Volumetric assessment of polycystic kidneys using computerized planimetry: analysis of accuracy and clinical significance

Background. Autosomal dominant polycystic kidney disease (ADPKD) is a hereditary disorder characterized by the progressive growth of multiple cysts in both kidneys. An increase in total kidney volume (TKV), reflecting the growth of both cysts and (to a lesser extent) noncystic parenchyma, is a key biomarker of disease progression. Accurate assessment of TKV is critical for: 1. Early prediction of the rate of decline in renal function. 2. Patient selection for clinical trials of new drugs. 3. Assessing the effectiveness of specific therapy aimed at slowing cyst growth (e.g., tolvaptan). 4. Assessing the effectiveness of transarterial embolization as part of pretransplant preparation in patients undergoing nephrectomy. Magnetic resonance imaging (MRI) is often used to measure TKV, but computed tomography (CT) remains a widely available and frequently performed imaging modality in patients with ARPKD (e.g., for evaluation of hematuria, urolithiasis, or suspected tumor). Manual planimetry on CT images has historically been considered the “gold standard” for measuring the volume of complex structures, such as polycystic kidneys. However, this method is time-consuming and labor-intensive.

Objective. Evaluation of the accuracy, reproducibility, and clinical utility of computerized planimetry for measuring TKV in ARPKD.

Materials and methods. This prospective study included 20 patients with ARPKD and ESRD (40 kidneys) who underwent multislice computed tomography of the kidneys and retroperitoneal organs. The volume of each kidney was measured using manual planimetry on a workstation (a Toshiba Aquilion Prime 160-slice volumetric CT system (Japan) and an Agfa digital system). Measurements were performed by two independent radiologists to assess interoperator agreement. To assess intraoperator agreement, one radiologist performed measurements twice, 2 weeks apart. The volumes obtained by planimetry were compared with those calculated using the ellipsoid formula (Length × Width × Thickness × π/6).

Results. The mean kidney volume by planimetry was 5208.9 ± 452 ml (range: 1065–17926 ml). Ellipsoid-based volume estimates systematically underestimated true volume (mean difference 7.14%, p < 0.001), especially in patients with severe cystic kidney deformation. Inter- and intra-observer agreement (ICC = 0.987) and inter-observer agreement (ICC = 0.994) were exceptionally high.

Conclusion. Computerized planimetry is a highly accurate and reproducible method for estimating the volume of polycystic kidneys, significantly outperforming ellipsoid-based estimates. This method provides an objective and reliable quantitative measure of total renal parenchyma and cyst volume, which is important for monitoring ARPKD progression, assessing response to therapy (e.g., tolvaptan), and predicting renal function, as well as for evaluating the efficacy of transarterial embolization to assess volume reduction in polycystic kidneys.