Introduction
Pseudomyxoma peritonei (PMP) is a rare malignant disease mainly originated from appendiceal mucinous tumors, which often produced mucinous and gelatinous masses [
1]. The clinical presentation of PMP varies widely and depends on the disease course; however, due to its low incidence rate, the diagnosis is always challenging [
2]. Cytoreductive surgery (CRS) in combination with hyperthermic intraperitoneal chemotherapy (HIPEC) is the active treatment for PMP [
3]; since then, the prognosis of PMP patients has been greatly improved, with both progression-free survival (
PFS) and overall survival (
OS) being significantly prolonged.
Completeness of cytoreduction is one of the most important prognostic factors for PMP [
4‐
6], as affirmed by our previous retrospective cohort study [
7]. The completeness of cytoreduction depends primarily on the surgical skills of the surgeon [
8] and the tumor load of PMP. The peritoneal cancer index (PCI) is employed to estimate the tumor load resulting from peritoneal metastasis, which exhibits a negative correlation with the chances of cytoreduction [
9]. Consequently, there is an urgent need to identify PCI cutoff point for predicting surgical resectability in PMP patients.
In clinical practice, the PCI cutoff point differs between low-grade and high-grade PMP patients, as low-grade PMP patients are more likely to achieve complete CRS than the high-grade subjects when the tumor load is equivalent. During the surgical removal of tumors, it is technically challenging to resect massive tumors with wide intra-abdominal involvement, including the hepatic hilum (region 2) and small intestine (regions 9–12) [
10,
11]. A former study confirmed that selected PCI regions (region 2 + 9–12), corresponding to the small intestine and hepatoduodenal ligament, were more predictive of complete resection in advanced epithelial ovarian cancer [
12]. The present study aimed to determine the optimal cutoff point for total and selected PCI to predict surgical resectability for both low-grade and high-grade PMP patients, which could contribute to preoperative patient selection and/or information.
Discussions
The present study identified a significantly higher PCI level in high-grade PMP patients compared to low-grade subjects. The discriminative ability of both total and selected PCI to predict surgical resectability for low-grade PMP patients demonstrated excellent performance, with optimal cutoff points of 21 and 5, respectively. Similarly, for high-grade PMP patients, both total and selected PCI exhibited good performance in predict surgical resectability, with optimal cutoff points of 25 and 8, respectively.
Completeness of cytoreduction emerged as the most critical prognostic factor for PMP [
5,
6,
17], with PCI is negatively correlated with the ability to achieve a complete CRS [
18]. Patient selection can be challenging in PMP cases with very high PCI [
19], as postoperative morbidity and mortality associated with CRS should be taken into account for such an extensive disease. Therefore, various studies have established PCI cutoff point to assist in preoperative patient selection for PMP.
Reviewing previous studies [
3,
9,
18,
20], we observed variations in PCI cutoff point for predicting resectability. We speculate that these differences stem mainly from variations in the included populations. For instance, Votanopoulos K. I. et al. found that 21% of high-grade PMP patients achieved a complete CRS when the PCI was ≥ 21. In our research, among 85 high-grade PMP patients with
PCI ≥ 21, only 14% (12/85) reached complete CRS. We speculate that this difference is mainly attributed to the different tumor loads of the included population, with our research showing a higher mean PCI of 28.8 compared to 14.8 in the former study. Additionally, the present study found a higher PCI level in high-grade PMP compared to low-grade PMP, which also result in the higher PCI cutoff point in high-grade PMP patients. This may be related to the biological behavior of the malignancy, since patients with a high PCI are likely to have more aggressive disease than those with a low PCI.
In 2002, Sugarbaker proposed that total gastrectomy with a temporary diverting jejunostomy could be employed to facilitate complete cytoreduction in patients with advanced PMP syndrome [
21]. By 2020, Kitai T. et al. [
3] reported that 69.0% (20/29) of extensive PMP patients underwent complete CRS, with all 20 patients who underwent gastrectomy based on the aforementioned surgical concept. However, this approach resulted in a high postoperative complication rate. Benhaim et al. reported that 54% (54/100) of extensive PMP patients achieved complete CRS. They concluded that technical progress contributed to increasing the complete CRS rate of PMP patients from 25 to 71% [
10]. Due to the potential adverse impact of gastrectomy on daily life after surgery [
22], our center prioritizes stomach-sparing surgery whenever possible, emphasizing that all treatments for PMP patients aim not only to keep them alive but in life [
23].
Overall, the determination of the optimal PCI cutoff point may be affected by the three key factors: the tumor load of PMP patients, the surgical approach, and the surgeon’s expertise. To establish a standardized PCI value for predicting surgical outcomes, our preliminary recommendation is to conduct multicenter research on a global scale. Moreover, it is crucial for all centers to be recognized as professional PMP centers.
The present study found that the discriminative ability of selected PCI and total PCI in predicting surgical resectability was similar for both low-grade and high-grade PMP patients. To our knowledge, this is the first attempt to compare the selected PCI with total PCI for PMP patients. The total PCI has certain limitations, which can only be assessed after complete lysis of all adhesions [
24], with complete abdominal examination [
10]; when confronting high tumor load patients, it is always time-demanding and cannot be obtained by laparoscopy [
12]. Although present study did not find a statistical difference between selected and total PCI in predicting complete cytoreduction for PMP patients, the selected PCI can be easily applied and time-saving to predict the surgical outcome for PMP patients. Hence, the selected PCI demonstrates better practicality in clinical practice.
According to the PCI cutoff point, the subgroups for both total and selected PCI of low-grade PMP patients exhibited different prognoses, while in high-grade PMP patients, no difference were observed. Through statistical analysis, it was found that only 25 high-grade PMP patients reached endpoint events. When combined with the Kaplan–Meier plots, the curves of the two subgroups showed intersection point. We consider that if the effective sample size of high-grade PMP patients is further increased, the PCI may also have predictive value.
There were several limitations in the present study. Firstly, PMP appears to be more common in women, who often present with rapidly enlarging ovarian masses [
25]. Nevertheless, the gender ratio (male/female) between included and excluded PMP patients in our study was 197/169 and 47/114 (
χ2 = 27.287,
p = 0.001), indicating a certain selection bias in the present research. Secondly, only 39% (143/366) of PMP patients underwent complete resection, a rate lower than that reported in foreign PMP centers. Thirdly, all participants were consecutively included over a 10-year period, and the complete cytoreduction rate increased during that time. Lastly, PCI can only be acquired during surgery, thus not contribute to preoperative patient selection. Fortunately, computer tomography (CT) determined PCI has been employed to accurately calculate PCI preoperatively by the experienced radiologists [
26]. In the future, there is a tremendous need to establish a prediction model to preoperatively calculate PCI.
In conclusion, the PCI cutoff point demonstrated good performance in predicting surgical resectability for both low-grade and high-grade PMP patients. The discriminative ability of total and selected PCI was similar; nevertheless, selected PCI was simpler and time-saving in clinical practice. In the future, new imaging techniques or predictive models may be developed to better predict PCI preoperatively, which might assist in confirming whether complete surgical resection can be achieved.
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