Role of diffusion-weighted magnetic resonance imaging and apparent diffusion coefficient in subtyping renal cell carcinoma

Abstract

BACKGROUND

Diffusion-weighted magnetic resonance imaging (DWI) has emerged as a non-contrast functional imaging technique for renal mass characterization. Its role in differentiating histopathological subtypes of renal cell carcinoma (RCC) remains an area of active investigation.

AIM

To evaluate the role of DWI and apparent diffusion coefficient (ADC) values in differentiating histopathological subtypes of RCC.

METHODS

In this prospective observational study, 127 patients with histopathologically proven RCC who underwent preoperative magnetic resonance imaging (MRI) including DWI were analyzed. Diffusion-weighted imaging was performed using b values of 0 second/mm² and 1000 seconds/mm², and ADC maps were generated. ADC values were measured from solid tumor components and compared among RCC subtypes. Statistical analysis included subgroup comparisons and receiver operating characteristic curve analysis to assess the ability of ADC values to differentiate clear cell RCC (ccRCC) from non-ccRCC.

RESULTS

Of the 127 RCCs, 97 (76.4%) were ccRCC, 24 (18.9%) papillary RCC, and 6 (4.7%) chromophobe RCC. The mean ADC value of ccRCC [(1.391 ± 0.271) × 10^-3 mm²/second] was significantly higher than that of papillary RCC [(0.876 ± 0.293) × 10^-3 mm²/second; P < 0.001] and chromophobe RCC [(1.059 ± 0.369) × 10^-3 mm²/second; P = 0.04]. No significant difference was observed between papillary and chromophobe RCC (P = 0.396). When grouped, ccRCC demonstrated a significantly higher mean ADC value compared with non-ccRCC [(1.391 ± 0.271) × 10^-3 mm²/second vs (0.915 ± 0.312) × 10^-3 mm²/second; P < 0.001]. Receiver operating characteristic analysis yielded an area under the curve of 0.889 (95% confidence interval: 0.804-0.975). An ADC threshold of 1.08 × 10^-3 mm²/second achieved 90% sensitivity and 83% specificity for identifying ccRCC.

CONCLUSION

DWI with quantitative ADC analysis reliably differentiates clear cell from non-ccRCC and demonstrates significant correlation with RCC subtype and tumor grade. DWI serves as a valuable adjunct to conventional MRI, particularly in patients with contraindications to contrast administration.

Key Words: Renal cell carcinoma; Diffusion-weighted imaging; Apparent diffusion coefficient; Magnetic resonance imaging; Tumor subtyping

Core Tip: Diffusion-weighted magnetic resonance imaging (MRI) provides functional information that complements conventional MRI in renal mass evaluation. In this study, quantitative apparent diffusion coefficient values showed significant differences among renal cell carcinoma (RCC) subtypes and demonstrated good diagnostic performance in differentiating clear cell from non-clear cell RCC. Apparent diffusion coefficient analysis reflects underlying tumor microstructure and cellularity and may be particularly useful when contrast-enhanced imaging is contraindicated. Incorporating diffusion-weighted MRI into multiparametric renal MRI protocols can enhance noninvasive preoperative characterization of RCC.

INTRODUCTION

Renal cell carcinoma (RCC) represents the most common malignant neoplasm of the adult kidney, accounting for approximately 80%-90% of primary renal malignancies[1]. Despite advances in imaging and surgical techniques, RCC remains the most lethal urological cancer, with a substantial proportion of patients eventually dying from disease progression[2]. The clinical presentation of RCC is frequently nonspecific, and the classic triad of flank pain, hematuria, and palpable mass is now uncommon, leading to an increasing number of tumors being detected incidentally during imaging performed for unrelated indications[3].

RCC is a heterogeneous disease with distinct histopathological subtypes that differ in biological behavior, prognosis, and response to therapy. Clear cell RCC (ccRCC) is the most prevalent subtype, followed by papillary and chromophobe RCC, as described in the Heidelberg classification[4-6]. These subtypes originate from different segments of the nephron and demonstrate characteristic histological and molecular features. Tumor grade, most commonly assessed using the Fuhrman nuclear grading system, remains a key prognostic factor, with higher grades associated with poorer outcomes[7-9]. However, histopathological grading is subject to interobserver variability, highlighting the need for reliable noninvasive imaging biomarkers.

Imaging plays a central role in the detection, characterization, and staging of RCC. While contrast-enhanced computed tomography is the primary imaging modality, magnetic resonance imaging (MRI) serves as an important alternative, particularly in patients with contraindications to iodinated contrast media or impaired renal function[10-12]. Traditionally, gadolinium-enhanced MRI has been favored in such patients; however, concerns regarding nephrogenic systemic fibrosis in those with severe renal dysfunction have led to increasing restrictions on gadolinium use and renewed interest in non-contrast MRI techniques[13,14].

Diffusion-weighted MRI (DWI) is a functional imaging technique that evaluates the microscopic motion of water molecules within tissues and does not require contrast administration. Quantitative assessment using the apparent diffusion coefficient (ADC) reflects tissue cellularity and microstructural complexity, with restricted diffusion commonly observed in hypercellular tumors[15]. Early renal DWI studies demonstrated the ability of ADC measurements to differentiate normal renal parenchyma from various pathological conditions, including cystic and solid renal lesions[16].

Subsequent investigations expanded the role of DWI in renal oncology, suggesting that ADC values may aid in distinguishing benign from malignant renal masses and, more importantly, in differentiating RCC subtypes based on underlying histopathological architecture[17]. Several studies have reported lower ADC values in papillary RCC compared with ccRCC, attributed to increased cellular density and reduced extracellular space, while chromophobe RCC demonstrates intermediate diffusion characteristics[18-20]. Despite these promising findings, overlap of ADC values among RCC subtypes and variability in imaging protocols have limited the routine clinical application of DWI. Further prospective studies correlating ADC measurements with histopathological subtypes are required to clarify the diagnostic utility of DWI in RCC characterization.

MATERIALS AND METHODS

Study design and patient population

This prospective observational study was conducted at a tertiary care academic institution after approval from the institutional ethics committee. Consecutive patients with radiologically suspected renal masses referred for MRI evaluation were included over the study period. Written informed consent was obtained from all participants prior to imaging. Patients who subsequently underwent surgical excision of the renal mass with histopathological confirmation of RCC were included in the final analysis. Patients with non-neoplastic renal lesions, purely cystic lesions without solid components, or those with inadequate image quality due to motion or susceptibility artifacts were excluded.

MRI acquisition protocol

Patients were imaged in the supine position. All MRI examinations were performed using a 3.0-T MRI system (GE Signa HDxt, GE Healthcare, WI, United States) with a phased-array body coil. Patients were imaged in the supine position. The MRI protocol included axial and coronal T1-weighted sequences, axial and coronal T2-weighted turbo spin-echo sequences, diffusion-weighted imaging, and dynamic contrast-enhanced T1-weighted imaging in patients without contraindications to gadolinium-based contrast agents. Contrast-enhanced imaging was performed using intravenous gadolinium chelates, with multiphasic acquisitions obtained according to institutional protocol. In patients with impaired renal function or contraindications to contrast, non-contrast sequences including DWI were emphasized.

Diffusion-weighted imaging technique

DWI was acquired in the axial plane using a respiratory-gated two-dimensional spin-echo echo-planar imaging sequence. Imaging parameters included a repetition time of 4286 milliseconds, echo time of approximately 66.6-68.8 milliseconds, slice thickness of 5 mm, interslice gap of 1 mm, matrix size of 128 × 128, and field of view of 35 cm × 35 cm. Diffusion sensitizing gradients were applied in 16 gradient directions with b values of 0 second/mm² and 1000 seconds/mm². The receiver bandwidth was 83.33 Hz/pixel, and eight signal averages were used. Each acquisition required approximately 1 minute 51 seconds, with total acquisition time varying due to respiratory triggering. The entire kidney was covered in the axial plane.

ADC measurement methodology

Quantitative ADC analysis was performed on dedicated ADC maps. Regions of interest (ROIs) were manually placed within the solid enhancing components of the renal mass, avoiding areas of necrosis, hemorrhage, calcification, or cystic degeneration. ROI size was adapted to tumor size and morphology rather than standardized to a fixed area. In tumors demonstrating intralesional heterogeneity, multiple ROIs were placed within the most diffusion-restricted solid regions, and the mean ADC value was calculated for statistical analysis. The most representative region was defined as the solid tumor component showing the lowest ADC value, as this is considered to best reflect tumor cellularity. This strategy was employed to reduce partial-volume effects and to account for known intratumoral heterogeneity across different RCC subtypes. For comparison, ADC measurements were also obtained from adjacent normal renal parenchyma in the contralateral kidney wherever feasible. Mean ADC values were recorded for each lesion and expressed in × 10^-3 mm²/seconds.

Histopathological evaluation

All patients underwent partial or radical nephrectomy following MRI evaluation. Surgical specimens were examined by experienced pathologists who were blinded to MRI diffusion findings. Tumors were classified into histopathological subtypes - ccRCC, papillary RCC, and chromophobe RCC - based on standard histological criteria. Nuclear grading was performed using the Fuhrman grading system. Histopathological findings served as the reference standard for comparison with imaging-derived ADC values.

Statistical analysis

Statistical analysis was performed using standard statistical software. Continuous variables were expressed as mean ± SD. ADC values among different RCC subtypes were compared using appropriate parametric or non-parametric tests depending on data distribution. Receiver operating characteristic (ROC) curve analysis was performed to evaluate the diagnostic performance of ADC values in differentiating ccRCC from non-ccRCC. The area under curve (AUC), optimal ADC cut-off value, sensitivity, and specificity were calculated. A P value < 0.05 was considered statistically significant.

RESULTS

Study population

During the study period, 549 patients underwent renal MRI. Of these, 217 patients were identified to have renal masses on imaging. Histopathological confirmation was available in 170 patients, among whom 141 were diagnosed with RCC. Complete MRI and diffusion-weighted imaging data were retrievable for 127 patients, who constituted the final study cohort. The age of patients ranged from 20 years to 80 years, with a mean age of 54.27 ± 12.51 years. The most common age group was 60-69 years (n = 38, 29.9%). There was a male predominance with a male-to-female ratio of 2.9:1. Most lesions were unilateral (98.4%), with a slight right-sided predominance (52.8%). Tumors involving more than one renal pole were more common than single-pole lesions (52.8%).

Histopathological distribution

Among the 127 RCCs, ccRCC was the most common subtype, accounting for 97 cases (76.4%) (Figure 1). Papillary RCC was identified in 24 cases (18.9%), and chromophobe RCC in 6 cases (4.7%) (Figures 2 and 3).

Figure 1 Imaging and histopathological correlation in clear cell renal cell carcinoma. A: Axial T2-weighted magnetic resonance image showing renal cell carcinoma; B: Diffusion-weighted image of the same lesion; C: Corresponding apparent diffusion coefficient map; D: Histopathological image confirming clear cell renal cell carcinoma (200 × magnification, scale bar = 50 μm).

Figure 2 Imaging and histopathological correlation in papillary renal cell carcinoma. A: Axial T2-weighted magnetic resonance image showing papillary renal cell carcinoma; B: Diffusion-weighted image of the lesion; C: Corresponding apparent diffusion coefficient map; D: Histopathological image confirming papillary renal cell carcinoma (200 × magnification, scale bar = 50 μm).

Figure 3 Imaging and histopathological correlation in chromophobe renal cell carcinoma. A: Axial T2-weighted magnetic resonance image showing chromophobe renal cell carcinoma; B: Diffusion-weighted image of the lesion; C: Corresponding apparent diffusion coefficient map; D: Histopathological image confirming chromophobe renal cell carcinoma (200 × magnification, scale bar = 50 μm).

Diffusion-weighted imaging characteristics

On qualitative assessment of diffusion-weighted images, restricted diffusion was observed in the majority of lesions. Patchy diffusion restriction was the most common pattern, seen in 85 lesions (66.9%), followed by solid (whole lesion) restriction in 32 lesions (25.2%) and peripheral restriction in 10 lesions (7.9%).

Quantitative ADC analysis

The mean ADC value of normal renal parenchyma measured in the contralateral kidney was (2.124 ± 0.207) × 10^-3 mm²/seconds, with values ranging from (1.71-2.75) × 10^-3 mm²/second. Mean ADC values of RCC subtypes are summarized in Table 1. ccRCC demonstrated the highest mean ADC value [(1.391 ± 0.271) × 10^-3 mm²/second], while papillary RCC showed the lowest mean ADC value [(0.876 ± 0.293) × 10^-3 mm²/second]. Chromophobe RCC demonstrated intermediate ADC values [(1.059 ± 0.369) × 10^-3 mm²/second].

Table 1 Mean apparent diffusion coefficient values of renal cell carcinoma subtypes.

RCC subtype	n	Mean ADC (× 10-3 mm2/second)	SD
Clear cell RCC	97	1.391	0.271
Papillary RCC	24	0.876	0.293
Chromophobe RCC	6	1.059	0.369

RCC: Renal cell carcinoma; ADC: Apparent diffusion coefficient.

Comparison between RCC subtypes

Statistically significant differences were observed between: (1) ccRCC and papillary RCC (P < 0.001); (2) ccRCC and chromophobe RCC (P = 0.04); and (3) No statistically significant difference was found between papillary and chromophobe RCC ADC values (P = 0.396). When RCCs were grouped as clear cell and non-clear cell subtypes, the mean ADC value of ccRCC [(1.391 ± 0.271) × 10^-3 mm²/second] was significantly higher than that of non-ccRCC [(0.915 ± 0.312) × 10^-3 mm²/second] (P < 0.001).

ROC curve analysis

ROC analysis was performed to evaluate the diagnostic performance of ADC values in differentiating ccRCC from non-ccRCC (Figure 4). The AUC was 0.889 (95% confidence interval: 0.804-0.975; P < 0.001). An optimal ADC cut-off value of 1.08 × 10^-3 mm²/second yielded a sensitivity of 90% and specificity of 83% for the diagnosis of ccRCC.

Figure 4 Receiver operating characteristic curve analysis demonstrating the diagnostic performance of apparent diffusion coefficient values in differentiating clear cell renal cell carcinoma from non-clear cell renal cell carcinoma. ROC: Receiver operating characteristic.

Fuhrman nuclear grade and ADC values

Based on Fuhrman nuclear grading, 78 tumors (72.2%) were classified as low grade (grades I-II) and 30 tumors (27.8%) as high grade (grades III-IV). The mean ADC value of low-grade RCC was (1.391 ± 0.306) × 10^-3 mm²/second, which was significantly higher than that of high-grade RCC [(1.241 ± 0.365) × 10^-3 mm/second] (P = 0.021).

DISCUSSION

The present study demonstrates that DWI with quantitative ADC analysis provides significant diagnostic value in the preoperative characterization of RCC subtypes. Among malignant renal tumors, ccRCC showed significantly higher ADC values compared with papillary and chromophobe RCC, while papillary RCC consistently demonstrated the lowest ADC values. These findings were statistically robust and were further supported by ROC analysis, which showed good discriminatory performance of ADC values in differentiating clear cell from non-ccRCC. Importantly, the results were obtained using a standardized DWI protocol with b values of 0 second/mm² and 1000 seconds/mm² and careful ROI placement, emphasizing the technical feasibility of incorporating ADC analysis into routine renal MRI protocols.

Diffusion characteristics and tumor microstructure

DWI reflects the Brownian motion of water molecules within tissues, which is influenced by cellular density, nuclear-to-cytoplasmic ratio, extracellular matrix composition, and microvascular perfusion. Hypercellular tumors with compact architecture restrict water diffusion, resulting in lower ADC values, whereas tumors with abundant cytoplasm, lipid content, or vascular spaces demonstrate relatively higher ADC values. In the present study, papillary RCC exhibited the lowest mean ADC value [(0.876 ± 0.293) × 10^-3 mm²/second], consistent with its histological architecture characterized by densely packed papillae, limited extracellular space, and relatively hypovascular stroma. ccRCC demonstrated the highest mean ADC value [(1.391 ± 0.271) × 10^-3 mm²/second], likely reflecting its lipid-rich cytoplasm, prominent vascular network, and relatively loose cellular arrangement. Chromophobe RCC showed intermediate ADC values, consistent with its mixed cellular and stromal composition. These observations align with the biophysical principles underlying DWI and support the hypothesis that ADC values serve as surrogate markers of tumor microstructure.

Comparison with prior studies

The findings of the present study are in agreement with multiple prior investigations cited in the thesis. Taouli et al[17] reported lower ADC values in papillary RCC compared with ccRCC and attributed this difference to reduced vascularity and increased cellularity in papillary tumors. Wang et al[19] evaluated RCC subtypes at 3.0-T and demonstrated significantly higher ADC values in ccRCC compared with papillary and chromophobe RCC, with no significant difference between papillary and chromophobe subtypes. Similar results were reported by Inci et al[21], who observed lower ADC values in papillary RCC and higher values in ccRCC. The present study corroborates these findings in a larger, histopathologically confirmed cohort and further strengthens the evidence that ADC values can reliably distinguish ccRCC from non-ccRCC. The absence of a statistically significant difference between papillary and chromophobe RCC ADC values in the current study is also consistent with prior reports[19,21], suggesting overlap in diffusion characteristics between these two subtypes.

Diagnostic performance and ROC analysis

ROC analysis in the present study demonstrated an AUC of 0.889 for ADC values in differentiating ccRCC from non-ccRCC. An ADC threshold of 1.08 × 10^-3 mm²/second achieved a sensitivity of 90% and specificity of 83%, indicating good diagnostic accuracy. These results are comparable to those reported by Wang et al[19], who demonstrated robust ROC performance of ADC values for RCC subtype differentiation, and by Rosenkrantz et al[20], who highlighted the utility of ADC in distinguishing low-grade from high-grade ccRCC. The relatively high sensitivity observed in the present study suggests that ADC may be particularly useful as a screening parameter for identifying ccRCC, which has important therapeutic and prognostic implications.

ADC values and tumor grade

In addition to subtype differentiation, the present study demonstrated a statistically significant difference in ADC values between low-grade and high-grade RCC. Low-grade tumors exhibited higher mean ADC values compared with high-grade tumors, reflecting increased cellularity and reduced extracellular space in higher-grade lesions. These findings are in accordance with Rosenkrantz et al[20], who reported significantly lower ADC values in high-grade ccRCC compared with low-grade tumors. Sandrasegaran et al[18] also observed a trend toward lower ADC values in higher-grade RCC, although statistical significance was limited by small sample size. The present study adds further evidence supporting the role of ADC as a noninvasive biomarker of tumor aggressiveness.

Normal renal parenchyma ADC values

The mean ADC value of normal renal parenchyma in the present study was (2.124 ± 0.207) × 10^-3 mm²/second, which was significantly higher than ADC values observed in malignant lesions. These values are consistent with prior studies by Cova et al[16] and Manenti et al[22], who reported normal renal parenchymal ADC values in a similar range. The clear separation between normal parenchyma and malignant lesions underscores the sensitivity of DWI in detecting pathological renal tissue and supports its role as an adjunct to conventional MRI sequences.

Technical considerations

Several technical factors merit consideration when interpreting DWI and ADC values in renal imaging. The choice of b values influences ADC measurements, with higher b values reducing perfusion effects but increasing susceptibility to noise. The present study employed b values of 0 second/mm² and 1000 seconds/mm², which have been widely used in prior renal DWI studies and provide a balance between diffusion sensitivity and signal-to-noise ratio. ROI placement is another critical factor. In the present study, ROIs were placed in the most diffusion-restricted solid component of the tumor while avoiding necrotic or cystic areas. This approach minimizes partial volume effects and improves reproducibility, as supported by prior methodological studies cited in the thesis.

Clinical implications

Histological subtype of RCC has significant clinical relevance, as ccRCC is generally associated with more aggressive behavior and poorer prognosis compared with papillary and chromophobe RCC. Furthermore, systemic therapies differ by subtype, with tyrosine kinase inhibitors such as sunitinib and sorafenib being more effective in ccRCC, while mammalian target of rapamycin inhibitors like temsirolimus are more commonly used in papillary RCC. The ability of DWI and ADC analysis to noninvasively predict RCC subtype may therefore aid in preoperative risk stratification, treatment planning, and patient counseling. Additionally, DWI is particularly valuable in patients with renal dysfunction in whom gadolinium-based contrast agents are contraindicated.

Limitations

The present study has certain limitations. First, although the overall cohort size was substantial, the number of chromophobe RCC cases was relatively small, which may limit statistical power for subgroup analysis. Second, ADC measurements may vary across scanners and imaging protocols, potentially affecting generalizability. Finally, overlap in ADC values between papillary and chromophobe RCC limits complete subtype separation using ADC alone. In addition, this study focused on the most common histopathological subtypes of RCC. Less common renal neoplasms such as collecting duct carcinoma, medullary carcinoma, and renal pelvic tumors with parenchymal invasion were not included. Therefore, the findings may not be directly generalizable to the full spectrum of renal tumors encountered in clinical practice. Future studies incorporating larger multicenter cohorts and standardized DWI protocols are needed to validate ADC thresholds and improve reproducibility. Advanced diffusion techniques, including ADC histogram analysis and intravoxel incoherent motion imaging, may further enhance RCC characterization[23].

CONCLUSION

DWI with quantitative ADC analysis provides valuable noninvasive information for the characterization of RCC. In the present study, significant differences in ADC values were observed among RCC subtypes, with ccRCC demonstrating higher ADC values compared with papillary and chromophobe RCC. ADC analysis also showed good diagnostic performance in differentiating clear cell from non-ccRCC and demonstrated a significant association with tumor grade. These findings support the role of DWI as a complementary tool to conventional MRI in the preoperative evaluation of renal masses, particularly in patients with contraindications to gadolinium-based contrast agents. While ADC values alone cannot completely distinguish all RCC subtypes due to overlap, their integration into multiparametric MRI protocols may enhance diagnostic confidence and assist in clinical decision-making.