Difference analysis in prevalence of incidental pancreatic cystic lesions between computed tomography and magnetic resonance imaging

We retrospectively reviewed consecutive patients who underwent an abdominal computed tomography (CT) or magnetic resonance imaging (MRI) examination at our institution during a 4-year period (January1 2013 to December31 2016). Four radiologists with 6 to 7 years of experience in abdominal imaging diagnosis reviewed the diagnostic reports in chronological order by radiology information system (RIS) and picture archiving and communication systems (PACS). If the diagnostic reports included pancreatic cystic lesions, the images would be re-evaluated by two radiologists together. The inclusion criteria for the study included: (a) availability of contrast-enhanced abdominal CT and MRI; (b) diagnosed with pancreatic cysts; (c) enough diagnostic quality of CT or MR images. Exclusion criteria consisted of: (a) patients who had a known or suspected history of pancreatic disease including pancreatic solid tumors, acute/chronic pancreatitis, etc.; (b) patients who had a history of non-pancreatic malignant tumors or cystic pancreatic lesion; (c) CT or MRI examinations for further evaluating PCLs; (d) patients who had non-specific abdominal symptoms which may be related to pancreas. The flowchart for the enrollment of this study is showed in Fig. 1.

Contrast-enhanced CT of the pancreas was as a part of the routine abdominal CT. Our study used several multi-slice CT (MSCT) equipment as follows: Siemens Somatom Sensation 16, Somatom Emotion 6, Somatom Definition AS, Siemens Healthcare, Erlangen, Germany; Toshiba Aquilion 64 or Aquilion one 320, Toshiba Medical Systems,Tokyo, Japan; GE Lightspeed VCT 64, GE Healthcare; United imaging Uihct128,China. A gantry rotation time of 0.5 s, a tube current of 150–200 mA, and a peak voltage of 120 kV were used for CT scanners. Contrast-enhanced abdominal CT including pre-contrast, arterial phase (30–35 s), portal venous phase (80s) and delayed phase (3 min) was performed in our hospital. The injection rate of contrast medium (Ultravist 300 mgI/ml or 370 mgI/ml,90-100 ml,Bayer Healthcare, Berlin, Germany) is 3-5 ml/s. Slice thickness was 1-5 mm.

Abdominal MR examinations were performed on either 1.5 T MR scanners (Magnetom Aera or Magnetom Avanto, Siemens Medical Solution, Erlangen, Germany; Uihmr1, United imaging, China) or 3.0 T MR scanners (Magnetom Verio, Siemens Medical Solution, Erlangen, Germany; Signa HDx, GE Healthcare, Milwaukee, USA; Uihmr770, United imaging, China). Avanto and Signa HDx were the most commonly used for abdominal examinations. The conventional abdominal MR included: (1) fat suppressed T2-weighted imaging (Avanto: repetition time (TR)/echo time (TE) = 3500/84 ms; slice thickness = 5 mm; slice gap = 1 mm; field of view (FOV) optimized to patients’ body habitus:285 × 214–308 × 380 mm; matrix = 168 × 320; Signa HDx: TR/TE = 4500–7100/88 ms; slice thickness = 5 mm; slice gap = 2 mm; field of view (FOV) = 400 × 300 mm; matrix = 320 × 224); (2) MR cholangiopancreatography (MRCP) (Avanto: TR/TE = 4500/758 ms; slice thickness = 4 mm; FOV = 340 × 340 mm; matrix = 180 × 320; Signa HDx: TR/TE = 7000/1228 ms; slice thickness = 5 mm; FOV = 300 × 300 mm;matrix = 288 × 288); (3) gradient echo (GRE) T1-weighted in-phase and opposed-phase imaging (Avanto: TR/TE = 6.8/2.35 (in-phase), 4.75 (opposed-phase) msec; slice thickness = 5 mm; slice gap = 1 mm; FOV = 85 × 214–308 × 380 mm; matrix = 180 × 320; Signa HDx: TR/TE = 207/2.31 (in-phase), 3.69 (opposed-phase) msec; slice thickness = 5 mm; slice gap = 1 mm; FOV = 400 × 400 mm; matrix = 192 × 256); (4) dynamic contrast-enhanced imaging (pre-contrast, arterial, portal venous and delayed phases) using 3D-GRE T1 weighted imaging with injection of gadopentetate dimeglumine (Magnevist, Bayer HealthCare, Berlin, Germany) at rate of 2-3 ml/s(Avanto: TR/TE = 5.04/2.31 msec, slice thickness = 3 mm, no slice gap, matrix = 250 × 512; Signa HDx: TR/TE = 4.1/1.4 msec, slice thickness = 3 mm, no slice gap, matrix = 200 × 352).

The following data were collected for each patient: age, sex and features of PCLs (size, location, number of cysts, communication to pancreatic duct and worrisome features such as enhancing mural nodule < 5 mm, thickened enhanced cyst walls, MPD size≥5 mm, abrupt change in the MPD caliber with distal pancreatic atrophy etc. [10]). If the lesions were multiple, only the largest one was recorded. According to guidelines, if patient has the absolute or relative indications for surgery such as jaundice, positive cytology, pancreatic duct ≥10 mm and enhancing mural nodules ≥5 mm etc., he/she should undergo surgery and/or be referred to a multidisciplinary group for further evaluation [10‐12]. If not, they could be followed up. If patient had undergone surgical resection and the pathologic result would be recorded. Samples of the PCLs detected by CT and MRI we reviewed are presented in Figs. 2 and 3.

A total of 90,599 abdominal CT and 63,769 MRI scans were performed between January 2013 and December 2016. Among of them, 54,210 of CT and 38,099 of MRI scans met our selection criteria. The mean age of the 54,210 individuals for CT and the 38,099 for MRI at that time was 55.16 ± 14.64 years and 53.94 ± 14.14 years respectively, and 50% were female in both CT and MR scans. Overall incidental detection rate was 2.51% (2320/92309) for PCLs on CT and MRI, with 10.2% (237/2320) patients with PCLs having multiple lesions. Furthermore, the PCLs were identified in 1038 patients with CT and 1282 patients with MRI examinations, representing the prevalence was 1.91% (95% CI, 1.8–2.0%) for CT and 3.36% (95% CI, 3.2–3.5%) for MRI, respectively.

Of all individuals with PCLs detected by CT, the median age of patients was 62 years (range, 8-96y) in comparison with 63 years (range, 16-94y) in the group with MRI, and about 60–61% of whom were women for CT and MRI. The mean age (62.26 ± 14.14y of CT and 60.74 ± 13.86y of MRI) of populations with PCLs was older than the mean age (55.16 ± 14.64 of CT and 53.94 ± 14.14 of MRI) of those without PCLs (P < .001, respectively). The prevalence of PCLs which were found by CT was 1.54% (418/27143) in males, compared with 2.30% (620/27067) in females with significant difference (P < 0.001), as well as MRI (2.63% = 498/18954 in males, 4.10% = 784/19145 in females, P < 0.001). Patient baseline characteristics were summarized in the Table 1.

The prevalence of PCLs increased with increasing age (P < 0.001). The maximal diameter of the largest lesions in CT and MRI varied from 1 mm to 165 mm (median size, 12 mm and 13 mm, respectively). PCLs (or the location of the largest one if lesions were multiple) detected by CT were located throughout the pancreas: uncinate (12.0%, 125/1038), head (20.2%, 210/1038), neck (13.3%, 138/1038), body (32.1%, 333/1038), tail (22.4%, 232/1038). The respective numbers for PCLs detected by MRI were 10.6%(136/1282), 22.8%(292/1282), 13.6%(175/1282), 31.9%(409/1282), 21.1%(270/1282). Furthermore, female patients had an increased frequency of PCLs within the distal gland (body and tail, P < 0.001 with CT and MRI, respectively). The location of PCLs and detection rate by age and gender were also provided in the Table 1 and Fig. 4.

The dilatation of the pancreatic duct was reported about 11% in patients both for CT and MRI. The portion of lesions communicated with the pancreatic duct in MRI was 16.5% (211/1282), compared to 10.6% (110/1038) in CT (P < 0.001). The detection rate of multiple lesions in CT and MRI was 8.0 and 12.0% (P = 0.001), respectively. About 1.4 and 3.1% of cysts had solid portion such as the thickened septa or mural nodularity which may represent malignant degeneration in CT and MRI (P = 0.008), respectively. The detection rate of worrisome features in CT and MRI was 25.7 and 31.7% (P = 0.001), respectively. Characteristics of pancreatic cysts were summarized in the Table 2.

Although population who underwent CT and MRI from 2013 to 2016 was increasing, there was no significant difference in the prevalence of pancreatic cystic lesions in the CT group (P = 0.796). The difference in MRI was also not obvious from 2013 to 2016 (P = 0.213). After 2013, the prevalence of pancreatic cysts between CT and MRI had a statistically significant difference (P < 0.001). Moreover, the prevalence of PCLs smaller than 20 mm in MRI was higher than CT (2.56% vs.1.20%, P < 0.001), but there was no difference in the prevalence of PCLs larger than or equal to 20 mm between CT and MRI (0.71% vs.0.80%, P = 0.122). The difference between CT and MRI from 2013 to 2016 was shown in the Table 3 and tendency of the detection rate was provided in the Fig. 5.

As before, if patient has the absolute or relative indications for surgery, he/she should undergo surgery or be referred to the multidisciplinary group for further evaluation [10‐12]. Finally, 312 cases underwent surgical resection. The maximum diameter of all resected lesions was larger than or equal to 20 mm and there was no difference in mean size between CT and MRI (P = 0.857). Interestingly, although there was no significant difference in the prevalence of cysts larger than or equal to 20 mm between CT and MRI (0.71% vs.0.80%, P = 0.122), we found significant difference for surgical resection rates of PCLs between CT and MRI (37.0% 143/387 vs. 55.2% 169/306, P < 0.001). The pathologic results were: benign (n = 283, 90.7%), borderline (n = 2, 0.6%), malignant (n = 27, 8.7%). The incidence of malignancy in the CT group was 4.9%, compared to 13.0% in MRI (P = 0.014).