Frequency distribution of histologic subtypes in CRC
71,810 CRC patients were included in this study from SEER registers during 2010 – 2012. According to the ICD-O-3 codes and description, 49,131 (68.4%) cases were classified as adenocarcinoma NOS, not otherwise specified. The rest 22,679 (31.6%) patients were analyzed for frequency distribution of histologic subtypes in CRC (Supplementary Table 1). Except for adenocarcinoma with neuroendocrine differentiation (Code 8574), which is a distinct subtype but accounts for a very small population, all the others (n=22,636, 99.8%) were included and categorized into four histologic subtypes: CA, MAC, SRCC, and AM. The most common subtype was CA, with 15,812 cases accounting for 69.9%. The numbers and frequencies of MAC, SRCC and AM were 5,689 (25.1%), 814 (3.6%), and 321 (1.4%), respectively (Table 1). Both SRCC and AM are relatively rare with their frequencies of less than 5%.
Comparisons of clinicopathologic differences between histologic subtypes
MAC was more common in female (P< 0.001) and older patients (P< 0.001) compared with CA. There was no significant difference in the distribution of gender and sex between CA and SRCC or AM. Compared with CA, all the other three subtypes were found less common in rectal cancer (all P< 0.001). In addition, MAC, SRCC,and AM, were significantly associated with some features of aggressiveness, including poor tumor differentiation, large size of primary tumors, high level of carcinoembryonic antigen (CEA), advanced T stage and N stage, distant metastasis, high positive rates of circumferential resection margin (CRM) involvement and perineural invasion, as well as frequent presence of tumor deposits (all P< 0.001, Table 2, P1 using “CA” as the reference).
We further compared the clinicopathologic differences between AM and the other two relatively more aggressive histologic subtypes: MAC and SRCC. Compared with MAC, AM was found more frequently in males (P= 0.05). AM was also associated with aggressive tumor characteristics including poor differentiation (P< 0.001), more advanced T and N stage (P< 0.001), distant metastasis (P= 0.002), higher positive rates of CRM (P< 0.001), perineural invasion (P< 0.001), and frequent presence of tumor deposits (P< 0.001). As for the comparison between AM and SRCC, AM was associated with better differentiation (P< 0.001), distant metastasis (P< 0.001), and perineural invasion (P= 0.003). No differences were found in other clinicopathologic characteristics between AM and SRCC. Detailed information was shown in Table 2 (P2, using “MA” as the reference).
The prognostic value of histologic subtypes for CRC specific OS
We compared the 3-year CRC specific OS rates between histologic subtypes (Table 3) in the general population and subgroups stratified by TNM stage (0+Ⅰ/Ⅱ/Ⅲ/Ⅳ), tumor location (Colon / Rectum), sex (Male / Female) and age(≤ 66 / > 66). The 3-year OS rates was 90.3 ± 0.004%, 71.6 ± 0.01%, 38.0 ± 0.06% and 49.8 ± 0.06% for CA, MAC, SRCC, and AM, respectively. MAC, SRCC, and AM showed significantly poor survival rates compared with CA. And this difference sustained in most of the subgroups, except for certain TNM stage subgroups. For instance, in stage IV patients, MAC did not show significant difference in the 3-year OS rate compared with CA. Additionally, there was no obvious difference in the 3-year OS rate when comparing SRCC and CA in stage 0+Ⅰ patients. So was when comparing AM and CA in stage Ⅱ patients. Compared with AM, MAC showed significantly better 3-year OS in the general population as well as in most subgroups, while no prognostic differences were found between AM and SRCC (Table 3, P1 using “CA” as the reference, P2 using “MA” as the reference). The CRC-specific OS of the four subtypes estimated using the Kaplan-Meier method were shown in Figure 1. Stratified by TNM stage, AM remained to present significantly worse CRC-specific OS compared with CA in stage 0+Ⅰ, stage Ⅲ, and stage IV groups (Supplementary Figure 1, P=0.04, P<0.001, P=0,001), but not in stage Ⅱ (Supplementary Figure 1, P=0.43).
We then conducted univariate and multivariate analysis to test the prognostic differences in CRC specific OS between histologic subtypes. By univariate analysis, besides histologic subtypes (P< 0.001), other significant prognostic factors including age (≤ 66 / > 66, P< 0.001), tumor location (Colon / Rectum, P< 0.001), grade (Well differentiated / Moderately differentiated / Poorly differentiated or undifferentiated, P< 0.001), TNM stage (0+Ⅰ/Ⅱ/Ⅲ/Ⅳ, P< 0.001), race (American Indian/Alaska Native / Asian or Pacific Islander / Black / White, P< 0.001), insurance status (Insured / Others, P< 0.001), marital status (Married / Widowed / Others, P< 0.001), CEA level (Normal / Borderline / Elevated, P< 0.001), CRM (Negative / Positive, P< 0.001), perineural invasion (Negative / Positive, P< 0.001) and tumor deposits (Absent / Present, P< 0.001) were identified. All the significant prognostic factors identified by univariate analysis were included for multivariate Cox regression analysis. Factors remained as independent prognostic factor included age (P< 0.001), grade (P= 0.001), TNM stage (P< 0.001), marital status (0.003), CEA (P< 0.001), CRM (P< 0.001), tumor deposits (P< 0.001) and histologic subtype (P< 0.001). After adjusting for confounding factors, MAC didn’t have a significantly different prognosis (P= 0.20, hazard ratio (HR) and 95% confidence interval (95% CI): 1.14 (0.93 – 1.39)), while the inferior prognosis of SRCC and AM remained significant (P< 0.001 and P= 0.003, HR and 95% CI: 1.88 (1.37 – 2.58) and 1.89 (1.25 – 2.85), respectively) compared with CA (Table 4). In addition, compared with AM, MAC had a significantly better prognosis (P = 0.01, HR and 95% CI: 0.60 (0.40 – 0.90)), while no survival difference was found between AM and SRCC (P = 0.98, HR and 95% CI: 0.99 (0.65 – 1.53)) (Table 4).