Renal cell carcinoma

Detection of renal masses has increased remarkably in recent times owing to wide-spread use of cross-sectional imaging. Imaging plays an important role in characterizing renal masses and is indispensable in treatment planning in renal malignancies.

The most commonly encountered malignant renal masses in practice include renal cell carcinoma (RCC), urothelial carcinoma, lymphoma and metastases. Urothelial carcinoma and lymphoma have been covered in detail in separate chapters.

Imaging of renal masses

Scan protocols

For characterization of renal masses, a multiphase CT or MRI study is recommended. CT study constitutes an unenhanced study followed by contrast injection and acquisition of corticomedullary phase at 40 seconds, nephrographic phase at around 100 seconds and delayed phase at around 5 minutes. MRI protocol includes T2-weighted single-shot fast spin-echo 2D sequences, axial T1-weighted 2D sequence with in-phase and opposed-phase gradient echo imaging, precontrast and postcontrast imaging with a 3D T1-weighted spoiled gradient recalled sequences in corticomedullary phase at 30 seconds, nephrographic phase at 100 seconds, 180–210 seconds and delayed phase imaging at 5 minutes and diffusion-weighted imaging with multiple b-values 0–50, 400–500 and 800–1000 s/mm².

Descriptors that help characterize renal masses

The ACR White Paper on CT imaging of incidental renal mass recommends using the following descriptors for characterizing renal masses:

• • Size
  
• • Attenuation
  
• • Homogeneity versus heterogeneity
  
• • Enhancement (>20 HU increase in contrast attenuation on CT; or >15% increase in signal intensity/visible enhancement on subtraction images on MRI)
  
• • Complexity of cystic lesions (Bosniak classification)
  
• • Growth and morphological change

Imaging features of common histological subtypes of RCCs

Given the prognostic implications, it is worthwhile for radiologists to know the imaging features that may help discriminate between the common histological subtypes of RCC on various imaging modalities.

Clear cell RCC.

This is the most common histological type of RCC, accounting for about 70% of cases. These are exophytic tumours with a heterogeneous appearance due to the presence of necrosis, intratumoural haemorrhage, cystic components with septations and calcific foci and hence have a more heterogeneous appearance on cross-sectional imaging than the other subtypes (Fig. 11.25.1.1). Necrosis is seen more often in larger masses and higher tumour grades (Fig. 11.25.1.2). On CT, depending on the tumour composition, these are seen as heterogeneous lesions, show marked contrast enhancement in the corticomedullary phase with washout on nephrogenic phase (Fig. 11.25.1.2). On MRI, these characteristically show high signal intensity on T2W sequences (Fig. 11.25.1.3). The presence of intracytoplasmic fat in the clear cells of the tumour is reflected in the loss of signal in opposed phase images on chemical shift imaging, which is seen in nearly 60% of these tumours. A pseudocapsule may be seen, which is best appreciated on T2-weighted imaging on MRI; the presence of a pseudocapsule has a high negative predictive value for perinephric extension (Figs. 11.25.1.3 and 11.25.1.4). Conversely, larger tumours with higher grades often have interrupted pseudocapsule and hence tend to be irregular, spread into the perinephric fat with renal vein and inferior vena cava (IVC) invasion (Figs. 11.25.1.5 and 11.25.1.6). On diffusion-weighted imaging, clear cell RCCs have been seen to have higher ADC values than nonclear cell RCCs, and lower-grade tumours have been seen to have higher ADC values than higher-grade tumours. The clear cell variant has worse prognosis than the papillary and chromophobe subtypes, presenting at a more advanced stage and being more likely to recur or metastasize (Fig. 11.25.1.7), with lower 5-year survival rates at 44%–69% as compared to 78%–92% for the other two.

Papillary RCC.

Papillary RCC comprises about 10%–15% of RCCs. These are slow-growing tumours and hence are well-marginated in contrast to clear cell RCC. As they are hypovascular, their enhancement on CT is significantly less than clear cell RCC (Fig. 11.25.1.8). These show characteristically low signal on T2W images on MRI (Fig. 11.25.1.9). Intracytoplasmic or macroscopic fat is less often seen on MRI imaging as compared to the clear cell variants. Larger tumours tend to be more heterogeneous (Fig. 11.25.1.10). These tumours may sometimes show cystic appearances, mural projections or blood degradation products. Calcifications are more common in papillary variants than clear cell RCC. Multifocality and bilaterality are also more common in these tumours than clear cell variants.

Chromophobe RCC.

These account for about 5% of RCCs. These are less aggressive, more homogeneous and hypovascular lesions than clear cell variants, with intensity of contrast enhancement on cross-sectional imaging being midway between clear cell and papillary variants. They show low to intermediate T2 signals on MRI. A characteristic feature is the presence of a central scar and spoke-wheel enhancement, the latter being a histopathological and imaging similarity between these tumours and oncocytoma (Fig. 11.25.1.11).

Less common subtypes.

Other subtypes of RCC are much less common but some may show distinct imaging findings. Multilocular cystic RCCs have excellent prognosis and lack mural nodules within the cystic components, unlike clear cell RCCs with cystic degeneration which show mural nodules. Collecting duct carcinomas, on the other hand, are aggressive tumours with poor prognosis and have medullary origin, and therefore appear similar to transitional cell carcinomas on imaging. Medullary RCCs are associated with sickle cell disease and sickle cell trait and are seen as infiltrating intracalyceal obstructive lesions with associated nodal disease.

Imaging work-up of suspected RCC – what the treating physicians expect from radiologists

The imaging work-up of a suspected RCC is aimed at:

1. 1. Ruling out benign lesions and other malignant lesions:
   
   
   

   
   
   • The first step in incidentally diagnosed small renal lesions is to rule out imaging differentials. In solid renal lesions, angiomyolipomas (AMLs), oncocytomas and inflammatory pseudotumours are the most common benign imaging differentials while urothelial carcinoma, lymphoma and metastases are the other malignant imaging differentials.
     
   • Lipid-rich AMLs show classic appearances of macroscopic fat content on USG (echogenic lesions), CT (fat density in lesions) and MRI (signal drop on fat-suppressed images with India ink artefact at the lesion-renal parenchyma interface) (Fig. 11.25.1.12). The lipid-poor AMLs, on the other hand, may present a challenge, may contain microscopic or no fat, hence may or may not show signal drop on chemical shift imaging in opposed phases. These lesions generally show hyperenhancement.
     
   • Oncocytomas show classic delayed enhancing central scar in one-third of cases, but this is seen in the chromophobe variant RCC as well (Fig. 11.25.1.13). Oncocytomas may show segmental enhancement inversion – an admixture of enhancing areas on corticomedullary phase which washout on delayed phase on one hand, and hypoenhancing areas on corticomedullary phase with delayed phase enhancement on the other hand. Oncocytomas and oncocytoma-chromophobe RCC hybrid lesions show an uptake in ⁹⁹Tc-Sestamibi SPECT-CT scans, which can help distinguish them from other renal lesions.
     
   • In general, ADC values of RCCs have been found to be lower than benign lesions.
     
   • Apart from RCC, other malignant tumours of the kidney which need to be considered in differential diagnosis include urothelial carcinoma, lymphoma and metastases.
     
   • Although earlier stage urothelial carcinomas can be readily characterized as such due to their appearances of wall thickening and filling defects in the renal pelvicalyceal system, advanced urothelial carcinomas are difficult to differentiate from RCC due to their infiltrative nature, which gives it a similar appearance as advanced RCC (Fig. 11.25.1.14). One may look for skip lesions in the rest of the urinary tract if urothelial carcinoma is suspected.
     
   • Renal lymphomas are most often seen as secondary involvement in non-Hodgkin’s lymphoma (NHL) and may be seen as solitary or multiple masses or infiltrative renal disease. These show homogeneous appearance and are hypoenhancing compared to the renal parenchyma on CT and MRI. Retroperitoneal involvement including nodal disease can also be seen.
     
   • The common sites of primary metastasizing to kidney include lung, breast, female genital tract, head and neck, colon and prostate. These are typically located at the corticomedullary junction and are ill-defined. They show lesser enhancement as compared to RCC; however, hypervascular primaries such as thyroid and choriocarcinoma may show metastases with brighter enhancement.
   
   
2. 2. Deciding if biopsy is needed:
   
   
   

   
   
   • Despite advances in imaging, there could be a need for biopsy in a renal mass to decide further course of action. The decision for biopsy is taken by giving due consideration to the age of the patient, imaging appearances, interval change in lesions and implications for patient management and imaging. AUA 2017 guidelines have described a wider role for renal mass biopsy and recommend considering biopsy if haematologic, metastatic, inflammatory or infectious aetiology is suspected. Biopsy can be CT-guided or USG-guided and can be decided as per imaging appearances and institutional practices.
   
   
3. 3. Planning the optimum treatment for the patient taking into account the stage of disease and preexisting conditions if any:
   
   
   

   
   
   • The American Joint Committee classification on cancer staging system gives staging of RCC (Tables 11.25.1.2 and 11.25.1.3).

TABLE 11.25.1.2

Staging of Renal Cell Carcinoma

PRIMARY TUMOURS (T)
TX	Primary tumour cannot be assessed
T0	No evidence of primary tumour
T1	Tumour ≤7 cm in greatest dimension, limited to the kidney
T1a	Tumour ≤4 cm in greatest dimension, limited to the kidney
T1b	Tumour >4 cm but ≤7 cm in greatest dimension, limited to the kidney
T2	Tumour >7 cm in greatest dimension, limited to the kidney
T2a	Tumour >7 cm but ≤10 cm in greatest dimension, limited to the kidney
T2b	Tumour >10 cm, limited to the kidney
T3	Tumour extends into major veins or perinephric tissues but not into the ipsilateral adrenal gland and not beyond the Gerota’s fascia
T3a	Tumour grossly extends into the renal vein or its segmental (muscle-containing) branches, or tumour invades perirenal and/or renal sinus fat but not beyond the Gerota’s fascia
T3b	Tumour grossly extends into the vena cava below the diaphragm
T3c	Tumour grossly extends into the vena cava above the diaphragm or invades the wall of the vena cava
T4	Tumour invades beyond the Gerota’s fascia (including contiguous extension into the ipsilateral adrenal gland)
REGIONAL LYMPH NODE (N)
NX	Regional lymph nodes cannot be assessed
N0	No regional lymph node metastasis
N1	Metastasis in regional lymph node(s)
DISTANT METASTASIS (M)
M0	No distant metastasis
M1	Distant metastasis

TABLE 11.25.1.3

Anatomic Stage/Prognostic Groups

Stage	T	N	M
I	T1	N0	M0
II	T2	N0	M0
III	T1–2	N1	M0
	T3	NX, N0 or N1	M0
IV	T4	Any N	M0
	Any T	Any N	M1

Localized renal cancer

Localized renal cancer is defined as a disease confined to the renal capsule and refers mainly to stage I and II disease.

Nearly 70% of RCCs, especially the lower-stage lesions, are incidentally diagnosed on cross-sectional imaging. Also, amongst incidentally diagnosed renal lesions less than 4 cm in size, about 20% turn out to be benign on histopathology. Asymptomatic incidentally diagnosed small renal masses have an indolent course and better prognosis. Nephron-sparing surgery (NSS) has gathered momentum in recent years due to promising results and prognoses in small lesions. The 2017 AUA guidelines for localized renal masses describe restricted and well-defined indications for radical nephrectomy, bigger role of nephron-sparing procedures such as partial nephrectomy, tumour enucleation and thermal ablation, as well as increasing role for biopsy as well as active surveillance of such lesions. Hence, imaging findings in these lesions become critical in charting management of these patients.

The imaging features of common histopathological subtypes of RCCs have already been discussed above. Signal intensity on T2W images and corticomedullary phase enhancement have been seen to be independent predictors of clear cell and papillary RCCs. Further, T2 signal homogeneity can be a predictor for slower growth rate. Hence, in general, multiparametric MRI studies have been shown to be effective in small renal mass characterization and can subsequently guide decisions regarding biopsy, surgery or surveillance. CT is a good alternative in patients with contraindication to MRI.

For cystic renal lesions, the Bosniak classification, which stratifies the risk of neoplasia in cystic renal lesions based on the complexity of their appearance (wall thickness, septations, solid component), can be used effectively to decide further course of management. The Bosniak classification originally applies to CT findings but can logically be extrapolated to MRI, USG and Contrast Enhanced Ultrasound (CEUS) as well. Bosniak I and II cysts are benign while Bosniak IIF, III and IV cysts show progressively increasing risk of neoplasia. Given the more indolent course of cystic RCCs as compared to solid lesions and possible complications of interventions, lately there has been a case for even the type III and IV cysts, which previously would be operated, to be followed up, especially if patient has existing comorbidities or if the solid component is minimal. Initial follow-up would be at 6 months, followed by annual imaging.

AUA 2017 guidelines recommend considering renal mass biopsy if haematologic, metastatic, inflammatory or infectious aetiology is suspected.

Once the need for surgery is established in a localized disease, NSS may be considered for stage Ia and Ib disease. To predict perioperative outcomes in NSS, various scoring systems have been proposed for preoperative renal mass evaluation, such as R.E.N.A.L. nephrometry score, PADUA score (Preoperative aspects and dimensions used for anatomical classification), C-index method and mathematical tumour contact surface area (CSA). The popular R.E.N.A.L. nephrometry score takes into account various tumour descriptors that help decide the technical feasibility of NSS and predict surgical outcomes. These include tumour radius, exo/endophytic location, nearness to collecting system or renal sinus, anterior/posterior location and location with reference to polar lines. These descriptors need to be commented upon diligently while reporting renal masses (Table 11.25.1.4). Higher scores are seen to correlate with ischaemia time, postoperative urologic complications, higher grade and mortality.

Larger organ-confined disease and locally advanced RCC

These include organ-confined tumours more than 7 cm in size, or tumours of any size which show regional nodal involvement and/or invasion of perinephric tissues but confined to Gerota’s fascia. These include renal vein and IVC invasion (luminal and mural). Generally, radical nephrectomy is recommended by the National Comprehensive Cancer Network (NCCN) in stage II and III RCCs.

Both CT and MRI are effective in diagnosing locally advanced disease in RCC. The loss or interruption of pseudocapsule is seen in more infiltrative and aggressive disease and is best seen on MRI. Local infiltration may be in the form of contiguous spread to perinephric tissues or discrete deposits in perinephric fat (Fig. 11.25.1.15). IVC or renal vein invasion could be in the form of intraluminal thrombosis or invasion of the vessel wall and have implications on surgical approach and outcomes. Tumour thrombus can be distinguished from bland thrombus by confirming vascularity within the thrombus on imaging. Doppler evaluation, CEUS, CT and MRI can all be helpful for the same. MRI is better than other modalities for venous evaluation, especially for mural invasion and IVC invasion (Figs. 11.25.1.16 and 11.25.1.17). Right-sided tumours, anteroposterior IVC diameter of 2.4 cm or more at the level of renal hilum and complete IVC occlusion at this level are associated with higher risk of IVC resection.

Sample reporting template for renal mass

Treatment of RCC

RCC is a relatively radiation-resistant tumour, and treatment options tilt in favour of surgical procedures for localized and locally advanced diseases. For metastatic disease, post-risk assignment, the treatment has to be planned. Treatment can be offered as per size and the clinical stage.

Related posts:

Percutaneous cholecystostomy Pyonephrosis Parasitic infestation in gut Ureter Recent advances in reproductive system Radiological techniques in hepatobiliary imaging

Stay updated, free articles. Join our Telegram channel

Join