994
Editorial correspondence
Radiologic evaluation of renal scars To the Editor: I read with interest the recent article regarding the comparison of methods of evaluation of renal scarring. 1 I agree that dimercaptosuccinic acid (DMSA) scanning is the best single means of assessing renal scars, but some comment is required. All imaging methods are highly dependent on the technique employed and on the care with which the study is performed and interpreted. Shanon et al. 1 describe a meticulous approach to the DMSA renal scan, including blood flow, dynamic, tomographic, and planar views. On the contrary, the intravenous urography (IVU) that they use is limited to two views, which may be expected to be less accurate than the DMSA scan as they perform it. Limiting the IVU reduces radiation exposure but may reduce diagnostic accuracy. Thus direct comparison of the techniques is rendered unequal. The authors suggest that DMSA scanning gives a lower absorbed radiation dose than IVU. This distinction depends on the techniques employed and should not be regarded as universal. For example, estimates of the effective whole body dose for a I-year-old child with the use of the DMSA dosage suggested by Shanon et al.1 would be 28 mrad, compared with 30 mrad for a four-film IVU. 2, 3 The IVU absorbed dose can be reduced with dose-reduction techniques, and satisfactory planar imaging with DMSA can be achieved by using smaller injected doses than those employed by Shanon et al. Accepting the lack of laboratory proof of pathologic changes, the authors confirmed that the detailed DMSA study is accurate in the identification of renal scars. As they stated, DMSA scanning provides additional benefit in the early documentation of renal parenehymal injury caused by pyelonephritis and allows accurate assessment of relative renal function. However, the IVU provides pyelographic information unavailable with scintigraphy, is quicker to perform, is less expensive, and does not require sedation. Ultrasonography alone is insufficient for the accurate delineation of renal scarring but, being without ionizing radiation risk, remains invaluable in the initial evaluation of urinary tract infection and in the subsequent assessment of renal growth. Initial reports of the relative accuracy of DMSA scanning and IVU in the documentation of renal scars were conflicting. With improved technology, DMSA scanning now has the advantage, and it is likely that high-resolution single-photon emission computed tomography will be adopted as the new standard for evaluation of both acute and chronic renal injury. Shanon et al. 1recommend that DMSA scans and IVU be repeated for at least several years. Although routine duplication of examinations is not advocated, I agree that IVU is indicated at some time during follow-up because most information on the long-term outcome of renal scarring has
The Journal of Pediatrics December 1992
used the IVU as gold standard. On the basis of previous experience, performing the IVU at perhaps 7 years of age would enable valid outcome prediction until such time as sufficient data accrue from longitudinal studies using DMSA.
Michael A. Sargent, MRCP, FRCR Department of Radiology British Columbia's Children's Hospital Vancouver, British Columbia V6H 3V4, Canada 9/35/41670 REFERENCES
1. Shanon A, Feldman W, McDonald P, et al. Evaluation of renal scars by technetium-labeled dimercaptosuccinic acid scan, intravenous urography, and ultrasonography: a comparative study. J PEDIATR 1992;120:399-403. 2. Webster EW, Alpert NM, Brownell GL. Radiation doses in pediatric nuclear medicine and diagnostic x-ray procedures. In: James AE, Wagner HM, Cooke RE, eds. Pediatric nuclear medicine. Philadelphia: WB Saunders, 1974:34-58. 3. Chervu LR, Blaufox MD. Renal radiopharmaceuticals: an update. Semin Nucl Med 1982;12:224-39.
Reply To the Editor: We appreciate Dr. Sargent's comments and are pleased that he concurs with our observation that DMSA scanning has the advantage over IVU in the documentation of renal scars. We should clarify one point raised by Dr. Sargent when he said that we '!recommend that DMSA scans and IVU be repeated for at least several years." We did, in fact, make that statement but were suggesting that repetition of DMSA scans and IVU be studied as a way of determining whether the "scars" identified early by DMSA scanning but missed by IVU were, in a sense, false-positive findings (i.e., that these might in fact be zones of edema associated with infection, rather than true scars). Although we did not say so explicitly, we meant these studies to be repeated as part of a research project to answer the "false-positive" question, not as a guide to clinioians in everyday practice.
William Feldman, MD, FRCPC Professor of Paediatrics and of Preventive Medicine and Biostatistics University of Toronto Head, Division of General Paediatrics Hospital for Sick Children Toronto, Ontario MSG 1X8, Canada 9/35/41669
Editorial correspondence
Radiologic evaluation of renal scars To the Editor: I read with interest the recent article regarding the comparison of methods of evaluation of renal scarring. 1 I agree that dimercaptosuccinic acid (DMSA) scanning is the best single means of assessing renal scars, but some comment is required. All imaging methods are highly dependent on the technique employed and on the care with which the study is performed and interpreted. Shanon et al. 1 describe a meticulous approach to the DMSA renal scan, including blood flow, dynamic, tomographic, and planar views. On the contrary, the intravenous urography (IVU) that they use is limited to two views, which may be expected to be less accurate than the DMSA scan as they perform it. Limiting the IVU reduces radiation exposure but may reduce diagnostic accuracy. Thus direct comparison of the techniques is rendered unequal. The authors suggest that DMSA scanning gives a lower absorbed radiation dose than IVU. This distinction depends on the techniques employed and should not be regarded as universal. For example, estimates of the effective whole body dose for a I-year-old child with the use of the DMSA dosage suggested by Shanon et al.1 would be 28 mrad, compared with 30 mrad for a four-film IVU. 2, 3 The IVU absorbed dose can be reduced with dose-reduction techniques, and satisfactory planar imaging with DMSA can be achieved by using smaller injected doses than those employed by Shanon et al. Accepting the lack of laboratory proof of pathologic changes, the authors confirmed that the detailed DMSA study is accurate in the identification of renal scars. As they stated, DMSA scanning provides additional benefit in the early documentation of renal parenehymal injury caused by pyelonephritis and allows accurate assessment of relative renal function. However, the IVU provides pyelographic information unavailable with scintigraphy, is quicker to perform, is less expensive, and does not require sedation. Ultrasonography alone is insufficient for the accurate delineation of renal scarring but, being without ionizing radiation risk, remains invaluable in the initial evaluation of urinary tract infection and in the subsequent assessment of renal growth. Initial reports of the relative accuracy of DMSA scanning and IVU in the documentation of renal scars were conflicting. With improved technology, DMSA scanning now has the advantage, and it is likely that high-resolution single-photon emission computed tomography will be adopted as the new standard for evaluation of both acute and chronic renal injury. Shanon et al. 1recommend that DMSA scans and IVU be repeated for at least several years. Although routine duplication of examinations is not advocated, I agree that IVU is indicated at some time during follow-up because most information on the long-term outcome of renal scarring has
The Journal of Pediatrics December 1992
used the IVU as gold standard. On the basis of previous experience, performing the IVU at perhaps 7 years of age would enable valid outcome prediction until such time as sufficient data accrue from longitudinal studies using DMSA.
Michael A. Sargent, MRCP, FRCR Department of Radiology British Columbia's Children's Hospital Vancouver, British Columbia V6H 3V4, Canada 9/35/41670 REFERENCES
1. Shanon A, Feldman W, McDonald P, et al. Evaluation of renal scars by technetium-labeled dimercaptosuccinic acid scan, intravenous urography, and ultrasonography: a comparative study. J PEDIATR 1992;120:399-403. 2. Webster EW, Alpert NM, Brownell GL. Radiation doses in pediatric nuclear medicine and diagnostic x-ray procedures. In: James AE, Wagner HM, Cooke RE, eds. Pediatric nuclear medicine. Philadelphia: WB Saunders, 1974:34-58. 3. Chervu LR, Blaufox MD. Renal radiopharmaceuticals: an update. Semin Nucl Med 1982;12:224-39.
Reply To the Editor: We appreciate Dr. Sargent's comments and are pleased that he concurs with our observation that DMSA scanning has the advantage over IVU in the documentation of renal scars. We should clarify one point raised by Dr. Sargent when he said that we '!recommend that DMSA scans and IVU be repeated for at least several years." We did, in fact, make that statement but were suggesting that repetition of DMSA scans and IVU be studied as a way of determining whether the "scars" identified early by DMSA scanning but missed by IVU were, in a sense, false-positive findings (i.e., that these might in fact be zones of edema associated with infection, rather than true scars). Although we did not say so explicitly, we meant these studies to be repeated as part of a research project to answer the "false-positive" question, not as a guide to clinioians in everyday practice.
William Feldman, MD, FRCPC Professor of Paediatrics and of Preventive Medicine and Biostatistics University of Toronto Head, Division of General Paediatrics Hospital for Sick Children Toronto, Ontario MSG 1X8, Canada 9/35/41669
