Patients
Ethical approval was obtained from Institutional Review Board of the Xiangya Hospital, Central South University. This retrospective study was performed in accordance with the provisions of the Declaration of Helsinki. The requirement for informed consent was exempted by the Institutional Review Board of the Xiangya Hospital, Central South University as all data was analyzed anonymously (number 2018111101). From June 2011 to November 2020, 17 patients pathologically diagnosed with MESTK (13 women and 4 men,the median age was 45.12 ± 10.77 years (range from 21 to 61y), who underwent preoperative multidetector CT (MDCT) within 15 days before the surgical procedure were recruited. The clinical information (the follow-up time, gender, age, clinical symptoms, menstrual status, surgery methods (PN and RN), and and R.E.N.A.L. Nephrometry Score (R.E.N.A.L.-NS)) were collected (R= Radius (tumor size as maximal diameter); E = Exophytic/endophytic properties of the tumor; N = Nearness of tumor deepest portion to the collecting system or sinus; A = Anterior (a)/posterior(p) descriptor; and L= Location relative to the polar line(13)). For R.E.N.A.L-NS, the range of 4 to 6, 7 to 9 and 10 to 12 were deemed low, moderate and high complexity lesions, respectively. The beginning and end of the follow-up period was the time the patient had surgery and the latest MDCT or ultrasound examination in our hospital.
MDCT techniques
All patients carried out plain and contrast enhancement MDCT scan in our hospital (128 slices, Somatom Definition, Siemens Healthineers, Erlangen, Germany; 320-slice, Toshiba Aquilion ONE, Canon Medical Systems, Otawara, Tochigi, Japan). The scanning parameters were as follows: slice thickness of 1mm, slice gap of 0mm, the pitch of 1.2, 100kVp and 200 mA for Somatom Definition and Aquilion ONE. For enhanced MDCT, a non-ionic iodinated contrast agent (iopromide, Ultravist; Schering AG, Berlin, Germany) was used at a rate of 3.5-4ml/s (1.5mL/kg, 80-100ml). The corticomedullary phase (CMP) (30s), the nephrographic phase (NP) (70s), and the excretion phase (EP) (3- 5 min) were obtained for each subject after injection of the contrast agent.
Imaging analysis
All MDCT images were transferred into the imaging workstation (Advantage
Workstation 4.4, GE Healthcare, Buc, France) and image post-processing (sagittal and coronal images) was performed. Two experienced radiologists (J.C. and G.P, reviewers 1 and 2 with 5 and 7 years of clinical experience in kidney MDCT, respectively) assessed the imaging characteristics without knowing the clinical and pathological information,including the maximal diameter (MD), shape, location, calcification, septa state, mural nodule, capsule of the tumor, renal sinus fat invagination (SFI), and the enhancement degree and pattern. SFI is defined as the direct contact of the tumor with the renal sinus stroma or fat cell (14), which appears as an invasion of the fat tissue of the renal sinus on MDCT. But it is not considered invasive if the tumor impinges on (but is separated from the fat by a connective tissue layer) the perinephric or renal sinus fat (15). Enhancement patterns were evaluated and analyzed with the CT attenuation for each phase for all MESTK. A region of interest (ROI) was drawn on the solid component of the tumor (size: 40- 60 mm2) on EP images, avoiding the cystic and calcified parts in the tumor. Then, it was copied to plain, CMP, and NP images of the same slice. For each subject, the above measurements were carried out two more times on different occasions within a week, and the average CT attenuation was calculated for each phase to obtain the enhancement pattern (wash out or delayed enhancement). Gradual enhancement pattern was considered present when the tumor attenuation in the nephrographic phase was at least 20 HU greater than that in the corticomedullary phase (16), while gradual washout pattern was defined when the CT value of the subsequent phase was reduced to less than 20 Hounsfield units (HU) (17). The degree of enhancement was defined as the difference between the attenuation value of the unenhanced scan and the CMP. A difference higher than 50 HU was classified as marked, between 20 and 50 HU as moderate, and less than 20HU as weak enhancement (6).
A threshold value of 25% (proportion of solid components in a cyst-solid tumor) is used in the Bosniak classification (version 2019) of renal cystic masses with solid components (18, 19). The Bosniak classification can be used for patients with cyst-solid mass when the proportion of solid part ≤ 25%. But, the Bosniak classification cannot be applied when the percentage of the solid element > 25%. However, it is difficult for radiologists to estimate - qualitatively - the volume percentage of the solid part on MDCT. The volume of mass and solid components are relative to their respective diameter (D) (V = (4/3)πR3 and R = D/2), So, the maximal diameters (MD) were used to replace the corresponding volume in our study, and the equation is as follow:
Ratio = MDsolid / MDtumor (1)
where MDtumor is the maximal diameter of tumor in the slice in which the mass has the greatest size and MDsolid is the maximal diameter of the solid part in the same slice. When the ratio of the MD of the solid part in mass ≤ 63%, it was classified into type B (can be classified with Bosniak classification), and into type A when the ratio > 63% (cannot be classified with Bosniak classification). The detailed illustration is shown in Fig.1.
For lesions classified into type B, the number of septa, septal thickness, septal enhancement, wall thickness, wall enhancement, and mural nodularity on MDCT were assessed, and Table 1 gives the reference standard during image analysis. Lesions were classified into I-IV category with Bosniak Classification(18).
Statistical analysis
An independent-sample t-test was used to assess differences in the MD, age, and R.E.N.A.L.-NS between type A and type B. The Fisher exact test was used to analyze gender, clinical symptoms, surgery methods, shape, RSI, calcification, septa, mural nodules, capsules, and enhancement patterns in the two groups. Statistical analyses were performed with SPSS 18.0 (SPSS Inc., Chicago, IL, USA). P-value > 0.05 was considered not statistically significant