New insights in the pathology of peritoneal surface malignancy
Review Article on New Technologies

New insights in the pathology of peritoneal surface malignancy

Norman John Carr

Peritoneal Malignancy Institute, Basingstoke and North Hampshire Hospital, Basingstoke, UK

Correspondence to: Norman John Carr, FRCPath. Peritoneal Malignancy Institute, Department of Histopathology, Basingstoke and North Hampshire Hospital, Aldermaston Road, Basingstoke, RG24 9NA, UK. Email: Norman.carr@hhft.nhs.uk.

Abstract: Pathology is central to the management of peritoneal surface malignancy. This article highlights some recent advances that have had an impact on patient management or could do so in the near future. Malignant peritoneal mesothelioma, particularly the epithelioid subtype, is amenable to radical therapy in selected cases, and factors such as ki67 proliferation index, expression of BAP1 and mutation in CDKN2A show promise as prognostic indicators. Our understanding of multicystic mesothelioma has improved in recent years; it is a true neoplasm for which surgery may be indicated. Serous carcinomas involving the peritoneum are now known to originate from tubal epithelium. They are of two distinct types, high grade and low grade, which are now recognized as different neoplasms with distinctive features, oncogenesis and behavior. Pseudomyxoma peritonei (PMP) is an unusual condition that usually arises from an appendiceal mucinous neoplasm. Recent consensus in the classification and nomenclature of these lesions is discussed, including the distinction between low grade and high grade appendiceal mucinous neoplasms (HAMN), and the diagnostic criteria for appendiceal adenocarcinoma. PMP is divided into four prognostic groups: acellular mucin, low grade mucinous carcinoma peritonei, high grade mucinous carcinoma peritonei, and high grade mucinous carcinoma peritonei with signet ring cells. The pseudomyxoma microbiome is a promising area for clinical intervention but has been the subject of little research activity. Goblet cell adenocarcinoma (previously known as ‘goblet cell carcinoid’) is a distinctive type of appendiceal adenocarcinoma. Its behavior correlates with histologic features, but no general consensus for classification has been reached.

Keywords: Appendiceal neoplasms; peritoneal neoplasms; pseudomyxoma peritonei (PMP); mesothelioma; serous carcinoma


Submitted Mar 18, 2020. Accepted for publication Jun 08, 2020.

doi: 10.21037/jgo-2020-01


These are exciting times in the pathology of peritoneal surface malignancy. On one hand, the management of patients depends increasingly on accurate pathologic classification and the identification of prognostic and predictive factors. On the other hand, scientific advances are leading to new insights in the genetics and oncogenesis of these lesions. Pathology is central to concept of personalised medicine, which in some cases already has a role in treatment. This article highlights some areas in which recent advances have had an impact on patient management or have the prospect of doing so in the near future.

Prognostic and predictive factors in peritoneal malignant mesothelioma

Malignant mesothelioma of the peritoneum has traditionally been associated with a dismal prognosis. However, studies have shown that cytoreductive surgery (CRS) combined with heated intraperitoneal chemotherapy (HIPEC) represents effective treatment in selected cases, and there is consequently a need for markers that could predict response to such treatment. Differences between pleural and peritoneal mesotheliomas suggest that we cannot simply extrapolate findings from pleural tumors and apply them uncritically to peritoneal lesions (1,2).

Morphology

Malignant mesothelioma is classified as epithelioid, sarcomatoid or biphasic (3). The epithelioid subtype has the better prognosis and is most likely to respond to cytoreduction and HIPEC (4-9). Stage is also a prognostic factor; lymph node metastases confer a worse prognosis (7,8).

In epithelioid malignant mesothelioma, no validated grading system exists. Features reported to be associated with shorter survival include a solid pattern of growth (Figure 1) and ‘minimal invasion’ (10-13). However, assessment of features such as these are subject to considerable inter-observer variation (14). A histologic grading system of prognostic significance has been described (15), a topic warranting further investigation.

Figure 1 Epithelioid malignant mesothelioma of peritoneum. There is a solid pattern on the left and a tubulopapillary pattern on the right. Hematoxylin and eosin, ×10.

Most malignant mesotheliomas are diffuse, but on very rare occasions localized malignant mesotheliomas may be encountered (16,17). Localized malignant mesotheliomas are cytologically indistinguishable from the diffuse kind, but they are solitary, well circumscribed and show no evidence of diffuse spread. The prognosis appears good and it has been suggested that complete surgical excision may be curative. However, information about their behavior is scanty on account of their rarity.

Well differentiated papillary mesothelioma is a rare neoplasm with characteristic histologic and genetic features (18,19). It is a distinct entity with a good prognosis and should not be confused with epithelioid malignant mesothelioma showing a tubulopapillary pattern of growth (20).

Mitotic count and ki67 proliferation index

The proliferation fraction in tumor cells as assessed by immunoexpression of ki67 has been shown to be related to prognosis (6,7,11,21). In a study of 117 patients, the authors found that ki67 >9% was associated with a poor response to CRS and HIPEC (7). At the Peritoneal Malignancy Institute, Basingstoke, we take the ki67 proliferation index into account when assessing patients’ suitability for surgery (22). Other studies have found that high mitotic count is associated with increased mortality (5,10,23).

Immunoexpression of p16

Expression of p16 is lost in some cases of malignant mesothelioma, correlating with mutation in its encoding gene, CDKN2A (24). Loss of p16 is associated with worse survival (5).

BAP1 mutation

Mutation in BAP1 is common in malignant mesothelioma and is more frequent in peritoneal than pleural primaries (25). BAP1 mutation leads to loss of BAP1 protein expression in tumor nuclei. In pleural mesothelioma, BAP1 mutation and loss of expression have been associated with improved survival (26), and a paper from France reports similar findings in peritoneal mesothelioma (27). Some patients have a germline mutation in BAP1, and such patients have improved survival compared with sporadic cases (28).

Targeted therapy

The elucidation of mutations such as those described above raises the prospect of treatments targeted at the abnormalities (29-31). A case in point is ALK rearrangement, which is found in some peritoneal mesotheliomas and tends to occur in younger women without asbestos exposure (32). ALK rearrangements are associated with strong immunoexpression of the encoded protein, anaplastic lymphoma kinase. We have recently seen a patient with a peritoneal mesothelioma harboring abnormal ALK who showed a striking response to the tyrosine kinase inhibitor ceritinib (personal observations).


Multicystic mesothelioma: controversial issues

Multicystic mesothelioma is a controversial entity. It is an uncommon condition that usually affects women of child-bearing age, but there is a wide age range and it also affects men. The most frequent site is the peritoneum of the pelvis and lower abdomen (33-37). No association with asbestos has been documented. Some patients are asymptomatic but others may have abdominal pain which may be related to areas of inflammation or necrosis in the tumor. Histologically, lesions are characterized by cuboidal or flattened mesothelial cells lining thin fibrous walls (Figure 2) (33,35,38).

Figure 2 Multicystic mesothelioma of peritoneum. This low power view demonstrates thin-walled cysts of varying size, typical of the neoplasm. Hematoxylin and eosin, ×2.

One controversy is whether multicystic mesothelioma is a true neoplasm. It has been argued that lesions are reactive to chronic irritation and should be designated ‘peritoneal inclusion cysts’. However, multicystic mesothelioma can be associated with other neoplasms such as adenomatoid tumor and well differentiated papillary mesothelioma, and it can be progressive and often recurs after surgery (18,33-35,39). Furthermore, lesions have been shown to harbor clonal chromosome abnormalities with fusion transcripts (40). Although it is possible that non-neoplastic mesothelial inclusion cysts could occur as a consequence of inflammation or previous surgery, such lesions are likely to be solitary cysts no more than 5 mm diameter (33). Multilocular lesions forming a distinct mass should be considered neoplastic.

Another controversial area surrounds management. Given the tendency to recur after surgery, CRS and HIPEC has been recommended, although recurrence even after this treatment has been documented (33-35,38,41). Whether asymptomatic patients should be subjected to such treatment is another question. Rare cases in which there is evidence of progression to malignancy have been reported, but this is also controversial (42,43).


Serous carcinoma involving the peritoneum

Ovarian epithelial neoplasms are not a single entity but a number of separate tumors with distinctive biological features (44). A consequence is that studies in which these different diseases are combined into a single group for survival analysis may be of limited value. Regarding serous carcinomas in particular, it has become clear that they are of two distinct types: low grade serous carcinoma and high grade serous carcinoma.

High grade serous carcinoma

It now well established that the vast majority of high grade serous carcinomas arise not from the ovary but from the fallopian tube, in particular from a pre-malignant precursor designated serous tubal intraepithelial carcinoma (STIC) (45). Studies from the era before the significance of STIC was appreciated are likely to have erroneously designated the ovary or peritoneum as the primary site because STIC was not sought or identified. Nevertheless, it is likely that primary peritoneal high grade serous carcinoma, although rare, could exist, possibly arising from implanted tubal epithelium. The 2014 FIGO staging classification allows for this possibility by having fallopian tube, ovarian, “tubo-ovarian” and peritoneal primary sites (46).

Histologically, high grade serous carcinomas are characterized by papillary and solid growth of pleomorphic cells with prominent nucleoli (Figure 3). Over-expression of p53 and p16 is common. The principal means of distinction from low grade serous carcinoma is the degree of cytologic atypia (44).

Figure 3 High grade serous carcinoma of the peritoneum. This high power view shows characteristically pleomorphic tumor cells. A mitosis is arrowed. Hematoxylin and eosin, ×40.

High-grade serous carcinoma involving the peritoneum may be treated with CRS and HIPEC (47). Improved recognition of the primary site of these lesions and their distinction from other histologic types of ‘ovarian’ cancer will allow more accurate understanding of how they behave.

Low grade serous carcinoma

Low grade serous carcinomas are less common than high grade serous carcinomas. They present at a relatively young age and, although relatively resistant to chemotherapy, prolonged survival is usual (48,49). Histologically, low grade serous carcinomas exhibit a complex papillary, micropapillary and/or cribriform architecture, often with psammoma bodies, which may be very numerous (Figure 4). Cytologic atypia is mild or moderate. Unlike high grade tumors, TP53 mutations are generally lacking.

Figure 4 Low grade serous carcinoma of the peritoneum. There are papillae, nests and cribriform structures. Numerous psammoma bodies are present (arrow). Hematoxylin and eosin, ×10.

It has been suggested that low grade serous carcinomas develop from serous borderline tumors of the ovary, which are themselves derived from implants of tubal epithelium (50,51). Thus, low grade as well as high grade serous carcinoma is ultimately of tubal origin.

Regarding treatment, our understanding has been restricted by case series that include these neoplasms with other types of ovarian cancer in a ‘one size fits all’ approach. However studies focussed on low grade serous carcinomas have shown that hormone therapy and CRS have a role in selected cases (52).


Classification of pseudomyxoma peritonei (PMP) and its appendiceal primary tumors

PMP is a clinical syndrome in which there is progressive accumulation of mucinous material within the peritoneal cavity due to a mucinous neoplasm (53,54). Its distinguishing characteristic is the redistribution phenomenon, whereby the mucin and the neoplastic cells it contains are redistributed through the peritoneal cavity by following the physiological flow of peritoneal fluid to sites of reabsorption, such as the omentum, paracolic gutters and inferior surface of the diaphragm, where the tumor accumulates. Lymphatic and hematogenous metastases are unusual and mostly confined to high grade disease. The vast majority of cases arise from a mucinous tumor of the appendix, but rare primary sites include mucinous tumors of the urachus, pancreas, biliary tract and cervix (55-59). Mucinous tumors arising in mature teratomas of the ovary are also a rare source, as are tailgut cysts (60,61). Rarely, colonic mucinous adenocarcinomas can behave as PMP (62).

Although PMP has been recognised as a neoplastic condition for over a century, its nomenclature has been problematic and the source of considerable controversy, with many different classification systems proposed over the years. One reason is the histologically bland appearance of the mucinous epithelium, despite its malignant behavior (63). For example, Ronnett et al. introduced the term ‘adenomucinosis’ for well differentiated (‘benign-looking’) lesions and used the term ‘adenoma’ for the appendiceal precursors (64). However, alternative terminology of ‘mucinous carcinoma peritonei’ was proposed by Bradley et al. (65) reflecting the morphologic and behavioral spectrum of PMP, while ‘low grade appendiceal mucinous neoplasm’ (LAMN) was introduced by Misdraji et al. as an alternative for lesions otherwise termed ‘adenoma’, ‘cystadenoma’ or ‘mucinous tumor of uncertain malignant potential’ (Figure 5) (66).

Figure 5 Low grade appendiceal mucinous neoplasm. The neoplastic epithelium shows an undulating pattern with scattered filiform villi. Cytologic atypia is minimal. The muscularis propria is indicated by a star. Hematoxylin and eosin, ×4.

Given the plethora of contradictory classifications, a modified Delphi process sponsored by the Peritoneal Surface Oncology Group International (PSOGI) was instigated, bringing together international experts in pathology and surgical oncology, including supporters of the principal classification systems then in use. A consensus on terminology was reached, and its use has facilitated comparison of results published by different institutions (54). It also forms the basis of the classification of these lesions in the 2019 World Health Organization (WHO) tumor classification (67,68). Table 1 shows the classification of mucinous neoplasms of the appendix; Table 2 shows the classification of PMP.

Table 1
Table 1 Summary of classification of mucinous appendiceal neoplasms with corresponding WHO grades (54,67,68)
Full table
Table 2
Table 2 Diagnostic classification of pseudomyxoma peritonei with corresponding WHO grades (67-69)
Full table

A detailed analysis of histologic criteria is beyond the scope of this article and readers are referred to recent reviews (69,70). However, there are a number of important points that are worth highlighting.

  • The distinction between ‘pushing’ and ‘infiltrative’ invasion is central to the classification (Table 1). Pushing invasion is characterised by broad-front extension into the appendiceal wall that can mimic a diverticulum (Figure 6). It may be associated with dense fibrosis but there is no true desmoplastic reaction. Histologic features of infiltrative invasion include small angulated glands, tumor budding, desmoplasia, and the ‘small cellular mucin pool’ pattern (Figure 7).
    Figure 6 Low grade appendiceal mucinous neoplasm showing pushing invasion. The neoplastic epithelium (arrow) makes a diverticulum-like structure pushing towards the serosal surface (arrowhead). Hematoxylin and eosin, ×2.
    Figure 7 Mucinous adenocarcinoma of appendix, moderately differentiated. The ‘small cellular mucin pool’ pattern of invasion is visible. Clumps of tumor cells (arrow) surrounded by mucin invade the appendiceal wall. Hematoxylin and eosin, ×4.
  • High grade appendiceal mucinous neoplasm (HAMN) is a new entity proposed by the PSOGI consensus for those lesions that do not show infiltrative invasion but have high grade cytology (54). Such lesions had not been previously identified separately, and were designated as either LAMN or adenocarcinoma in previous case series (66,71,72). The limited evidence available suggests they may be more likely to progress to PMP if there is extra-appendiceal mucin at the time of appendectomy (72). They are also more likely to contain TP53 mutations than LAMN (73). Progression of LAMN to HAMN may be associated with activation of the Wnt/beta-catenin pathway (74).
  • HAMN is staged as adenocarcinoma by the AJCC. Specifically, pT1 and pT2 are used to categorize HAMNs that would be classified pTis (LAMN) if low grade (75). The scanty evidence to date suggests that HAMNs confined to the appendix may have a low risk of progression to PMP (76). If larger studies confirm this finding, it may be better to align the staging of HAMN with LAMN.
  • In the appendix, the term ‘adenocarcinoma’ is reserved for lesions with infiltrative invasion. Although the implication is that they are more likely to spread via lymphatics or bloodstream than lesions with pushing invasion, peritoneal metastases are still more common than nodal metastases in patients with appendiceal adenocarcinoma (77).
  • Lesions with signet ring cells are classified separately because of good evidence they have a worse prognosis (78-81). Since it is sometimes difficult to distinguish degenerating cells in mucin pools from true signet ring cells, it has been suggested that at least 10% of the cells in PMP should show signet ring morphology before classifying a lesion as such (78).
  • Although the grade of the appendiceal primary and the peritoneal disease is usually the same, on rare occasions it may differ. Such cases are said to exhibit discordant histology (62,64).
  • The classification shown in Table 2 is prognostically significant (82). Of particular note, if no neoplastic epithelial cells are found histologically despite reasonable sampling, the risk of progressive disease is low (Figure 8) (81,83-85).
    Figure 8 Acellular mucin within the peritoneal cavity derived from a ruptured low grade appendiceal mucinous neoplasm. Hematoxylin and eosin, ×4.

Novel avenues of research into PMP

The pseudomyxoma microbiome

There is a highly conserved population of bacteria within PMP tissue, including some that are potentially pathogenic, and a greater density of bacteria is found in high grade than low grade PMP (86,87). Antibiotics not only decrease the density of bacteria but also affect beta-catenin expression within the tumor cells (88). Furthermore, some of the bacteria could interact with the mucin, and in one study levels of bacterial 16s ribosomal RNA were directly correlated with MUC2 expression (87). These findings have not yet led to the publication of clinical trials. Treatment with antibiotics is straightforward, relatively low-risk and inexpensive. Perhaps it is time to investigate the potential role of antibiotics in more detail.

New molecular biomarkers of prognosis

The need for biomarkers to help guide patient management is clear, but we cannot simply extrapolate from colorectal cancer since there are many important differences in the oncogenesis, pathology and behavior of colorectal and appendiceal neoplasms (89,90). For example, in appendiceal mucinous neoplasms KRAS mutations are found in the great majority, GNAS mutations are common, and BRAF mutations are rare. Individual case series of patients with PMP found prognosis was related to immunoexpression of p53, carbonic anhydrase II and SMAD4 (91-93), and gene expression profiling has been used to identify gene clusters of prognostic significance (94). None of these studies has yet led to the introduction of such techniques into general clinical practice, and more research is required.

Organoid models

Organoid culture allows cells to be grown in an environment that mimics their physiological niche in vivo (95). They represent a means of investigating genetic and epigenetic mechanisms underlying the neoplastic phenotype, and can be used for drug discovery. Their use in PMP has been limited so far, but the creation of organoids from peritoneal metastases of appendiceal neoplasms has been validated (96). A study using organoids derived from primary mucinous appendiceal adenocarcinomas showed that MUC2 expression could be reduced by celecoxib, an action mediated by reduced binding of CREB transcription factor to the MUC2 promotor (97).


Goblet cell adenocarcinoma

Goblet cell adenocarcinomas are rare neoplasms that are almost always primary in the appendix. They have distinctive morphology and are characterized by tight clusters of cells, sometimes with a small lumen (98-100). Most of the cells have mucin-filled cytoplasm that compresses the nucleus against the peripheral cytoplasmic membrane, but scattered among them small numbers of cells showing neuroendocrine features can usually be found (Figure 9). Common metastatic sites are peritoneum, omentum and ovaries (101).

Figure 9 Goblet cell adenocarcinoma. This lesion shows the typical nests of cells (group A in the Tang classification). Rare neuroendocrine cells with red granular cytoplasm are present (arrow). Hematoxylin and eosin, ×10.

These lesions were called ‘goblet cell carcinoid’ for many years, an ambiguous name that has caused them to be confused with neuroendocrine tumors, whereas in fact they are a type of adenocarcinoma. The 5th edition of the WHO classification now designates them ‘goblet cell adenocarcinoma’, reflecting their true nature, and ‘goblet cell carcinoid’ is not recommended (102). They are staged as adenocarcinomas by the AJCC (75).

The behavior of goblet cell adenocarcinomas is related to histologic features. The first published description of a grading system was by Burke et al. (98) and dates from 1990, but a number of others have been published since. The Tang classification divides goblet cell adenocarcinomas into three groups: group A has classical histologic features and the best prognosis; group B shows discohesive growth, increased atypia, irregular clustering and/or desmoplasia and has an intermediate prognosis; group C is characterized by poorly differentiated features and has the worst prognosis (101). Others have described two-tier grading systems that may be less prone to inter-observer variation than the Tang system (100,103). Taggart et al. described a grading system based on the proportion of tumor showing adverse histologic features reminiscent of the system of Burke et al. (104). A three-tier grading system based on the percentage of tumor showing tubular morphology has also been suggested (105).

None these grading methods has achieved universal acceptance. More research into prognostic value and interobserver reproducibility is required (106). The scanty evidence regarding ki67 proliferation index suggests it does not have a role in predicting behavior (107).

The genetic profile of goblet cell adenocarcinomas is strikingly different from that of appendiceal mucinous neoplasms or colorectal adenocarcinoma. Mutations in KRAS, APC, SMAD4 and BRAF are rare (108-111). Abnormalities in genes encoding the Wnt signaling pathway such as USP9X, NOTCH1, CTNNA1, CTNNB1 and TRRAP have been identified (108). These findings are potentially significant in the choice of chemotherapy.

Regarding treatment, recommendations are based on the results of small series and cannot be considered definitive (112,113). Patients with Stage I disease have good survival and can probably be treated with appendectomy alone if the margins are clear, but even this is an uncertain point (114).


Colorectal peritoneal metastases

The colorectum is the commonest primary site for peritoneal metastasis (115). Pathologic features associated with an increased frequency of peritoneal disease in colorectal adenocarcinoma are mucinous histology, poor differentiation, pT4 status and nodal metastases (116-118). Ovarian involvement is common in women with peritoneal carcinomatosis of colorectal origin, even if the ovaries appear normal macroscopically (119). Although peritoneal carcinomatosis was traditionally associated with a uniformly dismal prognosis, the introduction of radical therapy such as CRS and HIPEC has improved outcome in selected cases (120). This approach is consistent with the fact that in up to 25% of patients with peritoneal metastases there are no other clinically apparent sites of metastatic spread (121).

Signet ring cells and mucinous histology are associated with decreased survival in patients treated by CRS and HIPEC (122). Survival may also be reduced if free cancer cells are found in cytologic specimens of peritoneal fluid (123).

There are numerous molecular markers associated with peritoneal metastasis (121). A few have been investigated as potential prognostic or predictive factors, or targets for therapy. Angiogenesis plays an important part in the growth of metastatic tumor deposits, and in one study multivariate analysis showed that overall survival after CRS and HIPEC was negatively correlated with high expression of vascular endothelial growth factor (VEGF) (124). Another study found that loss of expression of the stem cell marker CD133 was associated with reduced disease-free survival although the benefit of chemotherapy appeared to be greater (125). Interestingly, although microsatellite instability is generally associated with worse prognosis in patients with nodal or solid organ deposits, it is associated with a better prognosis if peritoneal disease is dominant (121,126).


Conclusions

A common theme running through this article has been the importance of conceptualizing peritoneal malignancies as distinct pathological conditions rather than extrapolating from other types of neoplasm. For example, PMP is different from other types of mucinous neoplasia, and primary peritoneal mesothelioma shows some important differences from pleural primaries. Another theme has been the relative paucity of data available for the neoplasms discussed. Compared with more common tumors, we know relatively little about the basic biology and optimum management strategies for peritoneal malignancies. The answer is more research; the prize will include not only improvements in patient management but also insights into the biology of neoplastic growth and spread in general.

However, the benefits of research are only maximized if workers use common terminology allowing the results from different institutions to be compared. In the field of appendiceal neoplasia and mucinous carcinoma peritonei, the WHO Classification represents the way forward in this respect. We also need to apply insights from previous work to future research. For example, there can be no excuse for designing a project that looks at ‘ovarian cancer’ as a single entity when we know that there are different types of ovarian neoplasm that can be classified histologically, have distinctive genetic abnormalities and behave differently.


Acknowledgments

Funding: None.


Footnote

Provenance and Peer Review: This article was commissioned by the Guest Editors (Paul H. Sugarbaker and Kurt Van der Speeten) for the focused issue “Intraperitoneal Chemotherapy for Peritoneal Metastases: HIPEC, EPIC, NIPEC, PIPAC and More” published in Journal of Gastrointestinal Oncology. This article has undergone external peer review.

Conflicts of Interest: The author has completed the ICMJE uniform disclosure form (available at http://dx.doi.org/10.21037/jgo-2020-01). The focused issue was sponsored by the Peritoneal Surface Oncology Group International (PSOGI). The author has no other conflicts of interest to declare.

Ethical Statement: The author is accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.


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Cite this article as: Carr NJ. New insights in the pathology of peritoneal surface malignancy. J Gastrointest Oncol 2021;12(Suppl 1):S216-S229. doi: 10.21037/jgo-2020-01

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