Clinical features
The theory that SPTP may originate from germinal ridge-ovary primordia-associated cells during embryogenesis can also explain the higher diagnosis of the disease in women [
7]. SPTP is of unknown pathogenesis and low malignancy potential, and yet is rarely metastatic and has good prognosis after surgical excision [
8]. Although rare, SPTP is the most common pathological type representing 72.4% of pancreatectomy cases in patients under 40 years of age [
9]. In this study, patients developed the disease at an average age of 39 years, slightly higher than the average of 34 years reported by Hu et al. [
10] and yet significantly higher than the average age of 29 years reported by Zhan et al. [
11]. In addition, 66.7% (18/27 cases, including 14 females and 4 males) of patients were aged under 40 years, which was consistent with the higher diagnosis in young women [
12]. The male-female ratio in this study was 1:3.5, comparable with the 1:5.9 and 1:10 reported by other scholars [
7,
11].
SPTP has atypical clinical manifestations such as abdominal pain or discomfort, lumbar and back pain, and other tumor compression symptoms. Half of the patients in this study were diagnosed via physical examination. Since the tumor tissue is soft enough not to oppress the pancreatic and bile duct and cause obstructive symptoms, only one case developed secondary pancreatic duct dilation in the giant pancreatic head tumors, but with no bile duct dilation and obstructive jaundice symptoms.
Because CT examination has the characteristics of wide application, rapidity, and clear anatomical display, it is crucial for the detection of both symptomatic and asymptomatic SPTP tumors, but it is very easy to miss the diagnosis when the difference in density between the tumor and the surrounding normal pancreatic parenchyma is not obvious. Among the 10 cases in this group in which no calcification was seen, 4 cases of tumors smaller than 4 cm were isointense on CT plain scanning, and it was difficult to detect the lesions by CT plain scanning alone. Enhanced CT with target scanning can help to detect these tumors.
The iterative reconstruction (IR) algorithm is a commonly used reconstruction algorithm for CT images, which can reduce the radiation dose. Target scanning refers to the method of scanning after local magnification of the area of interest, which can significantly improve the spatial resolution of the image, more realistically respond to the density and anatomical relationship of the tissue, and improve the clarity of small lesions, and is mainly used for scanning lung nodules, and has not yet been reported to be applied to the pancreas. In this group of cases, target scanning was used in the parenchymal phase of the pancreas, and iterative reconstruction was used in the plain scanning, arterial phase, portal phase and delayed phase to effectively reduce the radiation dose.
General CT findings of SPTP
(1) Location: Although SPTP can occur in any part of the pancreas, it is most commonly diagnosed in the pancreatic tail. In this study, the distal pancreas accounted for 59.2% (16/27 cases), which was close to the 57.1% reported by Rai et al. [
5].
(2) Size: The average size of the tumor was 4.3 cm, close to the 4.4 cm reported by Chen et al. [
9] and significantly smaller than the 7 cm reported by Zhan et al. [
11] In this study, the tumor in male patients was smaller than that in female patients, and the gender difference was statistically significant.
(3) Morphology: Most of the tumors were quasi-circular, and a few were lobulated. In this study, the 5 cases of lobulated tumors were larger than 4 cm in diameter, suggesting that the tumors were in multicentric expansive growth when they were large.
(4) Growth pattern: The tumor center can be located inside the pancreas or protrude outward; tumors in exocentric growth often need to be distinguished from lymph node tumors or retroperitoneal neurogenic tumors. The enhanced scanning showed a “flared” change in the boundary between the tumor and the pancreas (Figs.
2 and
6). This often suggests that the tumor is from the pancreas, which is helpful for the localization diagnosis of the tumor.
(5) Capsule: A tumor with a complete capsule appears on the CT image as a clear capsule and has a clear boundary with the surrounding tissue; when the tumor perforates the capsule, an incomplete tumor capsule and an unclear boundary show on the CT image. Enhanced scanning showed that = the capsule is obviously enhanced (Fig.
6a). Wang et al. [
13] found in their study that tumors larger than 6 cm were related to capsular invasion. However, in this study, 4 of the 5 cases of tumors with incomplete capsules were smaller than 6 cm (except for only 1 case where it was larger than 6 cm), which is inconsistent with literature and this requires further study with large-sample data.
(6) Cystic or solid and CT findings: Due to the heterogeneity of the tumor, there can be bleeding, necrosis, or cystic degeneration in the tissue, which appear as iso- and high-density, high- and low-density, and low-density changes on the plain CT scanning. In this study, 67.7% (4/6 cases) of the tumors that were smaller than 3 cm were solid, and 100% (4/4 cases) of those larger than 5 cm were cystic-solid. This may be because bleeding, necrosis, or cystic degeneration occur with the growth of the tumor while none of them are found in small tumors. Miao et al. [
14] found that when cystic and solid components were in similar proportions, the solid components were flakey and were obviously enhanced after enhancement, showing “floating cloud signs” against the low-density cystic components; when cystic and solid components were alternatively distributed, the cystic wall nodule showed a “pseudopapillary structure”; when the solid components dominated, the cystic components showed a small subcapsular circular structure.
While the CT showed that the solid components were mamillary or flocculent located on the periphery, the pathology results showed that a solid region consisted of fibrous blood vessels and nestlike or patchy tumor cells, as well as a pseudopapillary region consisting of tumor cells around the connective tissue such as thin blood vessels; the centric cystic components consisted of bleeding and degenerative tissue containing fibrin components [
15]. This explains, pathologically, why the solid components are progressively enhanced on the CT enhanced scanning while the cystic components are not [
16‐
19]. In this study, the 24 cases of tumors were progressively enhanced to a degree lower than the surrounding normal pancreatic parenchyma, and 3 cases were progressively enhanced but were isoenhanced during the delayed phase.
(7) Calcifications: Central scattered punctate calcifications and marginal semiarc eggshell calcifications may occur in the tumor, with septal calcifications in some tumors [
20]. The calcification rate of 59.3% (16/27 cases) in this study was far higher than the 35.3% (12/34 cases) reported by Li et al., [
21] but this may be related to the selection of the cases. SPTP should be considered first when the solid components of the tumor are progressively enhanced, with calcifications observed in the lesions.
Vascular invasion and metastasis: Although SPTP is classified as a malignancy, it does not invade the arteries and veins around the tumor, and does not cause vascular stenosis or truncation, which is also a feature different from the duct adenocarcinoma. SPTP may metastasize to the regional lymph nodes, mesentery, omentum majus, and peritoneal metastasis, while liver metastasis the most common. In this study, liver metastasis occurred in only 3.7% (1/27 cases) of patients, and no other metastases were found.
Comparison of CT signs between male and female patients
There were statistically significant differences between the male group and the female group only with respect to the tumor size. The tumors in the male group were slightly smaller than those in the female group; 83.3% (5/6 cases) of male patients who showed no obvious clinical symptoms were incidentally detected with pancreatic lesions during the physical examination, and 16.7% (1/6 cases) visited doctors due to abnormal pain for two weeks. This suggests that male patients pay more attention to physical examination and are detected with tumors earlier than female patients. Therefore, advocating physical examination is of significance in the detection of tumors such as SPTP that have slow growth and no typical clinical symptoms.
Pathology examination results
Microscopically, it was found that the solid components of SPTP mainly consist of the solid patchy region, the pseudopapillary region, and the transition region of the two; the tumor cells were quasi-circular and middle-sized; the cells were less atypical, and the tumor cells could form the characteristic dendroid pseudopapilla around the blood vessels. The cystic region was mainly composed of bleeding, necrosis, and mucoid degeneration [
22]. Hu et al. [
10] conducted the IHC test on 132 SPTP patients and found that 98.5% (130/132 cases) developed nuclear positive expression of lymphatic enhancer factor-1 (LEF-1), while no LEF-1 expression was found in other pancreatic tumors and surrounding normal pancreatic tissue, with a specificity of 100%. In this study, the positive expression rates of LEF-1 and β-catenin in SPTP were 100%, which is consistent with literature.
Comparison of CT signs and pathology
With pathology examination as the golden standard, CT examination had an accuracy of 92.6% and 96.3% in the evaluation of the integrity of pancreatic capsule and the cystic or solid tumors. The two cases whose CT findings showed incomplete capsules may be because the enhanced scanning failed to completely show the capsule, due to which the boundary between the local part of the tumor and the pancreatic parenchyma was unclear and thus led to misjudgment. For one case with cystic-solid tumors, the plain CT scanning showed slightly low-density, and the target scanning during the pancreatic parenchymal phase showed relatively low enhancement, while the pathology results showed solid tumors. The reason may be that the mildly enhanced part of the solid region of the tumor was mistaken as there being no enhancement.
Treatment and follow-up
With plain CT scanning and enhanced scanning, doctors can accurately locate the tumor, and evaluate its adhesion and invasion to the surrounding tissue and blood vessels, thereby being able to select the most appropriate surgical plan. Patients with pancreatic head tumors may undergo radical gastroduodenectomy, or pylorus-preserving pancreaticoduodenectomy based on the relationship between the tumor and adjacent organs, thus helping improve the quality of life post-surgery. For smaller tumors on the pancreatic body, segmental pancreatectomy is an option to reduce postoperative complications. Patients had satisfactory prognosis, with few recurrences or metastases [
23,
24].
CT features are mainly used to distinguish between SPTP and pancreatic neuroendocrine tumors (pNETs). The degree of enhancement of SPTP is generally lower, while that of the latter is significantly higher during the arterial phase or portal vein phase, compared with the surrounding normal pancreatic parenchyma. However, it is difficult to distinguish between the above two when SPTP is as isoenhanced as the pancreatic parenchyma. In this study, 3 cases showed isoenhancement during the delayed phase, including 1 case that was misdiagnosed with pNETs based on preoperative CT.