Active surveillance of small renal masses

Abstract

Most renal masses incidentally detected by cross-sectional images are benign, being mainly cysts, and if they are malignant, they are indolent in nature with limited metastatic potential. Enhanced renal masses less than 4 cm in size are known as small renal masses (SRMs), and their growth rate (GR) and the possibility of developing metastasis are extremely low. Delayed intervention of SRMs by closed and routine imaging follow-up known as active surveillance (AS) is now an option according to urological guidelines. Radiologists have a key position in AS management of SRMs even unifocal and multifocal (sporadic or associated with genetic syndromes) and also in the follow-up of complex renal cysts by Bosniak cyst classification system. Radiologists play a key role in the AS of both unifocal and multifocal (sporadic or associated with genetic syndromes) SRMs as well as in the follow-up of complex renal cysts using the Bosniak cyst classification system. Indeed, radiologists must determine which patients with SRMs or complex renal cysts can be included in AS, establish the follow-up radiological test algorithm to be used in different scenarios, perform measurements in follow-up tests, and decide when AS should be discontinued. The purpose of this article is to review the indications and management of AS in SRMs, especially focused on specific scenarios, such as complex renal cysts and multifocal renal tumors (sporadic or hereditary). In this work, the authors aimed to provide a thorough review of imaging in the context of active surveillance of renal masses.

Keywords: Delayed intervention; Diagnostic imaging; Elderly; Renal cell carcinoma; Small renal mass.

Fig. 1

Definition, classification, and therapeutic approaches of localized renal masses. Localized renal masses were considered to be all the masses limited to the kidney which means no perinephric or renal sinus fat invasion. T1a, T1b, and T2 correspond to the TNM classification

Fig. 2

The main differences between active surveillance (AS) and watchful waiting (WW) in the monitoring of small renal masses. Active surveillance is defined as the initial management including the monitoring of renal tumor size by serial imaging with delayed treatment in case of progression. In WW, close monitoring with imaging techniques is not routine, curative treatment is not administered, and active treatment is only indicated on the appearance of symptoms

Fig. 3

Renal masses that do not need to be followed. a Axial contrast-enhanced CT showing a right renal too small to characterize mass (TSTC) (arrow); the renal mass is 8 mm in size, and the CT slice thickness is 5 mm, making the renal mass size less than twice the reconstructed CT thickness (8 mm/10 mm). b Axial contrast-enhanced CT showing a hypodense left kidney lesion measuring 3 HU (less than 20 HU), which is likely a simple cyst (arrow). c Axial non-enhanced CT showing a hyperdense left kidney cyst with an attenuation value of 85 HU greater than 70 HU, consistent with a hemorrhagic cyst (arrow). d Contrast-enhanced US of a Bosniak I cystic lesion showing no wall thickening or septa (arrow). e Unenhanced (right) and contrast-enhanced (left) US of a renal Bosniak II cystic lesion with thin septa that did not enhance in the post-contrast study (arrows)

Fig. 4

Indeterminate renal mass by CT in the right kidney. a Axial CT in the arterial phase depicts a renal tumor with 58 HU consistent with the indeterminate lesion (arrow). b Unenhanced and (c) contrast-enhanced ultrasound (US) of the same renal tumor showing the presence of a cyst with thin septa with no contrast uptake corresponding to a Bosniak II lesion (arrow): follow-up was not required

Fig. 5

Indeterminate renal mass by CT in an 82-year-old patient. a Axial unenhanced and (b) enhanced CT showing a renal mass with attenuation greater than 20 HU in unenhanced CT that enhances less than 20 HU after contrast administration, considered indeterminate by CT (arrows). c Unenhanced and (d) contrast-enhanced ultrasound (US) of the same lesion clearly depicting enhancement of this lesion consistent with a solid renal tumor (arrows), thereby allowing an indeterminate CT renal mass to be reliably classified as a solid mass

Fig. 6

Solid renal masses in two elderly patients with regular and irregular borders. a Axial contrast-enhanced CT in the arterial phase shows a hyperdense right renal mass with smooth margins and regular shape (arrow) which, if indicated, can be managed with active surveillance. b Axial contrast-enhanced CT scan in the nephrographic phase depicting a hypodense renal mass with irregular borders (arrow). Active surveillance is not indicated in this case

Fig. 7

Suggested active surveillance management algorithm. Contrast-enhanced computerized tomography (CECT), ultrasound (US), and magnetic resonance (MR)

Fig. 8

Comparison of maximum tumor diameter (MTD) in the axial plane of a small renal mass (SRM) with different imaging modalities (arrows). a Sagittal ultrasound (US) view. b Axial contrast-enhanced CT scan and (c) axial contrast-enhanced T1-weighted MR of the same renal lesion demonstrating there are no significant differences between US, CT, and MR with the use of the same plane for measuring MTD (3.33 cm, 3.46 cm, and 3.29 cm, respectively). Note that the differences between different radiological tests are less than 0.2 cm

Fig. 9

Formula of linear growth rate (GR) per year. To define the linear GR per year, divide the difference between the maximum tumor diameter (MTD) into two time points by the difference of the number of months between the two CT times and then multiply by 12

Fig. 10

Linear GR calculation of a SRM in AS in a 73-year-old patient. a Contrast-enhanced CT of a renal mass (arrow) with a MTD of 1.7 cm (16.95 mm) at the beginning of AS and (b) 3 months later (arrow), showing a MTD of 2.4 cm (23.96 mm). Using the linear GR formula, the difference of MTD (2.4 cm − 1.7 cm = 0.7 cm) is divided by the time between the two CT scans (3 months), and the result (0.3 cm) is multiplied by 12, resulting in 2.6 cm. c Taking into account that the mass presented a GR of more than 0.5 cm/year, this mass should be considered as progressive, and AS should be discontinued. Following evaluation by a multidisciplinary committee, the patient underwent partial nephrectomy (arrow)

Fig. 11

Calculation of the linear growth rate (GR) of a lower third left kidney small renal mass (SRM) undergoing active surveillance (AS) in a 65-year-old patient (arrows). a Coronal contrast-enhanced CT of the renal mass showing an axial MTD of 2.5 cm (24.89 mm) at the beginning of the surveillance and (b) 3 which had only increased to 2.5 cm (25.44) 3 years later. With the use of the linear GR formula in centimeters, 2.5 cm − 2.5 cm = 0 cm divided by 36 months and multiplied by 12, the annual GR was shown to be 0 cm/year, indicating that the renal mass was stable. After 5 years of follow-up, the interval between imaging tests can be lengthened or even discontinued according to the decision of the multidisciplinary committee

Fig. 12

Comparison between linear and volumetric measurements of small renal mass (SRM) growth over a 6-month period. Upper- and bottom-left: axial contrast-enhanced CT showing a renal mass with MTD 3.9 cm at the beginning of AS and 5.3 cm in the next study 6 months later with a linear GR of 1.4 mm between the two studies corresponding to a linear GR of 3.8 cm/year (arrows). Volumetric representation of the renal mass in an axial plane (thick arrows) (upper- and bottom-middle) and coronal plane (arrowheads) (upper- and bottom-right). The software showed an initial volume of 21 cc and a final volume of 53 cc. The increase in volume was 32 cc, indicating that the mass had doubled in volume over a 6-month period representing a rapid growth, although there are no established volume values to define progression

Fig. 13

Definition of active surveillance progression according to the American Society of Clinical Oncology (ASCO) and American Urological Association (AUA) guidelines

Fig. 14

Schematization of Bosniak’s classification for cystic renal masses and associated malignancy rate. Bosniak I: hairline-thin wall without septa, calcifications, or solid components; malignancy rate of 0%. Bosniak II: few hairline-thin septa, fine calcifications in a short segment of the wall, or slightly thickened calcification; malignancy rate of 0%. Bosniak IIF: multiple hairline-thin septa, smooth minimal thickening of the wall or septa, and thick or nodular calcifications; malignancy rate of 5%. Bosniak III: thickened irregular wall or septa with enhancement after the administration of contrast agent; malignancy rate of 50–70%. Bosniak IV: soft tissue enhancing mass independent of the wall or septa; malignancy rate of 95–100%. Remember that Bosniak I–II cysts do not need follow-up, Bosniak IIF cysts need follow-up, and surgery is indicated for Bosniak III–IV cysts

Fig. 15

Bosniak IIF, differentiation between low and more complex lesions. a Axial contrast-enhanced CT depicting a large cyst with a solitary thick septum. The cyst should be followed because the septum is thick, but suspicion of malignancy is very low (arrow). b Axial contrast-enhanced CT demonstrating a cyst with multiple septa. This complex appearance does not completely rule out a possible cystic tumor (arrow)

Fig. 16

Bosniak IIIs (a, b) and IIIn (c) lesions. a Axial T2-weighted MR showing a multiseptated cystic renal lesion (arrow) and (b) axial T1 post-contrast MR of the same lesion showing septal enhancement (arrow) consistent with a Bosniak IIIs cyst. c Axial contrast-enhanced CT of a cystic renal mass with septae nodularity (arrow) considered to be a Bosniak IIIn cyst

Fig. 17

Suggested Bosniak cyst classification management algorithm

Fig. 18

Cystic Bosniak IIF renal mass (arrows) presenting growth over time and showing malignant features with an occurrence rate of 5%. a Unenhanced US showing a cystic renal lesion with thickened septa. b Unenhanced US of the same lesion 1 year later showing increasing nodularity in the previously thickened septa, consistent with progression

Fig. 19

Contrast-enhanced US image showing a cystic renal lesion in the wall of the left kidney suspected of being a solid lesion. a Coronal contrast-enhanced CT showing a cystic renal mass with a hard-to-define image in its wall (arrow). c Unenhanced and (d) contrast-enhanced US clearly depicting enhancement on this solid nodule (arrows) in the cyst wall, leading to the lesion being defined and classified as a Bosniak II lesion

Fig. 20

Image suspected of being a complex hemorrhagic cyst larger than 3 cm in size, which was classified as Bosniak IIF, and then later reclassified as a Bosniak II cyst by contrast-enhanced US. a Axial T2 MR showing a complex renal cyst with thick septa (arrow). b Unenhanced US of the same lesion showing cystic component and echogenic material within the cyst (arrow). c Contrast-enhanced US showing the absence of enhancement of septa (arrow). The lesion was reclassified as a Bosniak I simple cyst not requiring follow-up

Fig. 21

Multifocal small renal masses (SRM), genetic syndromes classification, type of associated renal tumors, and suggested follow-up by active surveillance (AS)