Inside an incidental solid renal mass

An 89-year-old male patient, complaining of unspecific abdominal pain and hematuria, came to the hospital for a checkup. Physical examination and laboratory tests suggested a prostatic hyperplasia as the cause of hematuria. For further assessment and to rule out any other potential cause of hematuria, an abdominal contrast CT scan on a dual source photon-counting CT (PCCT), NAEOTOM Alpha®, was performed.

Standard CT images (Monoenergetic Plus images displayed at 70 keV, corresponding to conventional CT images acquired at 120 kV) showed an incidental renal mass in the right kidney, homogeneously enhanced (75 HU), well marginated with an imperceptible wall, measuring 2.0 x 1.6 cm² in size. The enhancement pattern in the arterial, venous, and delayed phases showed no remarkable differences. In the virtual unenhanced (VNC) images, the mass was slightly hyperdense (49 HU), with no calcifications present. Monoenergetic Plus images displayed at 40 keV revealed a significantly enhanced small lesion (112 HU), dorsally in the mass, invisible in the standard CT images, measuring 0.4 x 0.5 cm² in size. In the iodine maps fused with VNC images, the iodine density was measured to be 0.92 mg/mL for the mass and 1.8 mg/mL for the small lesion. In the 3D visualization, using thin maximum intensity projection (MIP) and cinematic volume rendering techniques (cVRT), a tiny branch of the renal artery supplying the mass was depicted. CT findings were compatible with a renal neoplasm. However, due to the patient’s age and physical condition, no immediate surgery nor further examinations were performed. Medication for the prostatic hyperplasia and surveillance for the renal mass were recommended.

Fig. 2: A cVRT image (Fig. 2a) and a thin MIP image (Fig. 2b) show a tiny branch of the renal artery supplying the mass (arrows).

Incidental findings in the kidneys are common and most of these are renal masses. [1] Solid masses are defined as those that contain little or no fluid attenuating (<20 HU) components and usually predominantly consist of enhancing tissue. [2] When hyperdensities are identified in small renal masses, it is of utmost importance to differentiate between a cyst, with only hyperdense content (hemorrhage or proteins), and a tumor with contrast enhancement. In a clinical routine, using conventional CT scanners, additional non-contrast CT or MRI must be performed to confirm or exclude post-contrast enhancement. If an incidental renal mass is visualized in contrast scans, such as this case, the patient needs to be re-scheduled for a non-contrast scan. For a proper diagnosis and subsequent patient management, a small renal mass needs to be completely characterized, which can be challenging and requires optimal iodine contrast-to-noise ratio (CNR).

This case is performed on NAEOTOM Alpha, a newly developed dual source CT scanner with photon-counting detectors (QuantaMax®), providing energy-resolved CT data with inherent spectral information and improved tissue contrasts in routine scans. [3] Owing to the improved iodine CNR, due to the missing down-weighting of the lower energy X-ray photons, the absence of the electronic noise, and the inherent spectral information, the small, enhanced lesion inside the mass, invisible and likely missed in standard CT images, is clearly depicted in the Monoenergetic Plus images displayed at 40 keV, as well as in the iodine maps fused with VNC images. The VNC images are routinely derived from the contrast scan, avoiding the necessity of having to perform a non-contrast scan or to call the patient back. Furthermore, the improved spatial resolution of PCCT facilitates the characterization of the small enhancing lesion with a diameter of only few millimeters and the tiny branch of the renal artery that supplies it. In the routine workflow, viewing of different image types, such as standard CT image, Monoenergetic Plus images displayed at different keV levels, VNC image, iodine image and iodine/VNC fused image, can be toggled interactively, using Interactive Spectral Imaging (ISI) technique, which greatly facilitates image reading and improves the efficiency of routine diagnosis. In this case, a conservative approach for the subsequent patient’s management is chosen, considering limited life expectancy and possible comorbidities, therefore the nature of the renal mass with the small lesion inside is not pathologically confirmed. However, this case demonstrates the great potential of PCCT in depicting such a small, enhanced lesion as well as a tiny supplying artery, helping the physicians to make a confident diagnosis.