Is the depth correction using the geometric mean really necessary in a Tc-99(m)-DMSA scan in the paediatric population?

Determination of the left to right dimercaptosuccinic acid (DMSA) uptake ra tio is theoretically one of the easiest quantitative procedures in nuclear medicine. The quantification can be performed on the posterior view, with o r without the lateral view for correction of kidney depth. The geometric me an can also be determined using both the anterior and the posterior views. The aim of this study was to evaluate the occurrence of remarkable differen ces in the results from quantification of the relative renal function using the geometric mean and those obtained using the posterior counts only. Mor eover, we evaluated to what extent the patient age influenced these differe nces. We reviewed 328 Tc-99(m)-DMSA scans. The difference between the relat ive renal function obtained using the posterior view and that obtained usin g the geometric mean was calculated and analysed statistically. For the pur pose of evaluating the value of performing the geometric mean calculation i n patients of different ages, patients were divided into four age groups (g roup I, less than or equal to2 years; group II, 3-9 years; group III, 10-18 years; group IV, >18 years). Using the Student's t test, no statistical di fferences were found in the relative renal function obtained by the two met hods (posterior projection and geometric mean) in groups I (t=0.01, P=0.992 ) and II (t=1.43, P=0.155), which consisted of patients younger than 10 yea rs (77% of the patients). In groups III and IV statistical differences were found (t=2.27, P=0.028 and t=2.170, P=0.038), respectively. We conclude th at for children under 10 years it is unnecessary to perform depth correctio n using the geometric mean except in rare cases of major malformations and position anomalies. ((C) 2001 Lippincott Williams & Wilkins).