Impact of Tumor Side on Clinical Outcomes in Stage II and III Colon Cancer With Known Microsatellite Instability Status

Background Tumor sidedness as a prognostic factor in advanced stage colon cancer (CC) is well established. The impact of tumor sidedness on the clinical outcomes of stage II and III CC has not been well studied. Methods The National Cancer Database (NCDB) was utilized to identify patients with pathological stage II and III primary adenocarcinoma of the colon from 2010 to 2015 using ICD-O-3 morphology and topography codes: 8140-47, 8210-11, 8220-21, 8260-63, 8480-81, 8490 and C18.0, 18.2,18.3, 18.5,18.6, 18.7. Univariate (UVA) and multivariable (MVA) survival analyses and Kaplan–Meier Curves with Log-rank test were utilized to compare overall survival (OS) based on tumor location and treatment received. Results A total of 35,071 patients with stage II (n = 17,629) and III (n = 17,442) CC were identified. 51.3% female; 81.5% Caucasian; median age 66 (range, 18–90). Majority of stage II and III tumors were right sided, 61.2% (n = 10,794) and 56.0% (n = 9,763). Microsatellite instability high (MSI-H) was more common in stage II compared to III, 23.3% (n = 4,115) vs 18.2% (n = 3,171) (p < 0.0001). In stage II MSI-H CC right was more common than left, 78.3% (n = 3223) vs 21.7% (n = 892). There was no significant difference in survival between stage II MSI-H left vs right (5-year OS 76.2 vs 74.7%, p = 0.1578). Stage II MSS CC right was more common than left, 56.0% (n = 7571) vs 44.0% (n = 5943), and survival was better in the left vs right (5-year OS 73.2 vs 70.8%, p = 0.0029). Stage III MSI-H CC was more common in the right than in the left, 75.6% (n = 2,397) vs 24.4% (n = 774) and survival was better in the left (5-year OS 62.5 vs 56.5%, p = 0.0026). Stage III MSS CC was more common in the right than in the left, 51.6% (n = 7,366) vs 48.4% (n = 6,905), and survival was better in the left vs right (5-year OS 67.0 vs 54.4%, p < 0.001). Conclusion Survival was better in left sided tumors compared to right in stage II MSS, stage III MSS, and stage III MSI-H CC.


INTRODUCTION
Colorectal cancer is the third most common cancer and third leading cause of cancer related mortality in the United States (US) (1). It is estimated that 104,610 new cases of colon cancer (CC) will be diagnosed in the US in 2020. Two thirds of patients present with locoregional disease, and primary tumor location could have a significant impact on the prognosis in CC across all stages (1)(2)(3). The predictive role of tumor sidedness was described in the locoregional (4)(5)(6) and metastatic setting (7,8). Embryologic and physiologic differences exist between the left and right sides of the colon. The portion of the large intestine from the cecum to the proximal two thirds of the transverse colon is derived from the midgut, and the distal third of the transverse colon to the upper anal canal is derived from the FIGURE 1 | Consort diagram.    (17,20,(24)(25)(26)(27). The prognostic role of tumorsidedness has been extensively studied in locoregional CC; however, MSI status was not included in these studies (4)(5)(6). Given the paramount importance of MSI status in locoregional CC management and the propensity for MSI-H tumors for the right side, it is imperative to analyze the impact of tumor sidedness with known MSI status. The aim of this study is to evaluate the impact of primary tumor side, left-sided (L) versus right-sided (R), on clinical outcomes based on known MSI status in patients with stage II and III CCs. We also sought to determine whether tumor side based on known MSI status is predictive of adjuvant chemotherapy (AC) benefit in stage II and III CCs.

PATIENTS AND METHODS
The National Cancer Database (NCDB) was utilized to identify patients with pathological stage II and III primary adenocarcinoma of the colon between years 2010 and 2015 who underwent resection. The NCDB contains clinical and demographic information on 70% of all incident cancers in the United States from >1,500 Commission-on-Cancer-accredited cancer centers. It is a joint quality improvement initiative of the American College of Surgeons Commission on Cancer and the American Cancer Society. Eligibility was obtained using the following ICD-O-3 morphology and topography codes: 8140-47, 8210-11, 8220-21, 8260-63, 8480-81, 8490 and C18.0, 18.2, 18.3, 18.5, 18.6, 18.7 ( Figure 1). Since portions of the transverse colon are within the left and right sides of the colon, tumors of the transverse colon were excluded. Patients that received neoadjuvant systemic/radiation therapy and adjuvant radiation were also excluded. Microsatellite stability status was divided into microsatellite stable (MSS) which included MSI stable (code 020) and MSI unstable low positive (code 040). Microsatellite unstable (MSI-H) status included MSI unstable high positive (code 050) and MSI unstable positive (code 060). Tumors without known MSI status were excluded. The primary outcome was OS difference between patients with right-sided tumors compared to left-sided tumors based on MSI status. The secondary outcome was OS of patients who received adjuvant chemotherapy compared to patients that received no treatment stratified by tumor side and MSI status. The following patientspecific covariates were included: sex, race, facility type, insurance status, year of diagnosis, AJCC pathologic stage, primary site, surgical margin status, microsatellite stability status, regional lymph nodes examined, Charlson-Deyo score, chemotherapy, type of surgery, age at diagnosis, and tumor size ( Table 1). No ethical approval was required for the study as deidentified patient information in the NCDB is legally accessible to the public.

Statistical Analysis
The clinical and demographic characteristics of the patients were summarized using descriptive statistics as appropriate for variable type and distribution. For numeric covariates, the mean, median, range, and standard deviation were presented. Frequency and its percentage were generated for categorical variables. For descriptive statistics, chi-square tests were performed for categorical variables and ANOVA for continuous variables. OS was defined as months from diagnosis to death or last contact, where those who were alive were censored at last contact. OS was estimated using the Kaplan-Meier method, and patient variables were compared

Patient Demographics and Tumor Characteristics
A total of 35,071 patients with resected pathological stage II (n = 17,629) and III (n = 17,442) CCs were identified (Figure 1). Baseline clinicopathological characteristics are summarized in   Table 2.
For stage III CC, survival was better in MSS compared to MSI-H (5-year OS: 60.5 vs 58.0%, p < 0.001) ( Figure 3A). However, after the adjustments of potential confounders in multivariable analysis, stage III MSI-H tumors were no longer associated with OS difference compared to MSS (HR 0.96, 95%  Table 3). Stage III MSI-H CC survival was better in L vs R (5-year OS 62.5 vs 56.5%, p = 0.0026) ( Figure  3B). Stage III MSS CC survival was better in L vs R (5-year OS 67.0 vs 54.4%, p < 0.001) ( Figure 3C).

DISCUSSION
The results of this study demonstrate that survival was better in the left compared to right-sided tumors among stage II MSS, stage III MSS, and stage III MSI-H CC patients. In stage II MSI-H CC, there was no difference in survival among the left versus right-sided tumors. This study confirms and emphasizes previous reports that bearing a left-sided tumor was associated with significantly improved survival (2,5,6). In two different SEER-Medicare studies, right-sided stage II cancers had higher overall survival than left-sided cancers and right-sided stage III CC had lower overall survival than left-sided CC (4,6). However, similar to prior studies, they did not have MSI status of the tumors. Results from a recent meta-analysis of 66 studies concluded that tumors originating in the left side of the colon were significantly associated with an absolute 19% reduced risk of death (2). Such a survival benefit was independent of race, stage (II, III, IV), year of publication, and type of study (2). Several studies have found that patients with MSI-H tumors have an improved prognosis and that MSI status is an independent predictor of overall survival (19)(20)(21)(22)(23). MSI-H is mostly seen in right-sided CC (17), with less than 5% seen in left-sided CC (14). The stage profile of MSI-H tumors is also more favorable (4). It is estimated that MSI-H accounts for 20-25% of stage II right-sided cancers and 15% of stage III right-sided tumors (28). MSI-H tumors have also been associated with a decreased risk of lymph node and distant organ metastases; providing further evidence that right-sided stage III cancers may be more biologically distinct from right-sided stage II cancers (22). Thus, primary tumor location can be used as a prognostic tool in CC in clinical decision-making processes especially with known MSI status as described in this study.
The results of this study demonstrated that there was no survival benefit from AC for stage II left-sided MSI-H patients; however, survival benefit from AC was observed for stage II right-sided MSI-H patients, left-and right-sided stage II MSS patients. Significantly more patients with left sided tumors received chemotherapy in all groups and the same survival findings were seen after adjustment of potential confounders by multivariate analysis. These results differed from those reached by Weiss et al., whereby no survival benefit was seen for either stage II right-sided or left-sided CC patients who received AC compared to those who did not (29 (29)(30)(31)(32)(33)(34)(35)(36)(37)(38). Survival benefit from AC is established for stage III CC (13); however, uncertainty exists for stage II patients (29). In resected stage II CC, the presence of MSI has been associated with a more favorable prognosis and lack of benefit from fluorouracil-based AC (39). Sinicrope et al. evaluated the prognostic impact of MSI status in patients with stage III CC enrolled in a randomized trial of FOLFOX-based AC and found that MSI-H proximal tumors (right-sided) had favorable disease free survival compared to MSS (40). In their analysis of five previous randomized trials of fluorouracil based AC, Ribic et al. found that there was no benefit from AC in stage II and III MSI-H CC in contrast to a benefit seen in MSS tumors (19). Given the previously identified relationship between tumor location and clinical outcomes without known MSI status, we sought to determine the impact of tumor location with known MSI status on the clinical outcomes of stage II and III CC patients.
To the best of our knowledge, this is the first study that describes the site of CC (right vs left) as an independent prognostic factor in the presence of known MSI status in stage II and III CC. This eliminates the potential bias associated with conclusions reached by other studies that utilized tumor location as a surrogate for MSI status. Despite the uniqueness of the analysis, this is a retrospective study with its inherent limitations. Patient treatment preferences and physician practice patterns are unmeasured factors that may play a role in clinical outcomes. Results of this study could be subject to unmeasured confounding

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

ETHICS STATEMENT
No ethical approval was required for the study as de-identified patient information in the NCDB is legally accessible to the public.

AUTHOR CONTRIBUTIONS
MA conceptualized the study, conducted the data curation, performed the formal analysis, conducted the investigation, was in charge of the project administration, supervised the study, conducted the validation, and wrote, reviewed, and edited the article. KZ conceptualized the study, conducted the data curation, performed the formal analysis, conducted the investigation, was in charge of the project administration, supervised the study, conducted the validation, and wrote, reviewed, and edited the article. RJ conducted the data curation, developed the methodology, provided the software, wrote the original draft, and wrote, reviewed, and edited the article. SW conducted the data curation, developed the methodology, provided the software, wrote the original draft, and wrote, reviewed, and edited the article. OA developed the methodology, conducted the investigation, wrote the original draft, and wrote, reviewed, and edited the article. WS developed the methodology, conducted the investigation, wrote the original draft, and wrote, reviewed, and edited the article. CW developed the methodology, conducted the investigation, wrote the original draft, and wrote, reviewed, and edited the article. MB conceptualized the study, was in charge of the project administration, conducted the investigation, validated the study, and reviewed and edited the article. BE-R developed the methodology, conducted the investigation, wrote the original draft, and wrote, reviewed, and edited the article. All authors contributed to the article and approved the submitted version.

ACKNOWLEDGMENTS
Part of the data presented in this study was presented at the 2020 ASCO Annual Virtual Meeting. The manuscript's abstract was