226 Background: Appendiceal adenocarcinoma (AA) is a rare and heterogenous cancer with marked differences in clinical course between high- and low-grade tumors. Unlike colorectal cancer (CRC) and other gastrointestinal malignancies, AA virtually never has hematogenous metastases, rather, metastasis is limited to the peritoneum. Here we present a retrospective, single institution study of AA to identify the prevalence of detectable ctDNA, evaluate the clinical predictive value of positive ctDNA, and assess what clinical, pathologic, or molecular features predict positive ctDNA. Methods: 160 blood samples from 147 patients with AA metastatic to the peritoneum were profiled with a CLIA approved 73 gene mutational panel as part of routine clinical practice. Paired tumor sequencing was available for 73 patients. Survival was measured starting from day ctDNA was drawn. Mutations that most likely represented clonal hematopoiesis were removed from analysis. Results: Out of 160 ctDNA samples, 120 were taken in the setting of radiographically apparent metastatic disease. Of these, 46 (38.3%) had any detectable mutation. 40 ctDNA tests were performed when patients had no radiographic evidence of disease (NED); 15 (37.5%) of which had any detectable mutation. High-grade tumors were more likely to have detectable ctDNA with detection rates of 10/46 (21.7%), 18/46 (39.1%), and 33/68 (48.5%) for well, moderately, and poorly-differentiated tumors, respectively. Restricting analysis to the 73 patients with paired tissue and blood samples and 73 genes sequenced in both, of 81 mutations detected in tumor only, 21 were detected in blood (sensitivity of 26%). Sensitivity was highest for mutation in TP53 (53.8%) suggesting these tumors may have a greater propensity to shed DNA into circulation. Overall, the sensitivity of ctDNA detection in metastatic AA was markedly less than what was observed in a cohort of 274 metastatic CRC patients from the same institution (288/581 = 49.6%). For AA patients with detectable ctDNA, variant allele frequency (VAF) in AA was significantly lower compared to CRC (median VAF 0.04% vs. 6%, p = <0.0001). Detectable ctDNA was associated with shorter overall survival (46.2 mo for positive ctDNA vs. not-yet-reached for negative ctDNA, HR = 2.5, p = 0.016) and shorter disease free survival (DFS) (60 mo vs. not-yet-reached, HR = 3.4, p = 0.05). As expected, patients with radiographic evidence of disease did worse than patients with NED (HR = 2.1. p = 0.08), but of note this hazard ratio was less than that for positive ctDNA. Conclusions: In patients with AA, the presence of detectable ctDNA is associated with shorter overall and disease-free survival. Sensitivity of ctDNA detection in metastatic AA is overall markedly lower than CRC, with detection more likely in high-grade tumors and higher sensitivity in tumors containing TP53 mutation.

163 Background: Appendiceal adenocarcinoma is both a rare and heterogenous tumor, with marked contrast in the natural history of low-grade and high-grade tumors (5-year OS 68% for low-grade vs. 7% for high-grade). While low-grade appendiceal adenocarcinoma is primarily treated with surgical resection sometimes followed by hyperthermic intraperitoneal chemotherapy (HIPEC), many inoperable candidates are treated with systemic chemotherapy although there is no prospective data supporting this practice. The purpose of our study was to objectively evaluate the effectiveness of systemic chemotherapy in low-grade mucinous appendiceal adenocarcinoma. Methods: A randomized crossover trial of surgically unresectable low-grade (well differentiated) mucinous appendiceal adenocarcinoma was performed with patients randomized to either 6 months observation followed by 6 months of chemotherapy (physician’s choice), or initial chemotherapy followed by observation. In this way each patient would serve as their own control. Enrollment of 30 patients was planned to have complete 6- and 12-month tumor measurements for 24 patients, providing 80% power at 0.05 significance level to detect a 5.0% difference in change in tumor size as measured by peritoneal RECIST in observation vs. treatment periods. Results: The trial closed early due to slow accrual. A total of 24 patients were enrolled. The majority of patients were treated with either 5FU or capecitabine (n = 15, 63%), bevacizumab was added for 3 (13%), and 3 were treated with doublet chemotherapy (FOLFOX/FOLFIRI). 15 patients who completed both treatment and observation periods were available for the primary analysis, the mean difference in tumor size was -4.5% (95% CI: -12.6, 3.7), indicating a slight trend towards faster growth on treatment than observation. This difference was not statistically or clinically significant (8.4% growth on treatment vs. 4.0% observation, p=0.26). Of the 18 patients who received any chemotherapy during the study period, zero achieved an objective response, 14 (77.8%) had stable disease during the entire year of follow up, and 4 (12.2%) had progression on study. Patient reported quality of life metrics identified that fatigue (p=0.02), peripheral neuropathy (p=0.014), and financial difficulty (p=0.0013) were all significantly worse while on treatment. There was not a significant difference in rate of bowel obstruction between the treatment first vs. observation first arms (12.5%, (n=3) vs 8.3%, (n=2)). Conclusions: These data from a prospective, randomized crossover trial indicate that patients with low-grade mucinous appendiceal adenocarcinoma do not derive benefit from 5FU based chemotherapy but do incur toxicity. These data further highlight the unique biology of low-grade appendiceal cancer and demonstrates the need to identify novel systemic therapies for this patient population. Clinical trial information: NCT01946854 .

223 Background: Goblet Cell tumors (GCTs) of the appendix are a rare, distinct, and under studied malignancy. Since 2019 the preferred World Health Organization (WHO) terminology is Goblet Cell Adenocarcinoma (GCA), but previously many terms have been used to describe these tumors including Goblet Cell Carcinoid and Adenocarcinoma ex goblet cell carcinoid as these tumors have a histological appearance that blends neuroendocrine and adenocarcinoma features. Historically goblet cell tumors have been considered one of the more aggressive subtypes of appendiceal cancer, but limited data exists and is mostly in the form of case reports. Here we present the retrospective analysis of a large single institution cohort. Methods: The internal database of the University of Texas MD Anderson Cancer Center (MDACC) was queried to identify all patients diagnosed with goblet cell appendiceal tumor. Patients were classified to two different histopathological groups, GCA (n=220) and GCA with signet ring adenocarcinoma (SRA) (n=146). Clinical, histopathological, and molecular data were extracted from the database in semi-automated fashion. Survival analysis were performed using Kaplan Meier methodology. Results: 366 patients with GCTs were identified from 1986 to 2022. 132 (36%) patients were seen during the last five years, with an average of 26 patients per year. Median follow up time was 54 months, while median age at diagnosis was 57 years. Tumor grade data was available for 294 patients. 95% of the patients had high grade tumors (moderately, moderately to poorly and poorly-differentiated) (n= 278), and 5% had low grade tumors (well and well to moderately-differentiated (n=16). The median overall survival was 85 months, and significantly different between the two groups, 118 months for the GCA group and 57 months for the GCA with SRA (p= 0.003). Lymph node (LN) status was known for 168 patients, rate of LN involvement was 53% (n=89) and significantly different between the two groups with 41% (n=39) for GCA and 68% (n=50) for GCA with SRA (p= <0.0006). The internal database of MDACC was queried for LN status of Mucinous adenocarcinoma (MA) (n=242) and SRA (n=104) for comparison purposes, rate of LN positivity was 13% in MA and 76% in SRA. Node positive patients had significantly worse overall survival with median overall survival of 51 months vs 85 months for node negative patients (p<0.004). By multivariate analysis, both LN status and SRA component were independent predictors of overall survival. 107 patients had gene mutation analysis tested, TP53, SMAD4, GNAS and KRAS were the most commonly mutated with 13%, 9%, 4%, and 3% respectively. Conclusions: This study highlights the heterogenicity of GCTs of the appendix and the importance of the histopathological classification in this distinct entity. GCT are much more likely to spread to LN and have a distinct somatic mutation profile relative to MA.

221 Background: Due to the rarity of appendiceal adenocarcinomas (AA), systematic study of these tumors has been limited. Thus, guidelines for the diagnosis and treatment of AA are often based on other related tumor types such as colorectal cancer. However, given that AA has been shown to be molecularly and functionally distinct, there is a need for focused clinical data to guide disease management. In AA, tumor marker levels are used by some practitioners to monitor response to treatment and aid in diagnosis. This study evaluates the association of elevated tumor marker levels with survival outcomes. Methods: The MDACC database was queried to identify patients with AA between 1997 to 2022. Patients with reported values for the tumor markers CA-125 (n=1076), CA 19-9 (n=1060), and CEA (n=1249) were then selected for analysis. Elevation of tumor markers was defined as above the laboratory upper limit of normal (CA-125 > 37 U/mL, CA 19-9 > 37 U/mL, and CEA > 3 ng/mL and survival outcomes were compared with a log-rank (Mantel-Cox) test. This analysis was repeated while controlling for tumor grade, which was defined by low-grade: well, well to moderately differentiated and high-grade: moderate, moderate to poor, and poorly differentiated. Results: Elevated CA-125 was predictive of overall survival in all patients with median survival not-yet-reached for those with normal CA-125 vs. 87.4 months for those with elevated CA-125 (HR: 5.8, p < 0.0001). Similarly, elevated CA 19-9 and CEA were also predictive of overall survival (HR: 2.8, 4.6, respectively, p < 0.0001 for each). Given that tumor grade is the primary driver of prognosis in AA, survival analysis was repeated while controlling for tumor grade. While elevated levels of all tumor markers were predictive of overall survival for both low-grade and high-grade tumors, elevated CA-125 was an especially strong predictor of survival in patients with high-grade tumors (OS: 69.8 months vs. not-yet-reached, HR: 14.3, p < 0.0001). Moreover, high-grade patients with elevated CA-125 had a reduced 5-year survival rate of 56% vs. 91%. Elevated CA-125 also stratified 5-year survival rate in low-grade patients (83% vs. 99%). Elevated levels of CA 19-9 and CEA were strongly predictive of overall survival for patients with low-grade tumors (HR: 10.6, 26.8, respectively, p < 0.0001 for each). Notably, patients with low-grade tumors expressing normal levels of CA 19-9 or CEA had excellent 5-year survival rates of 99% and 100%, respectively. Conclusions: These results highlight the utility of using tumor marker levels in conjunction with tumor grade to more accurately predict prognosis in AA. CA 19-9 and CEA were particularly useful indicators of outcome in patients with low-grade AA, while CA-125 had greatest prognostic value in high-grade tumors.

196 Background: The association between tumor mutations beyond KRAS and BRAF for patients with peritoneal metastases (PM) from colorectal cancer (CRC) are poorly understood. Here we describe these associations with survival after diagnosis of PM. Methods: CRC patients with PM with or without synchronous liver or lung metastases treated at a single institution from 2016 to 2020 were retrospectively studied. Standard of care next-generation sequencing was performed on tissue specimens with a targeted panel of genes frequently mutated in cancer. Clinical data, including survival and last contact, were obtained from medical records. Multivariate Cox regression was used to identify associations between overall survival after diagnosis of PM (OS) with clinical, histopathologic and somatic mutation data. Results: Of 526 PM patients, 179 (34.2%) also had synchronous liver metastases, and these patients had worse OS compared to PM alone (HR 1.67, [1.26-2.22], p=0.0003). Lung metastases were identified in 57 (10.8%) and were not associated with worse OS (HR 1.34, [0.90-1.97], p=0.15). Poorly differentiated grade (HR 2.08, [1.42-3.04], p=0.0002), mucinous histology (HR 1.44, [1.07-1.94], p=0.016), male gender (HR 1.37, [1.05-1.78), p=0.019), and age (HR 1.02, [1.01-1.04], p=0.004) were independently associated with OS. Tumor sequencing data was available for 442 (87%) patients. TP53 was the most common mutation (61.5% of patients) and was associated with decreased OS (HR=1.51 [1.12-2.02], p=0.005). BRAF mutation (85% of which were V600E) was identified in 9.7% of patients and was also associated with decreased OS (HR=1.88 [1.23-2.86], p=0.003). In contrast, PTEN mutation (4.3%) was associated with improved OS (0.41 (0.17-0.99), p=0.049), while both KRAS (51.4%) and SMAD4 mutations (16%) were not (KRAS: HR 0.94, [0.72-1.24], p=0.68; SMAD4: HR 0.78, [0.53-1.16], p=0.21). Conclusions: Mutational data for PM patients has a distinct profile and unique associations with OS when compared to other sites of metastatic disease. TP53 and BRAF mutations were associated with worse survival, while PTEN mutation was associated with improved OS. KRAS mutation was more prevalent than expected in an unselected cohort, and not associated with OS. A better understanding of PM-specific associations with survival may influence treatment strategies for this difficult disease site.