Predictive value of the RENAL nephrometry score on oncological and functional outcomes in cT1 renal cell carcinoma treated with cone-beam CT-guided percutaneous cryoablation
Stage 1 (cT1) renal cell carcinoma (RCC) can be treated by cone-beam CT-guided percutaneous cryoablation (CBCT-guided PCA). The RENAL nephrometry scoring system provides a structured method for evaluating the complexity of renal tumours by classifying them based on specific anatomical features. The aim was to evaluate the clinical utility of this scoring system in predicting postoperative oncological and functional outcomes in patients undergoing CBCT-guided PCA as their primary treatment for biopsy proven cT1 RCC. 91 patients were retrospectively analyzed. A statistically significant association was found between the RENAL nephrometry score classification and local treatment failure (p = 0.005). The RENAL score was not found to correlate with local tumour recurrence, postoperative complications, or change in renal function. These findings suggest that the RENAL nephrometry score can be utilized for risk stratification and more precise patient counseling.
The incidence of localized renal cell carcinoma (RCC) has risen in the past two decades, driven by increased cross-sectional imaging, an aging population, and higher comorbidity rates [1, 2]. RCC accounts for about 3.5% of global malignancies [3]. Meanwhile, RCC mortality has declined due to earlier detection of low-stage tumours and improved treatments.
Partial nephrectomy (PN) is the standard for treating stage 1 renal tumours, preserving renal function [4]. However, image-guided percutaneous cryoablation (PCA) is gaining traction as a nephron-sparing alternative with good oncologic outcomes and lower morbidity [1]. The European Association of Urology (EAU) and American Urological Association (AUA) guidelines now endorse image-guided cryoablation for small tumours in frail or comorbid patients but not for tumours over 4 cm [5, 6].
A relatively novel and underutilized imaging-guided technique is the application of cone-beam computed tomography (CBCT). Compared to conventional computed tomography (CT) guidance, CBCT enables real-time three-dimensional needle tracking, trajectory planning, enhanced workspace sterility, and improved angulation and rotational flexibility due to its C‑arm configuration. These features enhance access to renal tumours, particularly those located in the upper pole or on the anterior aspect of the kidney [7].
The RENAL nephrometry scoring system provides a structured method to assess renal tumour complexity, categorizing tumours based on anatomical features such as size, exophytic/endophytic nature, proximity to the collecting system, anterior/posterior position, and location [8]. These factors help estimate the difficulty of PN, with multiple studies showing a correlation between the tumour scores and postoperative outcomes and complications [9, 10]. In comparison, its predictive value for oncological outcomes and complications after CBCT-guided PCA in stage 1 RCC is not well established, with limited research available [11].
The decision to use CBCT-guided PCA still relies heavily on tumour stage, patients’ comorbidities and overall general health. This study aimed to assess the clinical utility of the scoring system in predicting postoperative oncological and functional outcomes in patients who underwent CBCT-guided PCA as primary treatment for histopathologically confirmed stage 1 RCC. The findings could significantly impact patient counseling and support more personalized treatment approaches in the future.
Patients and methods
Patients who underwent CBCT-guided PCA for stage 1 RCC were retrospectively included between January 2014 and February 2023 after digital informed consent was obtained. The majority of the procedures were conducted by the same multidisciplinary team of medical professionals, consisting of urologists and interventional radiologists. Patients with biopsy-proven RCC were included in the analysis, no distinction was made in histopathological subtype. In addition, patients must have undergone CBCT-guided PCA as their primary treatment. Patients with histopathological features other than RCC (malign or benign) and lesions higher classified as stage 1 were excluded.
The RENAL nephrometry scoring system
The RENAL scoring system includes 5 key anatomical characteristics of a renal mass. Four of these are rated on a scale of 1, 2, or 3 points, while the fifth specifies whether the tumour is located anteriorly or posteriorly within the kidney [8]. The components of the score are (R)adius (maximal tumour diameter)—1 point for tumours 4 cm or smaller, 2 points for those over 4 cm but under 7 cm, and 3 points for tumours 7 cm or greater; (E)xophytic/endophytic properties—1 point for tumours that are 50% or more exophytic, 2 points for those less than 50% exophytic, and 3 points for completely endophytic tumours; (N)earness of the tumour’s deepest part to the collecting system or sinus—1 point for a distance of 7 mm or more, 2 points for a distance over 4 mm but less than 7 mm, and 3 points for a distance of 4 mm or less; (A)nterior (a)/posterior (p)/no designation (x) descriptor; (L)ocation relative to the polar line—1 point if the tumour is entirely above the upper or below the lower polar line, 2 points if it crosses the polar line, and 3 points if over 50% of the tumour lies between the polar lines or crosses the renal midline. Renal tumours are classified by complexity based on their RENAL score: 4–6 for low complexity, 7–9 for moderate complexity, and 10–12 for high complexity.
Endpoints
The endpoints included in this study were local treatment failure, local tumour recurrence, postoperative complication rate and renal function (eGFR) alteration. Tumours with technically unsuccessful ablation or those demonstrating evidence of local recurrence on the first follow-up imaging after treatment were classified as local treatment failures. Local tumour recurrence was characterized as the presence of a hyper-enhancing or enlarging soft tissue nodule within or adjacent to the ablation zone, as identified on CT or MRI imaging conducted at least three months post-ablation, except for the first imaging after procedure. Postoperative complications were categorized based on the Clavien-Dindo classification system. The possible change in eGFR is calculated based on the last eGFR measurement recorded prior to ablation and the first eGFR measurement obtained during the initial follow-up assessment.
Statistical analysis
Summary statistics were generated to describe the patient cohort, by number (percentage) for categorical data and median (interquartile range (IQR)) or mean (SD) for continuous data. The IQR is the difference between quartile 1 (Q1) and quartile 3 (Q3). The log rank test based on assumptions of the Kaplan-Meier method were used to assess differences in local treatment failure and local tumour recurrence among RENAL score categories. The Chi-Square test was used to examine the trend of increasing RENAL score categories (low, moderate and high tumour complexity) with postoperative complications. The Kruskal-Wallis test was used to test for an association of RENAL score with eGFR. Results with p < 0.05 were considered statistically significant, and all tests were two-sided. Statistical analyses were performed using SPSS Statistics version 28.0 software.
Results
Between January 2014 and February 2023, 91 patients underwent CBCT-guided PCA as the primary treatment modality for a solitairy kidney tumour. The cohort comprised 69.2% (63) male patients, with a mean age of 69 ± 9.1 years and a mean follow-up time of 33 ± 20.6 months. The median (IQR) tumour diameter was 28 (14) mm. Histological analysis revealed 83.5% (76) clear cell RCC (ccRCC), 11% (10) papillary RCC (pRCC) and 5.5% (5) chromophobe RCC (crRCC). The median (IQR) RENAL nephrometry score was 7 (3). 31 (34.1%), 53 (58.2) and 7 (7.7) patients were included in the low-, moderate- and high RENAL score complexity group, respectively. The baseline cohort characteristics are shown in Tab. 1.
Table 1
Baseline patient- and renal tumour characteristics
Patient characteristics
No. renal tumours, n (%)
91 (100)
Age, mean ± SD
69 ± 9.1
Gender, n (%)
– male
63 (69.2)
– female
28 (30.8)
eGFR preoperative, mean ± SD
72 ± 25.9
eGFR postoperative, mean ± SD
64 ± 25.6
eGFR difference, mean ± SD
−8.5 ± 18.5
Follow-up (months), mean ± SD
33 ± 20.6
Tumour characteristics
Side of tumour, n (%)
– right
50 (54.9)
– left
41 (45.1)
Diameter of tumour (mm), median (IQR)
28 (14)
Pathological biopsy result, n (%)
– clear cell RCC
76 (83.5)
– papillary RCC
10 (11)
– chromophobe RCC
5 (5.5)
RENAL-score, median (IQR)
7 (3)
RENAL-score tumour complexity, n (%)
– low (4–6)
31 (34.1)
– moderate (7–9)
53 (58.2)
– high (10–12)
7 (7.7)
Treatment failure
Complete ablation was achieved in 95.6% (87), while 3.3% (3) resulted in incomplete ablation. The mean RENAL score for cases with complete and incomplete ablation were 7.3 ± 1.5 and 9.7 ± 0.6, respectively (p = 0.007) (Tab. 2). A statistically significant correlation was observed between the RENAL nephrometry score classification and local treatment failure (p = 0.005) (Tab. 3).
Table 2
Endpoints with corresponding mean RENAL score
n (%)
Mean RENAL score (SD)
p (95%-CI)
Local treatment failure
0.007 (−4.124–−0.658)
– complete
87 (95.6)
7.3 (1.5)
– incomplete
3 (3.3)
9.7 (0.6)
Local tumour recurrence
0.043 (0.033–2.048)
– yes
10 (11)
8.3 (1.7)
– no
81 (89)
7.3 (1.5)
Postoperative complications
0.475 (−1.545–0.726)
– yes
8 (8.8)
7 (2.1)
– no
83 (91.2)
7.4 (1.5)
Table 3
Local treatment failure
RENAL score tumour complexity
Low
(4–6)
Moderate
(7–9)
High
(10–12)
No. renal tumours, n (%)
31 (34.1)
53 (58.2)
7 (7.7)
No. local treatment failure, n (%)*
1 (0)
1 (3)
2 (29)
*Log rank p = 0.005
Local tumour recurrence
Local tumour recurrence was observed in 10 patients (11%). The mean RENAL score for cases without local recurrence during follow-up was 7.3 ± 1.5, whereas for cases with local recurrence, it was 8.3 ± 1.7 (p = 0.043) (Tab. 2). Further analysis did not demonstrate a statistically significant correlation between higher RENAL score classifications and the occurrence of local tumour recurrence (p = 0.294) (Fig. 1).
Fig. 1
Local tumour recurrence by RENAL score complexity (low, moderate and high) (log-rank p = 0.294) and additional number at risk table for local recurrence by time
A total of 8 (8.8%) complications were observed following the procedure. All were Clavien-Dindo complication grade 1, with no additional details provided. Complications were distributed as follows: 3 cases in the low-score group, 5 cases in the moderate-score group and 0 cases in the high-score group. Postoperative complications were not significantly associated with higher average RENAL score, which were 7.4 and 7, respectively (p = 0.475) (Tab. 2). Furthermore, no significant trend was found between the RENAL nephrometry score classification and the occurrence of postoperative complications (p = 0.693) (Fig. 2).
Fig. 2
Number of complications by RENAL score: 3, 5 and 0 for low, moderate and high scores, respectively (p for trend = 0.693)
The mean eGFR prior to and following surgery was 72 ± 25.9 mL/min and 64 ± 25.6 mL/min, respectively. The average difference between preoperative and postoperative eGFR was −8.5 ± 18.5 mL/min (p < 0.001) (Tab. 1). However, no significant correlation was found in the change in eGFR between low, moderate and high complexity groups based on the RENAL score (p = 0.261).
Discussion
Since the introduction of the RENAL nephrometry score, numerous studies have assessed its predictive value regarding various clinical outcomes including surgical approach, oncological prognosis, postoperative complications, and renal function outcomes [8‐15]. Although the majority of these studies has focused on surgery, partial nephrectomy, as their primary intervention, the predictive value of the RENAL nephrometry score for minimal invasive percutaneous procedures remains questionable, particularly in the context of CBCT-guided PCA.
Schmit et al. treated 751 renal tumours with PCA (54%) and radiofrequency ablation (RFA) (46%). They reported local treatment failure rates of 3.5% (15) and 4% (13) for PCA and RFA, respectively Additionally, they found a significant association between the RENAL nephrometry score and overall local treatment failure (p < 0.001) [11]. The mean nephrometry score for tumours with local treatment failure was 7.6 ± 2.2, compared to 6.7 ± 1.9 for without treatment failure. In contrast, Konstantinidis et al. and Bhindi et al. did not observe a significant association between the RENAL score and local treatment failure [16, 17]. In our cohort, the mean RENAL score for cases with complete and incomplete ablation were 7.3 ± 1.5 and 9.7 ± 0.6, respectively (p = 0.007) (Tab. 2). A statistically significant correlation was observed between the RENAL nephrometry score classification and local treatment failure (p = 0.005) (Tab. 3).
We did not find a statistically significant correlation between the RENAL nephrometry score classification and the occurrence of local tumour recurrence (p = 0.294) (Fig. 1). These results are consistent with those of Konstantinidis et al., who reported mean RENAL score of 5.73 ± 0.4 and 5.57 ± 0.1 in patients with and without local tumour recurrence, respectively (p = 0.804). In contrast, Camacho et al. did find a significant correlation between a RENAL score > 8 and both overall and early recurrences (p = 0.0001) [18].
Previous research has reported a significantly higher incidence of postoperative complications in lesions with RENAL score > 8 (p = 0.0001) [18]. These findings are corroborated by other studies. Schmit et al. observed a significant association between the RENAL score classification and major postoperative complications, with a mean nephrometry score of 8.1 ± 2 in patients who developed complications, compared to 6.8 ± 1.9 in those without complications (p < 0.001) [11]. In contrast, Konstantinidis et al. identified 9 (5.5%) complications following treatment, but found no significant correlation with the mean RENAL nephrometry score (p = 0.097) [16]. Our analysis aligns with these results, as we found no significant trend between the RENAL nephrometry score classification and the occurrence of postoperative complications (p = 0.693) (Fig. 2).
The mean change in eGFR in our cohort was −8.5 ± 18.5 mL/min (p < 0.001) (Tab. 1). However, change of such magnitude may be considered of little clinical relevance. Complications due to renal function decline (e.g. dialysis) were not reported. No significant correlation was found between the change in eGFR and complexity groups classified according to the RENAL nephrometry score (p = 0.261). Similarly, studies by Schmit et al. and Konstantinidis et al. reported a statistically significant change in eGFR, but found no significant correlation with the RENAL score, with p-values of 0.197 and 0.936, respectively [11, 16].
This study has several strengths. The data represents the first comprehensive evaluation and validation of the RENAL nephrometry score’s utility in predicting both oncological and functional outcomes in patients with cT1 RCC treated with CBCT-guided PCA. Additionally, it is one of the few studies to assess the predictive value of the RENAL score for both oncological and functional outcomes following percutaneous focal therapy. Another strength of this study is that all lesions included were biopsy proven RCC.
The current study has several notable limitations. First, its retrospective, long-term design and single-center nature may increase susceptibility to statistical and selection bias, as well as the potential for data loss. Moreover, the comparison of primary outcomes between the complexity groups (low, moderate and high) based on the RENAL score was constrained by the limited number of cases in the high complexity group. This may be attributed to a reluctance to utilize cryoablation for the treatment of more complex tumours. Furthermore, this study did not evaluate the predictive value of the individual components of the RENAL score in relation to oncological and functional outcomes in patients with cT1 RCC primary treated by CBCT-guided PCA. Future research should address this gap.
Conclusion
A higher RENAL nephrometry score is significantly associated with an increased risk of incomplete ablation of renal tumours, leading to treatment failure. However, the RENAL score was not found to correlate with local tumour recurrence, postoperative complications, or change in renal function. These findings suggest that the RENAL nephrometry score can be utilized for risk stratification and more precise patient counseling. Nevertheless, the current findings should be interpreted with caution, and further validation through additional studies is recommended.
Funding
No funding was received for conducting this study.
Declarations
Conflict of interest
M. Duijn, A.E.C. Ruiter, A.D.M. van Swijndregt, V.P.M. van der Hulst and B.W. Lagerveld declare that they have no competing interests.
Ethical standards
Ethical approval was waived by the local Ethics Committee of OLVG Amsterdam in view of the retrospective nature of the study and all the procedures being performed were part of the routine care. Informed consent was obtained from all individual participants included in the study.
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Predictive value of the RENAL nephrometry score on oncological and functional outcomes in cT1 renal cell carcinoma treated with cone-beam CT-guided percutaneous cryoablation
Auteurs
Matthijs Duijn
Annebeth E. C. Ruiter
Alexander D. Montauban van Swijndregt
Victor P. M. van der Hulst
Brunolf W. Lagerveld
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