Renal parenchymal volume analysis: Clinical and research applications

Abstract

Background and objectives: In most patients, the renal parenchymal volumes in each kidney directly correlate with function and can be used as a proxy to determine split renal function (SRF). This simple principle forms the basis for parenchymal volume analysis (PVA) with semiautomated software, which can be leveraged to predict SRF and new-baseline glomerular filtration rate (NBGFR) following nephrectomy. PVA was originally used to evaluate renal transplantation donors and has replaced nuclear renal scans (NRS) in this domain. PVA has subsequently been explored for the management of patients with kidney cancer for whom difficult decisions about radical versus partial nephrectomy can be influenced by accurate prediction of NBGFR. Our objective is to present a comprehensive review of the applications of PVA in urology including their clinical and research implications.

Methods: Key articles utilizing renal PVA to improve clinical care and facilitate urologic research were reviewed with special emphasis on take-home points of clinical relevance and their contributions to progress in the field.

Results: There have been considerable advances in renal PVA over the past 15 years, which is now established as a reference standard for the prediction of functional outcomes after renal surgery. PVA provides improved accuracy when compared to NRS-based estimates or non-SRF-based algorithms. PVA can be performed in minutes using routine preoperative cross-sectional imaging and can be readily applied at the point of care. Additionally, PVA has important research applications, enabling the precise study of the determinants of functional recovery after partial nephrectomy, which can affect surgical approaches to this procedure.

Conclusions: Despite the wide availability of PVA, primarily for use in renal transplantation, it has not been widely implemented for other urologic purposes at most centres. Our hope is that this narrative review will increase PVA utilization in urology and facilitate further progress in the field.

Keywords: functional recovery; kidney cancer; new baseline glomerular filtration rate; parenchymal volume analysis; partial nephrectomy; radical nephrectomy; split renal function.

FIGURE 1

Various approaches to perform parenchymal volume analysis to obtain renal functional metrics, such as split renal function (SRF) and percent parenchymal volume preserved (PPVP). (A–C) Estimation of renal parenchymal volume based on linear measurements of the length, width and height (LWH) as described by Schober et al. and Feder et al. (A) Linear LWH measurements showing length (cephalad to caudad, coronal view); (B) width (lateral to medial, coronal view); (C) thickness (anterior to posterior measurement, sagittal). (D–F) Free‐hand scripting segmentation procedure utilized in CT/MRI volumetric analyses to estimate differential parenchymal volume and split renal function. Transverse measurements of areas are made at 3 mm intervals and then summed to give a direct estimate of the parenchymal volumes in the ipsilateral versus the contralateral kidney, as described in Mir et al. (D) segmentation to obtain the parenchymal volume of the contralateral kidney. (E) Segmentation to obtain the total volume of the ipsilateral renal parenchyma and tumour. (F) Segmentation to obtain the volume of the tumour alone, which is then subtracted from (E) to determine the preoperative ipsilateral parenchymal volume. (G–I) Semi‐automated software‐derived parenchymal volume analysis (PVA) to estimate the SRF (Fujifilm medical systems) as described in Rathi et al. (G) Measurement of the volume of the contralateral kidney. (H) Volume of the ipsilateral kidney + tumour. (I) Volume of the tumour alone, which is then subtracted from (H to determine the ipsilateral parenchymal volume. SRF is based on the relative amounts of parenchyma on each side normalized by total parenchymal volume. (J–L) Use of semi‐automated software to determine PPVP following PN as described in Kazama et al. (J) Tumour plus parenchyma. (K) Tumour alone. (L) Parenchyma present in postoperative state. These techniques demonstrate the evolution of PVA calculation methods as each iteration improved the technique of determining SRF and PPVP. With all these methods it is important to exclude cysts, central sinus, collecting system and other unrelated structures when measuring the parenchymal volumes on each side before and after surgery.

FIGURE 2

The utility of parenchymal volume analysis (PVA) for estimating split renal function (SRF) and predicting functional outcomes after radical nephrectomy (RN). (A–C) Show the accuracies of SRF‐ and non‐SRF‐based models for predicting new baseline GFR (NBGFR) after RN (Rathi et al. ⁵ ). The SRF‐based model is predicted NBGFR = 1.24 x preoperative GFR x SRF _{Contralateral} (Rathi et al. ⁶ ), with 1.24 representing the average amount of renal functional compensation observed in adults after RN. B) Non‐SRF‐based model 1 is predicted NBGFR = 17 + preoperative GFR (×0.65) − age (×0.25) + 3 (if tumour>7 cm) − 2(if diabetes) (Aguilar Palacios et al. ²⁷ ). (C) Non‐SRF‐based model 2 is predicted NBGFR = 29.8–0.235 x age − 3.30 (if diabetes) + 0.457 x preoperative eGFR − 1.39 (if proteinuria) + 0.401 x size of renal mass (cm) + 0.148 x months post‐RN − 0.0020 x age x months post‐RN (Bhindi et al. ²² ). (D–F) Show the accuracies of different approaches for estimating SRF and, by extension, predicting NBGFR after RN (Rathi et al. Sci Rep. ²¹ ). (D) SRF was obtained from semi‐automated software‐derived parenchymal volume analysis. (E) SRF was obtained from Tc‐99 m MAG3 nuclear renal scans. (F) SRF was derived from PVA based on the product of linear measurements of the renal length, width and thickness. PVA from semi‐automated software estimates provides the most accurate estimates of SRF and NBGFR after RN (p values <0.05 when compared to the other methodologies used in E or F).