Appendiceal Neoplasms Research Articles

Appendiceal collision tumor presented as acute appendicitis: A dissonant scenario

Renal cell carcinoma arises from the epithelium of the renal tubules and accounts for approximately 3% of adult malignancies. Tumours occur most often in older individuals usually in sixth and seventh decades of life. Bilateral multifocal papillary type of RCC is rare and its presence should prompt the suspicion of some underlying hereditary genetic predisposition thus thorough cytogenetic evaluation and detailed family history could help in finding the etiology. Bilateral RCC could be synchronous or metachronous. We hereby present the autopsy findings of a case of sudden death of a 65 years old male. On histopathological evaluation, both kidneys showed papillary variant of renal cell carcinoma as an incidental finding.

Incidental finding of synchronous neuroendocrine tumor of appendix and rectal adenocarcinoma. A case report and literature review

Abstract Objectives Colorectal cancers represent the predominant malignancies affecting the gastrointestinal tract. In contrast, appendiceal tumors occur less frequently. Among them, appendiceal neuroendocrine tumors (ANETs) account for approximately 50 % of cases and are often detected incidentally during unrelated procedures. Case presentation We present an extremely rare case of an incidental neuroendocrine tumor of the appendix discovered during surgery for rectal adenocarcinoma and provide a review of the relevant literature. A 70-year-old patient with radiologically and endoscopically diagnosed rectal cancer underwent a low anterior resection with total mesorectal excision and a temporary diverting ileostomy following preoperative chemotherapy and radiation therapy. Intraoperatively, a small appendiceal mass was discovered and excised. Histopathological examination confirmed a well-differentiated NET of the appendix measuring 0.4 cm in diameter. Conclusions This case report emphasizes the importance of thorough intraoperative examination during colorectal surgeries and highlights the need for increased awareness of appendiceal NETs among surgeons.

Appendiceal collision tumors: case reports, management and literature review.

Appendiceal tumors are incidentally detected in 0.5% cases of appendectomy for acute appendicitis and occur in approximately 1% of all appendectomies. Here, we report two cases of appendiceal collision tumors in two asymptomatic women. In both cases, imaging revealed right-lower-quadrant abdominal masses, which were laparoscopically resected. In both cases, histological examinations revealed an appendiceal collision tumor comprising a low-grade appendiceal mucinous neoplasm and well-differentiated neuroendocrine neoplasm (NEN). For complete oncological control, right hemicolectomy was performed in one patient for the aggressive behavior of NEN; however, histology revealed no metastasis. The other patient only underwent appendectomy. No further treatment was recommended. According to the latest guidelines, exact pathology needs to be defined. Proper management indicated by a multidisciplinary team is fundamental.

Low incidence of occult colorectal carcinoma in a cohort of 150 consecutive patients with diverticular strictures

BackgroundThe symptoms of stricturing diverticulitis can overlap with those of colorectal cancer. Additionally, the stricture itself may mask a “hidden” colorectal cancer. We aimed to describe the demographics, operative details and outcomes, including occult colorectal cancer, in a cohort of consecutive resections for presumed diverticular strictures. MethodsIn this single-center, retrospective cohort study, all patients undergoing resection for a presumed diverticular stricture between January 2010 and December 2015 were included. Preoperative imaging and colonoscopies were individually reviewed. Only patients with radiographically, endoscopically and/or intraoperatively benign-appearing strictures were included. ResultsOne hundred fifty patients (72.7% female, mean age = 70.4 ± 11.8 years, 62.7% elective) were included. Only 34 (22.7%) had a complete preoperative colonoscopy. In 95 (63.6% of cohort) patients, the stricture was non-traversable colonoscopically. Overall, 47 (31.3%) patients did not have complete preoperative imaging or a colonoscopy.In total, 53.3% were open procedures and 62% had non-diverted primary anastomosis. Eleven (14.7%) underwent resection of adjacent organs (5 appendixes/5 right colons/7 fallopian tubes ± ovaries/3 small bowel resections/2 partial cystectomies/1 spleen). The median length of stay was 7 (5, 12.5) days. Only 2 cancers (1.3% of patients) involving the stricture (1 invasive moderately differentiated sigmoid adenocarcinoma/1 lymphoma) were found. Three additional cancers were found in organs involved in the inflammatory process (20% of concomitantly resected organs, 1 ovarian carcinoma/1 leukemia in a lymph node/1 appendiceal tumor). ConclusionDespite approximately one-third of the cohort not having undergone successful preoperative colonoscopy or imaging, the rate of neoplasia involving diverticular strictures was only 1.3%. A relatively high cancer rate was found in concomitantly resected organs involved in the stricturing process.

Appendiceal Goblet Cell Carcinoma: Role of Cytoreductive Surgery (CRS) and Hyperthermic Intraperitoneal Chemotherapy (HIPEC).

Goblet cell carcinoma (GCC) encompasses a separate entity in appendiceal neoplasms with mixed glandular and neuro-endocrine pathological features. GCC mostly presents as an acute appendicitis duo to luminal obstruction or as an incidental finding on the surgical appendectomy specimen. In case of tumour perforation or presence of other risk factors, guidelines suggest additional treatment with a completing right hemicolectomy or cytoreductive surgery (CRS) with hyperthermic intraperitoneal chemotherapy (HIPEC). We report the case of a 77-year-old male with symptoms of appendicitis for which an appendectomy was performed. The appendix was ruptured during the procedure. There was an incidental finding of GCC on the pathological specimen. Because of possible tumour soiling, the patient received a prophylactic CRS-HIPEC. A literature review was performed to investigate the potential role for CRS-HIPEC as a curative treatment in patients with GCC. GCC of the appendix is an aggressive type of tumour with a high risk of peritoneal and systemic dissemination. CRS and HIPEC is a treatment option: both in a prophylactic setting and in patients with established peritoneal metastases.

Appendiceal neoplasms-A practical guide.

Appendiceal neoplasms (ANs) are a diverse group of pathologies that range from benign to malignant with widely varying prognoses. This article serves as an overview of the practical approach to evaluating and managing a patient with AN by reviewing the current literature and guidelines to provide a framework for the management of these nuanced pathologies.

Cecal-Colon Intussusception due to Appendiceal Mucinous Adenocarcinoma.

Intussusception is a rare presentation in adults and describes when one portion of the intestine telescopes into another portion. Intussusception is associated with malignancies serving as the lead point in adults. Appendiceal mucinous neoplasms are uncommon tumors often incidentally discovered during appendectomy procedures to manage acute appendicitis. Here we present a case report of an instance of mucinous adenocarcinoma of the appendix that manifested as a large bowel obstruction with intussusception limited to the colon, underscoring the possibility of concurrent intussusception and mucinous neoplasms. The case highlights the importance of meticulous diagnostic evaluation and management, particularly without well-defined treatment protocols. Appropriate diagnostic workup and management, including surgical intervention, are critical for patient outcomes and overall prognosis. The study recommends that patients diagnosed with confirmed or suspected appendiceal neoplasms undergo upfront oncologic resection where aggressive malignancy is a concern. Colonoscopy should be performed postoperatively for all patients to identify synchronous lesions.

Frequency, clinical picture, diagnosis, and differential diagnosis of the pathology of the digestive tract (esophagus, stomach, and bowel) in IgG4-related disease (autoimmune pancreatitis)

Despite the publication of European guidelines for the diagnosis and treatment of IgG4-related disease (IgG4-RD), this problem is very complicated from a practical point of view. There are many publications about autoimmune pancreatitis and liver and biliary tract lesions in IgG4-RD, but the data on the digestive tract lesions are not sufficient. Gastrointestinal manifestations of IgG4-RD are considered to be rare or very rare.
 IgG4-RD presenting as esophageal lesions is rare condition. Only single clinical cases are available. I. Obiorah et al. reported the highest number of cases of IgG4-related esophagitis (8 cases). Interestingly, 7/8 cases reported by I. Obiorah et al. were males (age range: 18 to 79 years), and most patients had dysphagia as the most prominent symptom, with erosions and strictures at endoscopy. Histologically, all patients had a lymphoplasmacytic infiltrate and storiform fibrosis, while obliterative phlebitis was observed in 3/8 patients. Hence, 2 of the 3 major histological “Boston criteria” of IgG4-RD were fulfilled in all patients. Besides, all patients had at least 50 IgG4-positive cells per HPF (range: 50–110), and IgG4/IgG ratios of at least 50% (range: 50–90%). Of I. Obiorah’s patients, s-IgG4 had only been measured in three — all of whom had values in the normal range. Esophageal strictures were identified in three patients, two patients had achalasia.
 Based on K. Notohara’s series and the other published cases, it can be concluded that gastric IgG4-RD often (but not always) seems to be clinically asymptomatic, that s-IgG4 is elevated in a considerable number of cases, and histologically, that at least two of the Boston criteria often are found. The recommended cut-off of 50 IgG4-positive cells and IgG4/IgG ratios of >40% was often fulfilled in the reported cases of IgG4-RD.
 There are few reports of IgG4-RD involving the duodenum, duodenal papilla, ileum with small bowel obstruction caused by pseudotumor or sclerosing disease of the small bowel with luminal narrowing.
 There are case reports of IgG4-RD with lesion of ileocecal area with an obstructing ileocecal mass and appendix, mimicking Crohn’s disease, appendicitis or appendiceal tumor. IgG4-related inflammatory bowel disease may — rarely — also be one of manifestations of IgG4-RD.
 Treatment should be conducted in accordance with European guidelines.

Cytoreductive Surgery and Hyperthermic Peritoneal Chemotherapy in Appendiceal and Colorectal Cancer: Outcomes and Survival.

We reviewed outcomes following cytoreductive surgery/hyperthermic intraperitoneal chemotherapy (HIPEC) for patients with appendiceal or colorectal neoplasms and evaluated key prognostic indicators for treatment. All patients who underwent cytoreductive surgery/HIPEC for appendiceal and colorectal neoplasms were identified from an IRB-approved database. Patient demographics, operative reports, and postoperative outcomes were reviewed. 110 patients [median age 54.5 (18-79) years, 55% male] were included. Primary tumor location was colorectal (58; 52.7%) and appendiceal (52; 47.3%). 28.2%, .9%, and 12.7% had right, left, and sigmoid tumors, respectively; 11.8% had rectal tumors. 12/13 rectal cancer patients underwent preoperative radiotherapy. Mean Peritoneal Cancer Index was 9.6 ± 7.7; complete cytoreduction was achieved in 90.9%. 53.6% developed postoperative complications. Reoperation, perioperative mortality, and 30-day readmission rates were 1.8%, .09%, and 13.6%, respectively. Recurrence at a median of 11.1 months was 48.2%; overall survival at 1 and 2years was 84% and 56.8%, respectively; disease-free survival was 60.8% and 33.7%, respectively, at a median follow-up of 16.8 (0-86.8) months. Univariate analysis of preoperative chemotherapy, primary malignancy location, primary tumor perforated or obstructive, postoperative bleeding complication, and pathology of adenocarcinoma, mucinous adenocarcinoma and negative lymph nodes were identified as possible predictive factors of survival. Multivariate logistic regression analysis showed that preoperative chemotherapy (P < .001), perforated tumor (P = .003), and postoperative intra-abdominal bleeding (P < .001) were independent prognostic indicators for survival. Cytoreductive surgery/HIPEC for colorectal and appendiceal neoplasms has low mortality and high completeness of cytoreduction score. Preoperative chemotherapy, primary tumor perforation, and postoperative bleeding are adverse risk factors for survival.

Omitting hemicolectomy for patients with appendiceal neuroendocrine tumours of 1–2 cm

Omitting hemicolectomy for patients with appendiceal neuroendocrine tumours of 1–2 cm

AB153. SOH23ABS_228. A retrospective review of patients with primary appendiceal neoplasms and the best practice guidance for the management of these tumors

Background: Appendiceal neoplasms include a heterogeneous group of tumors that exhibit varying malignant potential. Neuroendocrine tumors (NET) of the appendix are the most common type. The surgical outcomes of appendiceal neoplasms depend on the location, grade, size of tumor, positive lymph nodes and metastasis. For metastatic disease and peritoneal invasion, cytoreductive surgery (CRS) and hyperthermic Intraperitoneal Chemotherapy (HIPEC) have become the standard of care, but the need for this therapy can be a challenging decision. Multidisciplinary teams (MDTs) often have a large role to play in the discussion and management of individual patient care.

An unusual association between Burkitt's lymphoma and a Carcinoid tumor in a Syrian boy: a rare case report.

The authors report a case of a 15-year-old Syrian adolescent who was admitted to our hospital due to a persistent, severe generalized abdomen pain accompanied by nausea, vomiting, loss of appetite, and inability to pass stool or gas. An abdominal radiograph revealed dilated intestinal loops with air-fluid levels. The patient underwent emergency surgery through which a retroperitoneal mass was removed as well as part of the ileum and the appendix. The final diagnosis was consistent with intestinal BL associated with an appendiceal carcinoid tumor. The correlation between gastrointestinal carcinoids and other types of tumors was frequently reported. However, there have been few reports of carcinoid tumors being associated with lymphoreticular system cancers. BLs were classified into three variants: endemic, sporadic, and acquired immunodeficiency-associated BL while appendiceal neuroendocrine tumors were classified as the following: well-differentiated neuroendocrine tumors with benign or uncertain malignant potential; well-differentiated neuroendocrine carcinoma with low malignant potential; and mixed exocrine-neuroendocrine carcinoma. Our article demonstrates an unusual association between BL and an appendiceal carcinoid tumor that highlights the significant role of histological and immunohistochemical staining in confirming the diagnosis, as well as the role of surgery in treating the complications of intestinal BLs.

Open versus Closed technique for administration of heated intraperitoneal chemotherapy (HIPEC): Morbidity and Mortality outcomes from a high-volume centre

Background and aimsCytoreductive Surgery (CRS) and Hyperthermic Intraperitoneal Chemotherapy (HIPEC) is an established treatment in selected patients with peritoneal metastases, delivered in the UK in specialist centres. HIPEC can be administered via the open coliseum technique as first described by Sugarbaker (O-HIPEC) or using a closed technique (C-HIPEC). Data comparing the safety and outcomes of these different approaches is limited. This study aims to compare morbidity and mortality rates of O-HIPEC and C-HIPEC following CRS for peritoneal metastases from colorectal cancer and appendiceal tumours. MethodsConsecutive patients undergoing CRS with open (05/2019–04/2020) and closed (05/2020–04/2021) HIPEC were identified from a prospectively maintained database. Baseline data including primary pathology, HIPEC agent and major operative procedures were analysed using Chi-squared and Fishers exact tests to ensure comparability of groups. Primary outcomes were 30- and 60-day postoperative mortality and morbidity (Common Terminology Criteria for Adverse Events, CTCAE). Secondary outcomes were length of critical care and overall hospital stay. In addition, morbidity and mortality were compared between HIPEC agents (mitomycin and oxaliplatin/5-fluorouracil). Results99 patients (39.3%) and 153 patients (60.7%) underwent O-HIPEC, C-HIPEC respectively. Groups were well matched for baseline demographics, pathology, and HIPEC agent. In the O-HIPEC and C-HIPEC groups respectively, the incidence of 60-day complications (CTCAE 1–4) was 40.4% vs 39.3% (chi squared 0.94) and severe complications (CTCAE 3–4) 14% vs 13% (Fisher's exact p = 1) There was no perioperative mortality but one death in each group within the follow up period. There was no difference in morbidity or mortality between those receiving mitomycin or oxaliplatin. ConclusionClosed administration of HIPEC is safe with no difference in post-operative morbidity or mortality compared to open HIPEC administration. Differences in longer term oncological outcomes including overall survival and disease-free survival between open and closed HIPEC techniques are yet to be determined.

Appendiceal tumor in young adult

Appendiceal tumor in young adult

Prevalence of appendiceal tumors in post-operated appendectomy patients in the emergency room of the IPS Central Hospital from January through December 2020

Prevalence of appendiceal tumors in post-operated appendectomy patients in the emergency room of the IPS Central Hospital from January through December 2020

Signet Ring Cell Colorectal and Appendiceal Cancer: A Small Signet Ring Cell Component Is Also Associated with Poor Outcome.

Colorectal signet ring cell (SRC) carcinoma with ≥50% SRCs (SRC ≥ 50) has a poor prognosis, but the prognostic role of SRCs < 50% (SRC < 50) is unclear. The aim of this study was to provide a clinicopathological characterization of SRC colorectal and appendiceal tumours and analyse the importance of the SRC component size. All patients in the Swedish Colorectal Cancer Registry diagnosed with colorectal or appendiceal cancer in 2009-2020 at Uppsala University Hospital, Sweden, were included. The SRCs were verified, and the components estimated by a gastrointestinal pathologist. Of the 2229 colorectal cancers, 51 (2.3%) had SRCs, with a median component size of 30% (interquartile range of 12.5-40) and 10 (0.45%) had SRC ≥ 50. The SRC tumours were primarily localized in the right colon (59%) and appendix (16%). No patients with SRCs had stage I disease, and 26 (51%) had stage IV, of whom, 18 (69%) had peritoneal metastases. The SRC tumours were often high grade with perineural and vascular invasion. The 5-year overall survival (OS) rate for patients with SRC ≥ 50 were 20% (95% confidence interval (CI) 6-70), for SRC < 50, 39% (95% CI 24-61); and for non-SRCs, 55% (95% CI 55-60). Among the patients with SRC < 50 and <50% extracellular mucin, the 5-year OS was 34% (95% CI 19-61), while those with ≥50% extracellular mucin had an OS of 50% (95% CI 25-99). The 5-year recurrence-free survival rates were 51% (95% CI 13-83) for patients with SRC tumours, as compared to 83% (95% CI 77-89) and 81% (95% CI 79-84) for mucinous and non-mucinous adenocarcinoma, respectively. The presence of SRCs was strongly associated with aggressive clinicopathological features, peritoneal metastases, and poor prognosis, also when they make up <50% of a tumour.

Mucinous tumors arising from ovarian teratomas as another source of pseudomyxoma peritoneii: MR findings comparison with ovarian metastases from appendiceal mucinous tumors.

The origin of pseudomyxoma peritoneii (PMP) has been established as low-grade appendiceal mucinous tumors (AMT). However, intestinal-type ovarian mucinous tumors are known as another source of PMP. Recently, it is advocated that ovarian mucinous tumors causing PMP originates from teratomas. However, AMTs are often too small to detect on imaging; then, differentiating metastatic ovarian tumors of AMT from ovarian teratoma-associated mucinous tumors (OTAMT) is important. Therefore, this study investigates the MR characteristics of OTAMT compared to the ovarian metastasis of AMT. MR findings of six pathologically confirmed OTAMT were retrospectively analyzed compared to ovarian metastases of low-grade appendiceal mucinous neoplasms (LAMN). We studied the existence of PMP, uni- or bilateral disease, the maximum diameter of ovarian masses, the number of loculi, a variety of sizes and signal intensity of each content, the existence of the solid part, fat, calcification within the mass, and appendiceal diameters. All the findings were statistically analyzed using the Mann-Whitney test. Four of the six OTAMT showed PMP. OTAMT showed unilateral disease, had a larger diameter, more frequent intratumoral fat, smaller appendiceal diameter than those in AMT, and they were statistically significant (p < .05). On the other hand, the number, variety of size, signal intensity of loculi, and the solid part, calcification within the mass did not differ from each other. Both OTAMT and ovarian metastasis of AMT appeared as multilocular cystic masses with relatively uniform signal and size of loculi. However, a larger unilateral disease with intratumoral fat and smaller size of the appendix may suggest OTAMT. OTAMT can be another source of PMP, as AMT. MR characteristics of OTAMT were very similar to ovarian metastases of AMT; however, in cases with PMP combined with fat-containing multilocular cystic ovarian mass, we can diagnose them as OTAMT, not PMP caused by AMT.

Goblet cell adenocarcinoma of the appendix: A case report of three cases

Introduction and importanceAppendiceal goblet cell adenocarcinoma is in 0.3–0.9 % of appendectomy specimens. There is still controversy regarding whether surgery with dissection or additional resection is necessary for goblet cell adenocarcinoma and whether adjuvant chemotherapy is practical. We present three cases of goblet cell adenocarcinomas. Case presentationCase 1: A 30-year-old woman was diagnosed with appendicitis and underwent appendicectomy. Histopathological evaluation revealed a malignant neoplasm with goblet-like cells and tumour infiltration into the subserosa. The patient underwent laparoscopic ileocecal resection, and the main lymph nodes at the root of the feeding vessels were removed. Case 2: A 50-year-old man was diagnosed with appendicitis and underwent appendicectomy. Histopathological evaluation revealed a malignant neoplasm with goblet-like cells; malignant cells were found at the surgical resection margins. The patient underwent laparoscopic ileocolic resection. Case 3: A 60-year-old man undergoing treatment for malignant melanoma. He was diagnosed with appendicitis associated with an appendiceal tumour, and emergency laparoscopic caecal resection was performed and diagnosed as goblet cell adenocarcinoma. We decided to prioritize treatment for malignant melanoma, and the patient is under follow-up for goblet cell adenocarcinoma and no metastasis was detected. Clinical discussionWe performed additional resection in two case of goblet cell adenocarcinoma. Diagnosing appendiceal goblet cell adenocarcinoma is difficult, and the prognosis of patients with positive lymph nodes is poor. Surgical treatment should be considered for the advanced stages of this disease. ConclusionGoblet cell adenocarcinoma, diagnosed after appendectomy, additional resection including lymph node dissection may provide a long-term prognosis.

Discussion on the benefits of different treatment strategies in elderly and non-elderly patients with appendix MiNEN: a retrospective study based on SEER database.

To investigate the benefits of surgery alone and postoperative chemotherapy in elderly and non-elderly patients with appendiceal mixed neuroendocrine non-neuroendocrine neoplasms (MiNENs) and analyze the factors affecting the prognosis of patients with MiNEN of the appendix. Based on the Surveillance, Epidemiology, and End Results database (SEER) of the National Cancer Institute of the USA, 389 patients with appendiceal MiNENs from 2000 to 2016 were collected. All patients were distributed in the elderly group (≥ 60years old) and the non-elderly group (< 60years old) according to their age. The prognosis of the two groups of patients who received simple surgery and postoperative chemotherapy was analyzed and compared. The two treatment methods of the two tranches were matched by propensity score matching method. The effect of different treatment ways on the prognosis of sick persons was compared. The survivorship curves were painted by the Kaplan Meier method, log rank test was used to analyze the subsistence discrepancy of each group, and COX proportional risk model was used to analyze the factors affecting the prognosis of patients with appendiceal MiNENs. No matter the overall survival rate (OS) or cancer-specific mortality (CSM) of the two treatment schemes, the prognosis of patients in the only surgery group was meaningfully higher than that in the postoperative chemotherapy group, with statistically significant difference in component comparison (χ2 = 16.496, χ2 = 16.860, P < 0.001). After propensity score matching of patients in each group, there was no striking discrepancy in the OS of patients in the only surgery group compared with those in the postoperative chemotherapy group, regardless of whether they were elderly patients or non-elderly patients (χ2 = 3.205, χ2 = 1.521, all P > 0.05), the CSM consequences are fitting in with the OS. The consequences of the multivariate COX regression model showed that age (≥ 60years old), sex (female), high histological grade, and lymph node positive were all the influencing factors for the poor OS of patients with MiNEN; the CSM results are consistent with the OS. For patients with appendix MiNEN, whether elderly or non-elderly patients (especially for non-elderly patients), surgical treatment may be a better choice.

Abstract 6546: Peritoneal microenvironment promotes appendiceal adenocarcinoma tumor formation in PDX models

Abstract Background: Few preclinical models of appendiceal adenocarcinomas (AAs) exist. Orthotopic patient-derived xenografts (PDXs) better recapitulate the original tumor microenvironment. Therefore, we hypothesized that AA patient tumors should grow more successfully in orthotopic PDXs compared to standard flank implanted PDXs. Methods: After obtaining informed consent, tumor samples were obtained from 16 patients with AA undergoing surgical resection at MD Anderson Cancer Center. Tumors were implanted in both the flank and peritoneum of NSG mice; from each patient, at least 6 mice (n≥3 IP, n≥3 flank) were implanted with an approximately 0.5 cm3 tumor. Mice were assessed after 6 months for tumor formation. Whole transcriptomic RNAseq was performed on tumor samples taken from either the flank (n=3) or peritoneum (n=3) of two poorly differentiated (grade 3) patient tumors. The R package DESeq2 was used to conduct differential gene expression analysis comparing IP to Flank PDXs, controlling for the specific patient tumor. Finally, Gene Set Enrichment Analysis (GSEA) with the hallmark gene sets from MSigDB was implemented to study the relative up/down-regulation patterns of gene sets in each PDX cohort. Results: Tumor formation rate, regardless of grade, was greater in IP vs. flank implantation (31.4% vs 11.7%). Higher grade was associated with higher average tumor formation rates (IP—G1: 13.3%, G2: 33.8%, G3: 44.5%; Flank—G1: 0%, G2: 0%, G3: 31.2%). GSEA identified that cell proliferation and energy metabolism gene sets were significantly enriched in IP tumors relative to flank: E2F Targets, G2M Checkpoint, Oxidative Phosphorylation, MYC Targets V1, MYC Targets V2, and DNA Repair (NES = 2.91, 2.62, 2.60, 2.31, 1.61, 1.82, respectively; FDR q-val ≤ 0.01 for all). Meanwhile, gene sets associated with tumor environmental stress were enriched in flank tumors: Angiogenesis, Hypoxia, and TGF-β Signaling (NES = -2.00, -1.73, -1.62, respectively; FDR q-val ≤ 0.01 for all). Epithelial Mesenchymal Transition and TNF-α signaling via NF-κB were also enriched in flank tumors (NES = -2.28, -2.20; FDR q-val &amp;lt; 0.01). Conclusions: The peritoneal microenvironment promotes growth of appendiceal tumors in PDXs, with higher-grade being associated with greater tumor formation rate. GSEA also suggests that AAs preferentially grow in the peritoneal microenvironment in association with greater activation of cellular proliferation and energy metabolism pathways and lower activation of tumor environmental stress pathways. Citation Format: Vinay K. Pattalachinti, Ichiaki Ito, Saikat Chowdhury, Abdelrahman Yousef, Yue Gu, Keith F. Fournier, John P. Shen. Peritoneal microenvironment promotes appendiceal adenocarcinoma tumor formation in PDX models. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6546.