Clear cell carcinoma is the most common subtype; accounting for approximately 70% of RCCs and in our study, incidence of clear cell carcinoma was 63.6% [16]. Papillary renal carcinomas, the second most common subtype, comprise 10% to 15% of RCCs and in our study, it was 15.9% [17].
MRI is considered a useful tool for imaging solid renal mass and equivocal lesions and also for post-treatment follow‐up [18]. DWMRI is widely used in almost all discipline tumour detection, tumour characterization and the evaluation of treatment response in patients with cancer [19]. A quantitative analysis of ADC can be used to characterize tumours and assess response to treatment [20, 21]. DWI evaluates the Brownian movement of water molecules at microscopic level, which is related to tissue microstructure, cell membrane integrity and cellularity [22].
Altering the gradient amplitude, duration and time interval (b-value, measured in seconds per square millimetre) between paired diffusion gradients alters the sensitivity to the degree of water motion [19, 23]. There are three principle applications of DWI for renal imaging. First, low b-value DW images can replace fat-saturation T2-weighted images, decreasing total examination time. Second, the long b-value DW images may improve renal lesion detection. And last, DWI-derived parameter (i.e. ADC values) could potentially characterize renal lesions [24]. By performing DWI using different b-values, quantitative analysis, namely, the calculation of apparent diffusion coefficient (ADC) values, is possible and the ADC values can be displayed as a parametric map (ADC map) [19]. The gradients are characterized by their b-values, which express the amount of diffusion weighting [25, 26]. Restricted water diffusion demonstrates high signal intensity on DWI and lower ADC values on ADC map [19, 23]. Malignant tissues have increased cellularity with decreased extracellular space, resulting in restriction of water diffusion and thus leading to higher signal intensity on DWI and lower ADC value.
Liu B et al. conducted a meta-analysis to compare diagnostic performance of non-invasive modalities like CT, MRI, PET with 2-FDG and DWMRI for detecting metastatic lymph node in patients with cervical cancer. Among the 4 non-invasive modalities, DWI-MRI was found to have the highest sensitivity [8].
The role of DWI and ADC values in distinguishing benign and malignant lymph nodes is limited because cellular tissues such as lymph nodes have high signal intensity on DWI regardless of their biologic behaviour [27]. There is still considerable overlap in ADC values and lymph node evaluation in clinical practice, that mandate to rely on conventional features such as shape, size and growth patterns [28].
On cross-sectional imaging, a normal lymph node usually measures < 1 cm in diameter, has a smooth, well-defined border, shows homogeneous density or signal intensity and tends to have an oval or cigar shape. Metastatic disease can change the shape of the node by infiltrating nodal tissue and expanding the nodal capsule. Thus, rounded rather than oval nodes are suspicious to harbour malignancy [29].
Lin G et al. studied the DWI for detection of pelvic lymph node metastasis in patients with cervical and uterine cancers. They analysed that the relative ADC values between tumour and nodes were significantly lower in metastatic than in benign nodes (0.06 vs. 0.21 × 10–3 mm−2/s, p < 0.001; cut-off value 0.10 × 10–3 mm2/s). They concluded that the combination of size and relative ADC values is useful in detecting pelvic lymph node metastasis in patients with cervical and uterine cancers [9].
Kim et al. observed that the ADC values were significantly lower in the metastatic lymph nodes than in the non-metastatic lymph nodes of cervical cancer patients. The ADC was significantly lower in the metastatic lymph nodes (0.7651 × 10–3 mm2/s ± 0.1137) than in the non-metastatic lymph nodes (1.0021 × 10−3mm2/s ± 0.1859; p < 0.001). Measurement of ADC values may be useful, especially for detection of small metastatic lymph nodes compared with the limited sensitivity of CT and conventional MRI [30].
Luo N et al. studied on apparent diffusion coefficient ratio between axillary lymph node with primary tumour to detect nodal metastasis in 36 patients of breast cancers. They found that the mean ADC value of metastatic lymph nodes was significantly lower than those of benign lymph nodes (0.787 × 10–3 mm2/s ± 0.145 versus 1.043 × × 10–3 mm2/s ± 0.257; p < 0.05) [10].
Hasanzadeh F et al. also conducted a study on diagnostic value of diffusion-weighted magnetic resonance imaging in evaluation of metastatic axillary lymph nodes in a sample of Iranian women with breast cancer. They found that the mean ADC value of metastatic axillary lymph nodes was 0.824 ± 0.103 × 10–3 mm2/s and of non-metastatic axillary lymph nodes was 1.098 ± 0.23 × 10–3 mm2/s. There was statistical difference in mean ADC values between metastatic and non-metastatic axillary lymph nodes (p value < 0.001) [11].
Vallini V et al. conducted a study on carcinoma prostate to evaluate the usefulness of diffusion-weighted imaging (DWI) in patients with prostate cancer candidate to radical prostatectomy and extended pelvic lymph node dissection in 26 patients with pathologically proven prostate cancer. They observed mean fitted ADC values as 0.79 ± 0.14 × 10–3 mm2/s for metastatic lymph nodes and 1.13 ± 0.29 × 10–3 mm2/s in non-metastatic ones (p < 0.0001) [12].
In the present study, out of total 44 cases lymph nodes were found positive on histopathology, in 25 (56.8%) patients with lymph node mean ADC value of 1.16 ± 0.26 × 10–3 mm2/s. We found that lymph nodes with lower ADC value and ADC value below the cut-off (1.25 × 10–3 mm2/s) have higher probabilities harbouring cancerous cells (p = 0.05). Similar results were observed by Lin G et al., Kim et al., Luo et al., Hasanzadeh et al. and Vallini et al. studies [9,10,11,12, 30].
Si J et al. performed a study in 25 patients oral squamous cell carcinoma and compared the mean ADC value of 30 histo-pathologically proved reactive lymph nodes and 21 histo-pathologically proved metastatic lymph nodes. Their result showed significant difference between mean ADC values of reactive lymph node (1.037 ± 0.149) and metastatic lymph node (0.702 ± 0.197) [31].
He XU et al. analysed that the ADC value in positive lymph nodes was significantly lower than that with negative lymph nodes [SMD = 1.02, 95% confidence interval (CI) = 0.54–1.50, p < 0.001] in 687 cases with cervical tumour [32].
Barua et al. studied the primary penile tumour characteristics with DWMRI and its correlations with inguinal LN status and tumour positivity in LN dissection specimen within normal-sized LNs. In their study, primary tumour ADC ranged from 0.65 × 10–3 to 1.2 × 10–3 mm2/s (mean: 0.87 × 10–3 ± 0.11 × 10–3 mm2/s). The mean ADC values for grade 1, grade 2 and grade 3 tumour were 0.89 × 10–3, 0.82 × 10−3and 0.80 × 10–3 mm2/s, respectively. The ADC value of < 0.95 × 10–3 mm2/s was positively correlated with pathological LN presence within normal-sized LN with mean ADC value of 0.87 × 10–3 ± 0.11 × 10–3 mm2/s. The mean ADC value for higher-grade and -stage tumour was low. They conclude that the ADC value of primary tumour can help in prediction of LN metastasis in carcinoma penis with clinically and radiologically normal groin [13].
According to Studer et al., enlarged (> 1 cm) nodes are not necessarily metastatic but may be reactive, i.e. false positive (58%) which may be more common in necrotic tumours or tumours that involve the renal vein [33]. In our study, also 2 patients had lymph node > 1 cm in size that was benign on histopathology.
In the present study, when an ADC value of 1.25 × 10–3 mm2/s was taken as a cut-off for differentiating between benign and malignant lymph nodes, the results were accurate in 72.7% of patients with sensitivity of 63.1%, specificity of 80% and positive predictive value of 70.5%.
Basara et al. conducted a study on carcinoma breast in female. They found that mean ADC values were 1.00 × 10–3 mm2/s for the malignant and 1.39 × 10–3 mm2/s for the benign lymph nodes. The ADC values of malignant lymph nodes were significantly lower than the benign ones (p = 0.001). When 1.22 × 10–3 mm2/s was accepted as the cut-off ADC value, a sensitivity of 75.6% and a specificity of 71.1% were detected [34].
Lee et al. studied the 22 patients with head and neck cancer and found mean ADC values as 1.086 ± 0.222 10 3 mm2/s for benign lymph nodes and 0.705 ± 0.118 10 3 mm2/s for malignant lymph nodes (p 0.0001). When an ADC value of 0.851 10 3 mm2/s was used as a threshold value for differentiating benign from malignant lymph nodes, the best results were obtained with an accuracy of 91.0%, sensitivity of 91.3% and specificity of 91.1% [35].
In the study population, we also found that, out of total 44 cases lymph nodes were found malignant on histopathology, in 25 (56.8%) patients with mean renal tumour ADC value of 0.89 ± 0.18 × 10–3 mm2/s (p = 0.05). It was evident that primary renal tumour with lower ADC value has higher chances of harbouring malignant lymph nodes.
There is paucity of study comparing the ADC value of regional lymph node and its predictive value of foretelling malignant lymph node among genitourinary cancer. Although histological subtypes are being described based on renal tumour ADC values. Possibilities of harbouring malignant nodes in renal tumour were not studied in detail. Our study is probably the first in this direction to predict lymph node positivity in patients with radiologically organ-confined RCC.
In our study, while correlating the ADC value to that of lymph node positivity, it was observed that those lymph nodes with low ADC value and below cut-off ADC value were found to harbour malignant cell and are considered for standard lymph node dissection.