = D-d/D). It is difficult to decide how essential correction for magnification is because on plain radiographs the aortic--vertebral column distance is altered by both magnification and paramedian aortic location, where magnification increases the apparent aortic-vertebral distance and the antro-lateral aortic location to the vertebrae causes an apparent reduction in distance or even an overlap. A tradeoff exists where correction for magnification results in a more specific test, and not taking magnification into consideration results in a more sensitive one. Men tend to have more intra-abdominal fat, whereas women tend to have more subcutanous fat, (4) which may explain the unexpected measurement differences between men and women at L2 and L3.

By applying these findings to our lymphoma population, we were able to identify abnormal aortic displacement in 3 of 5 patients with retroperitoneal lymphadenopathy and a visible calcified aorta on a lateral radiograph. We do not propose that plain radiographs are suitable for screening or for diagnostic work-up of suspected retroperitoneal disease, since they lack sensitivity and specificity, and better modalities of imaging are available for these indications. However, evaluation for aortic displacement should be part of any plain radiographic interpretation.

TABLE

Measurements of Distance (in Millimeters) Between Abdominal Aorta and
Vertebral Column

Spinal                    Standard      95th
Level      Mean  Range    Deviation  Percentile

T12
   Total   2.5   0-6.9      1.43        5
   Female  2.4   0-6.9      1.36        4.7
   Male    2.7   0-6        1.5         5.6
L1
   Total   3.1   0-8.2      1.77        6.9
   Female  2.9   0-8.2      1.78        6.1
   Male    3.5   0.1-8      1.7         7
L2
   Total   4.25  0-11       2.15        8.1
   Female  3.5   0-8.2      1.7         6.2
   Male    5.5   0.7-11     2.28        9
L3
   Total   4.8   0.7-13     2.1         8.9
   Female  4.2   1.3-8.9    1.5         7.3
   Male    5.8   0.7-13     2.5         10

References

(1.) Havrilla TR, Reich NE, Haaga JR: The floating aorta in computerized tomography: a sign of retroperitoneal pathology. Comput Axial Tomogr 1977; 1:107-110

(2.) LaRoy LL, Cormier PJ, Matalon TAS, et al: Imaging of abdominal aortic aneurysms. AJR 1989; 152:785-792

(3.) Dixon AK, Lawrence JP, Mitchell RA: Age-related changes in the abdominal aorta shown by computed tomography. Clin Radiol 1984; 35:33-37

(4.) Dixon AK Abdominal fat assessed by computed tomography: sex difference in distribution. Clin Radiol 1983; 34:189-191

RELATED ARTICLE: KEY POINTS

* Conventional radiographs frequently reveal calcification of aortic plaques in the elderly.

* Displacement of posterior aortic wall calcified plaques from the spine by more than 10 mm suggests retroperitoneal disease.

* Separation of anterior aortic wall calcified plaques from the spine by more than 40 mm suggests aortic aneurysmal dilation or retroperitoneal disease.

From the Department of Radiology, Medical University of South Carolina, Charleston.

Reprint requests to Riyadh Al-Okaili, MD, Medical University of South Carolina, Department of Radiology, 171 Ashley Aye, Charleston, SC 29425.