The recent study underlines the great potential of PCCT in oncological imaging by enabling differentiation between adrenal adenomas and adrenal metastases, utilizing PCCT-derived image parameters from routine workflows, especially RE, which can be used in contrast enhanced scans to differentiate between adrenal adenomas and adrenal metastases.

While adrenal adenomas are typically smaller compared to metastases [21, 22], a significant difference between the adenomas (mean diameter: 15.4 mm) and metastases (mean diameter: 23.4 mm) included in the recent study was observed. Density values obtained from ea- and pv-contrast enhanced scans showed no significant differences between adenomas and metastases, which underlines the challenge of differentiating AINs in conventional contrast enhanced CT-imaging. However, density values measured from VNCa (p = 0.043) and VNCv (p = 0.027) exhibited significant differences between adenomas and metastases, possibly owing to their low microscopic or macroscopic fat [23]. Compared to further DECT-based studies like Loonis et al. [16] the recent study showed lower diagnostic performance of VNC reconstructions. For instance, this may be due to different scan parameters and calibrations. Furthermore, there is need for multicentric studies with lager cohorts. Several studies were able to demonstrate that PCCT-derived VNC reconstructions may substitute true non-contrast images in many cases [24,25,26]. However, the overall mean values of adenomas were in VNCa (20.93 HU) and VNCv (21.64 HU) higher than the suggested cut-off (20 HU) for benign adrenal lesions in unenhanced imaging suggested by the EURINE-ACT study [21]. Bette et al. demonstrated that VNC algorithms overestimate CT values compared to true-non-contrast images. Accordingly, they suggest an adapted threshold of 26 HU to differentiate adrenal adenomas from metastases [18]. In line with these findings the recent study showed density values < 26 HU for adrenal adenomas in VNCa and VNCv and values > 26 HU for adrenal metastases in VNCa (26.53) and in VNCv (28.26), results given in Table 2. Due to the limited differences in VNCa and the large standard deviations in the VNC measurements, there remains potential for improvement and a need for additional factors to achieve better differentiation of adrenal lesions. Concerning the iodine maps, no significant differences were observed between adrenal adenomas and metastases.

Several studies by Nagayama et al. and Loonis et al. introduced and validated factors as relative enhancement (RE), normalized iodine density (NID) and iodine/ VNC ratio (IVR) in DECT [15, 16]. However, there are no studies examining these promising parameters for the PCCT. These factors are calculated as described in the methods section. In this context, alongside the polyenergetic images, which are standard in diagnostics, RE also takes VNC reconstructions into account by computing a quotient based on the difference between the polyenergetic and VNC measurements relative to the VNC measurement. RE calculated from pv scans show significant differences between adenomas and metastases and show the highest AUC in our study. However, RE calculated from ea images show no significant differences between adenomas and metastases, this might be due to the fact that the lesions show no relevant contrast media uptake in the ea contrast phase, and this effects the quotient. Nevertheless, the results of the RE calculated from pv scan show a higher AUC than the conventional measurements in VNC reconstructions. Studies on DECT derived data sets from Nagayama et al. and Loonis et al. showed the same improvement by calculating RE [15, 16]. In NID no significant differences were observed between adenomas and metastases. However, NID can be used to calculate IVR, representing the quotient of NID and density in VNC. Looking at the results there is no significant difference between the lesions in ea contrast phase, which may due the small iodine uptake in this contrast phase. Measurements of pv contrast phase show significant differences between adenomas and metastases.

In total the recent study showed that PCCT derived VNC reconstructions, as RE and IVR from pv contrast phase can be used to differentiate between adrenal adenomas and metastases. These results are in line with the preliminary data from DECT [15, 16]. Given the recent results the best diagnostic performance was observed in RE (Sensitivity: 74%, Specificity: 68%). Best sensitivity was shown in VNCv density (100%) compared with a low specificity (39%). IVR derived from pv contrast phases showed also a good sensitivity (86%) with reduced specificity (55%) compared to RE. Since only two measurements and the VNC reconstructions are required for RE, it could be calculated quickly in clinical routine and provide an initial orientation for AINs. However, dedicated CT protocols for adrenal imaging can reach sensitivities and specificities up to 98% and 92% [27]. Additionally, chemical shift MRI allows for the calculation of the signal intensity index (SII), which has been shown to discriminate adenomas from metastases with an accuracy of 100% [28]. In this context, the spectral imaging measurements and calculations derived from DECT and PCCT are not yet capable of replacing these established techniques. Nevertheless, they offer a straightforward and efficient initial assessment without requiring additional imaging. Compared to MRI, PCCT provides superior spatial resolution and contrast-to-noise ratio, which enhances the visualization of small or low-contrast lesions [29, 30]. MRI, while effective in characterizing adrenal masses, particularly lipid-rich adenomas, can be limited by its lower spatial resolution and longer acquisition times [31, 32]. PCCT’s ability to generate virtual non-contrast (VNC) images and material decomposition techniques further aids in the accurate characterization of adrenal lesions, reducing the need for additional imaging studies [18, 33].Accordingly, the Canadian Urological Association and American Urological Association guidelines recommend non-contrast CT as the first-line imaging for incidental adrenal masses, with MRI as a secondary option for indeterminate cases [31]. PCCT’s advanced capabilities align well with these guidelines, offering improved diagnostic accuracy and potentially reducing the need for follow-up MRI.

In routine practice, PCCT can streamline workflows by providing comprehensive imaging data in a single scan, reducing the need for multiple imaging modalities. Its high spatial resolution and spectral capabilities allow for detailed tissue characterization, which is crucial for accurate diagnosis and treatment planning [34, 35]. Additionally, PCCT’s potential for reduced radiation dose is particularly beneficial for patients requiring frequent imaging [29, 30].

For clinical application, however, the small number of preliminary studies with comparatively small numbers of cases and the inferior specificity and sensitivity of special imaging represent a strongly limiting factor, so that dedicated imaging of the adrenal glands cannot simply be dispensed with as part of good clinical practice.

One of the main limitations of the present study is the relatively small patient cohort, which is primarily due to the limited number of PCCT examinations conducted to date. Furthermore, as the scans were obtained from routine clinical practice, true non-contrast (TNC) images were not available. This represents an additional limitation, as TNC imaging could have been used to validate the VNC reconstructions.

Despite these constraints, the current study serves well as a proof-of-concept and provides a valuable foundation for future research. In particular, expanding the sample size through multicentre studies would be an important next step. It would also be beneficial to include studies that contain native scans, which could also reach a sufficient cohort size through collaborative, multicentre efforts.

Overall, the introduction of PCCT represents a significant advance in adrenal imaging, offering the capability for adrenal mass characterization from routine scans which can be expected to improve patient safety and decrease overall radiation exposure.