Imaging approach
4.3 Incidental renal masses
Table 4.1 Features of classic renal carcinoma
Solid, may be higher or lower attenuation than normal parenchyma precontrast Clear enhancement, often very vascular and heterogeneous
Calcification common (20%)
Tendency for renal vein and caval invasion Nodal disease
Liver, bone lung metastases
MRI: medium signal intensity T1, may be hemorrhagic high signal areas;
heterogeneous high signal T2, heterogeneous enhancement following intravenous gadolinium compounds
◦ Technique
CT scanning should be done with images both before and after intravenous con- trast.
Because hypervascular liver metastases may occur, imaging in arterial as well as portal venous phases is preferred.
Such masses are easily detected, and have typical imaging features (Table 4.1).
• If a patient cannot tolerate intravenous iodinated contrast for any reason, MRI without and with intravenous gadolinium contrast including T1- and T2-weighted images is an excellent alternative, with diagnostic accuracy similar to CT and perhaps better staging accuracy.
• MRI is particularly effective at detecting or excluding venous invasion and graphically dis- playing the exact extent (Fig. 4.2). Most masses in this category are readily recognizable as malignant, typical renal carcinomas (RCC).
4.3 Incidental renal masses
• Very commonly, as non-invasive imaging is used for an ever wider variety of indications, a renal mass may be detected when unsuspected.
• An increasing proportion of RCC are discovered incidentally, and such tumors tend to be smaller, lower stage and more curable than those presenting symptomatically.
• There are many non-malignant etiologies of incidental renal masses.
• A solid mass or complex renal cyst seen at sonography may need further evaluation with CT (or MRI) done with images before and after contrast.
• Doppler evidence of flow within the mass, including flow within septations in a cystic mass, usually indicates the mass is malignant (Fig. 4.3), although benign renal lesions (including angiomyolipoma and oncocytoma) may show internal flow.
• The incidental renal masses should fall into one of several categories after diagnostic CT (or MRI): Clearly malignant (usually RCC), clearly benign (including typical simple cyst), or indeterminate.
(a)
(c)
(b)
Figure 4.2 A 61-year-old female with right abdominal mass on exam, gross hematuria and flank pain. A right renal mass with probable caval extension of uncertain degree was seen on CT. (a) An axial T1 MRI shows a heterogeneous medium signal intensity mass (arrows) arising from the right kidney (arrowheads remaining normal parenchyma). The mass bulges medially into the location of the inferior vena cava. (b) On fat suppressed T2-weighted image the tumor is of overall high signal and very heterogeneous typical of RCC. No liver metastases or adenopathy were evident. (c) Coronal gadolinium enhanced T1-weighted image shows the mass enhances, and clearly extends into the inferior vena cava, with enhancement of the tumor thrombus (arrows).
The upper extent of the tumor thrombus is clearly shown at the diaphragm (arrowheads) but not extending into right atrium. This large stage 3B clear cell carcinoma was successfully resected and the patient shows no recurrence 2 years later.
4.3 INCIDENTAL RENAL MASSES 57
(a)
(b) (c)
Figure 4.3 This 54-year-old female had sonography done because of abdominal pain later discovered to be diverticulitis. (a) There is a left lower pole hypoechoic mass(arrows) with flow in septations on color Doppler(arrowheads). (b) Unenhanced CT image shows the lobular mass with fleck of calcification; attenuation 41 Hounsfield units (HU). (c) Coronal reformatted image of post-contrast CT shows the exophytic mass with septal enhancement (attenuation 66 HU).
This typical cystic RCC should be classified Bosniak Type 4 by imaging.
◦ Often, it is readily recognizable that the mass is most likely a primary renal carcinoma, with typical features as described in Table 4.1 (Fig. 4.4).
◦ It is often possible to determine that a mass is a recognizable benign entity:
Benign cysts are very common and can usually be recognized as such.
The classification developed by Bosniak may be useful to place cysts in appropriate category according to risk of malignancy (Table 4.2) (Figs 4.3 and 4.4).
A renal mass containing true fat is almost always a benign angiomyolipoma (Fig. 4.5).
(a) (b)
(c) (d)
Figure 4.4 This 55-year-old male had routine contrast enhanced CT after motor vehicle collision.
(a) A probably simple cyst is seen at lower pole of left kidney. (b) A smaller lesion (arrow) at lower pole of right kidney appears similar in attenuation to renal parenchyma worrisome for neoplasm.
(c) Follow-up CT after contrast showed enhancement in the right lower pole mass (was 36 HU before and 78 HU after contrast). The left sided lesion showed no contrast enhancement (13 HU before and after contrast). A 1.8 cm clear cell carcinoma was removed with partial nephrectomy from the lower pole of the right kidney. The left renal lesion was followed up. (d) Follow-up CT 3 years later shows the cyst is stable although has developed peripheral calcification.
4.3 INCIDENTAL RENAL MASSES 59
Table 4.2 Bosniak Classification
Category 1: meets all criteria of simple cyst, 100% should be benign
Category 2: minimally complicated cyst-lack of through transmission, internal echoes, high attenuation on CT, thin septae, few small calcifications on septae or wall, but no enhancing solid component: 90–95% benign
Category 3: Moderately complex cyst-more internal echoes, thick or irregular septations, thick or nodular wall, extensive calcifications, questionable enhancement (8–15 HU change): 50%
malignant
Category 4: Cystic malignancy: complex largely cystic mass but with definite solid component with flow on US, or definite enhancement on CT or MRI
◦ Sometimes incidental renal masses do not fall into either clearly malignant or clearly benign lesions:
Complex cysts (Fig. 4.3) including hemorrhagic cysts or lesions with questionable enhancement.
Multilocular cystic nephroma produces a multiseptated solitary mass in a single kidney-although benign such lesions are difficult to distinguish from cystic RCC.
A multicystic dysplastic kidney in the adult is usually seen as a small collection of cysts in the renal fossa without function and usually no enhancement-these are usually incidental and can be followed (Fig. 4.6).
Enhancement after contrast is the most critical feature of a malignant lesion.
• An increase of over 15–20 Hounsfield units (HU) after contrast is virtually certain enhancement; a change of less than 10 is not considered significant.
• Small increase in attenuation (10– 15 HU) can result artifactually from volume averaging with small lesion, or pseudoenhancement.
• The majority of renal masses of soft tissue attenuation with clear enhancement are malignant, however, benign renal masses uncommonly occur with those charac- teristics, including oncocytoma, lipid poor angiomyolipomas and leiomyomas.
• PET is of limited use, as only about half of the malignant renal masses show abnormal uptake (Fig. 4.7).
• Oncocytomas
◦ Small oncocytoma demonstrate features and enhancement similar to small RCC
◦ Large oncocytoma often have central scar that simulates central necrosis, so that these are often treated as if RCC.
◦ Diagnosis may be possible with biopsy especially if immunostains are done, but it is controversial whether all solid renal masses should be biopsied.
• Lesions that remain indeterminate after optimal quality CT or MRI often are followed.
◦ Over time lesions may evolve, making diagnosis of RCC possible, and if small when presenting rarely metastasize until well over 3 cm.
(a) (b) (c)
(d) (e)
(g)
(f)
Figure 4.5 Sonography was requested due to abnormal liver enzymes. (a) An exophytic hyperechoic (3.4×3.7 cm) mass protrudes from the left kidney (arrows). (b) Internal flow is shown on color Doppler. (c) MRI T1 axial image shows hyperintensity within the mass (arrow).
(d) Opposed phase image shows areas of signal drop within the mass, indicating fat. (e) The mass is low attenuation on fat suppressed T2 image, excluding RCC. (f) One year follow-up was done with CT due to pain; unenhanced image shows the lesion is of fat attenuation. (g) Some enhancement is noted following contrast; note the markedly exophytic growth pattern with small footprint on the kidney, typical of many AML’s. Although this is clearly a benign AML, due to increasing size (4.8×5.2 cm) it was embolized.
4.3 INCIDENTAL RENAL MASSES 61
(a) (b)
Figure 4.6 Due to back pain, CT was done in this 30-year-old male. (a) A non-functioning atrophic left kidney with numerous cysts is noted. (b) Follow-up 3 years later shows the lesion is stable, with no enhancement on post-contrast images. This is typical appearance of multicystic dysplastic kidney presenting in adulthood.
(a) (b)
Figure 4.7 This 48-year-old female developed right flank pain. (a) Unenhanced CT was done searching for kidney stones. No stone or evidence of obstruction was seen, but a suspicious area of fullness in mid right kidney (arrowheads) which measured 32 HU was noted. (b) Further evaluation with enhanced CT shows the lesion is homogeneously enhancing, 104 HU, with no evidence of metastases. (c) Since CT showed a pulmonary nodule, a PET/CT was carried out. The mass showed no FDG uptake. Nephrectomy was done revealing Stage 1 granular cell RCC.
(c)
Figure 4.7 (Continued)
◦ How long a renal mass must be followed remains controversial; since some low grade RCC is slow growing. At least 2 years and possibly 5 years follow-up may be advised.