Methods
Study sample and histologic evaluation
Twenty-four cases of CIAM were retrieved from the Department of Pathology, Daehang Hospital, between March 2011 and March 2017. Ten were retrospectively collected from pathological data files of patients treated between March 2011 and December 2013, while 14 were identified prospectively between January 2014 and March 2017. All lesions were completely excised either endoscopically (n = 22) or surgically (n = 2); 1 of the latter lesions was removed by right hemicolectomy because of the presence of a synchronous huge adenoma, and the other was removed via low anterior resection because the physician suspected a malignancy. None of the patients had any history of inflammatory bowel disease (IBD) or familial adenomatous polyposis (FAP).
All specimens were routinely processed, stained with hematoxylin and eosin, and evaluated by a gastrointestinal pathology specialist (M.J.K.). The degree of dysplasia in the epithelial component was assessed according to the architectural complexity, extent of nuclear stratification, and severity of abnormal nuclear morphology, and was classified into low-grade dysplasia (LGD) or high-grade dysplasia (HGD) [
11]. Tumors exhibiting lamina propria invasion with no submucosal extension were diagnosed as intramucosal carcinomas.
The sizes of the polyps were measured by the pathologist, while the anatomical locations were identified and classified as the proximal colon (up to the splenic flexure) versus the distal colon/rectum.
The study was performed according to the Declaration of Helsinki, and was approved by the institutional review board at Daehang Hospital (approval number DH17–001). Obtaining additional informed consent for the use of patient samples was not required, as the specimens were coded to protect patient confidentiality.
Patients were assessed for clinicopathologic characteristics including age, sex, and pathology reports. Follow-up data (mean, 23.1 months) were available for 18 patients who underwent endoscopic procedures (n = 17) or surgery (n = 1) between March 2011 and May 2016. Two patients were lost to follow-up. The remaining samples (n = 6) were collected subsequently from patients who had not yet undergone routinely scheduled follow-up visits.
Immunohistochemical staining
Immunohistochemistry was manually performed by using formalin-fixed, paraffin-embedded blocks. Sections (3 μm) were cut, deparaffinized in xylene, and dehydrated in increasing concentrations of ethanol. Immunohistochemical staining was performed with anti-synaptophysin (clone Z66, 1:100 dilution; Invitrogen, Melbourne, Australia), anti-chromogranin (clone NS55, 1:100 dilution; Invitrogen), and mouse monoclonal anti-Ki-67 (clone 7B11, 1:100 dilution; Invitrogen) after routine microwave antigen retrieval. Negative control samples underwent the same procedure with the omission of the primary antibody. Slides were counterstained with Mayer’s hematoxylin.
Immunoreactivity for synaptophysin and chromogranin was evaluated as positive or negative. Negative protein expression was defined as the complete absence of cytoplasmic staining in the microcarcinoid component in the presence of positive labeling in non-neoplastic internal control cells, while the opposite staining pattern was considered positive expression. Ki-67 immunostaining was performed to determine the proliferative activity and grade of the microcarcinoid NE cells. Nuclear immunostaining at known proliferative locations, such as germinal centers and the basal half of the crypt epithelium, was used as an internal positive control for each sample.
Statistical analysis
Data analyses were performed using SPSS version 21.0 (SPSS Inc., Chicago, IL, USA). Student’s t-test was used to compare parametric distributions, while the χ
2 or Fisher’s exact test was used for frequency distributions. A P-value <0.05 was considered statistically significant.
Discussion
Since first described by Pulitzer et al. in 2006, CIAMs have been recognized as a rare intestinal neoplasm consisting of intermingled adenomatous and WD NE components [
1]. Unlike other mixed adenoneuroendocrine tumors of the large intestine in which the NE component occupies a substantial proportion of the tumor, the NE component of a CIAM occupies only a minute region of the polyp without disturbing the overall architecture [
1,
11]. The NE component found in CIAM differs from classic colorectal neuroendocrine tumors (NETs) in that it does not form a visible nodule and is always accompanied by a glandular neoplasm occupying the majority of the polyp by definition [
1]. Additionally, most microcarcinoids in CIAMs are reportedly located in the basal lamina propria, contrary to classic NETs in which the epicenters are located in the submucosa [
1,
11]. Therefore, the NE component in CIAMs is always incidentally found during the pathologic examination of adenomatous polyps, while rectal NETs are discovered during routine rectal examinations or endoscopies as submucosal masses or due to the presence of clinical symptoms such as rectal bleeding, pain, or constipation [
11]. The majority of colonic NETs are large, with average sizes of 4.9 cm; therefore, they are frequently symptomatic [
11].
To date, reports of CIAMs have been sporadic. The largest study was conducted by Salaria et al., who investigated 11 prospectively collected cases over a 7-year period [
1,
7‐
10]. However, their study was limited by the fact that most cases (
n = 9, 81.8%) were obtained by external consultation; therefore, clinicopathologic data regarding polyp location, procedure type, and immunohistochemical staining results for NE markers were lacking. Another study of 7 CIAM cases by Lin et al. included 5 in the large intestine and 2 in the duodenum [
7]. We excluded 2 duodenal cases from our literature review because our study includes CIAMs that occurred only in the colon and rectum. We also excluded 1 of Lin et al.’s remaining 5 cases because the lesion resembled a goblet cell carcinoid based on histologic photographs showing bland-looking glands with prominent goblet cells infiltrating the submucosa [
7]. A goblet cell carcinoid is a distinct form of mixed adenoneuroendocrine tumors, which usually shows aggressive biologic behavior despite its bland-looking histology [
11,
12]. We also excluded a study published by Estrella et al. because we could not clearly distinguish CIAM cases from adenoma/low-grade NETs [
13]; in their study, the majority of cases included in the adenoma/low-grade NET category are presumed to be mixed adenoma/classic NET because a substantial proportion of their cases (40%) invaded the submucosa. Additionally, the authors classified 4 cases arising from the duodenum under the same category as 19 colorectal cases, despite pre-existing evidence indicating that the anatomic site is one of the most important factors that affect the clinical behavior of NETs [
11]. Four patients known to have FAP were also included in the adenoma/low-grade NET category, raising our concerns over the pathogenetic heterogeneity of their cases, which can potentially affect the analysis of clinical outcomes. After excluding the above-mentioned cases, we summarized the clinicopathologic data of 21 previously reported CIAMs in Table
2. In our present study, we enrolled and analyzed CIAM cases that only occurred in the colon and rectum, and did not include any patients with a history of IBD or FAP to minimize the genetic and pathogenetic heterogeneity of the investigated cases (Table
1).
Table 2
Clinicopathologic features of the previous studies
1 | | 80/M | Ascending colon | Polypectomy followed by RHC | 30 | TVA, HGD | TVA and carcinoid tumor | NS | + | NS |
2 | Pulitzer et al., 2006 [ 1] | 77/F | Cecum | Polypectomy followed by RHC | <10 | TA, HGD | TA with HGD and endocrine neoplasia of UMP | NS | + | + |
3 | Pulitzer et al., 2006 [ 1] | 77/M | Cecum | Polypectomy | 13 | TA | TA with squamous metaplasia | NS | + | – |
4 | Pulitzer et al., 2006 [ 1] | 77/F | Cecum | Polypectomy followed by RHC | 20 | SIAC | TVA, HGD | NS | + | + |
5 | Pulitzer et al., 2006 [ 1] | 62/F | Descending colon | Polypectomy | 5 | TA | TA with microcarcinoid | NS | + | + |
6 | | 51/M | Rectum | Polypectomy followed by proctosigmoidectomy | 20 | TVA | TVA with microcarcinoid | 1 | + | + |
7 | | 59/M | Rectum | Polypectomy followed by proctosigmoidectomy | 13 | Adenoma | Adenocarcinoma | 1 | + | + |
8 | | 66/F | Cecum | Polypectomy followed by RHC | 15 | TA | TA and microcarcinoid | 1 | NS | NS |
9 | | 56/M | Sigmoid colon | Polypectomy followed by transanal excision | 25 | TVA | TVA and microcarcinoid | 1 | NS | NS |
10 | | 55/M | Right colon | NS | 53 | TA | Suspicion of invasive carcinoma | 2 | + | NS |
11 | | 54/F | Transverse colon | NS | 14 | TA, HGD | TA, HGD | 1 | NS | NS |
12 | | 81/F | Left colon | NS | 12 | TVA, HGD | TVA, HGD | 3 | + | – |
13 | | 28/F | Right colon | NS | 34 | TVA, HGD | TVA, HGD, with squamous morules | 7 | + | – |
14 | | 82/M | Right colon | NS | 15 | TVA | Suspicion of invasive carcinoma | 5 | + | – |
15 | | 72/F | NS | NS | 15 | TVA | NS | 5 | NS | NS |
16 | | 60/F | Right colon | NS | 7 | TVA, HGD | TVA, HGD | 3 | – | – |
17 | | 55/M | Left colon | NS | 12 | TA | TA with squamous morules | 7 | NS | NS |
18 | | 48/M | Left colon | NS | 15 | TVA | TVA with squamous morules | 4 | + | – |
19 | | 62/M | Left colon | NS | 25 | TVA | TVA with squamous morules | 4 | NS | NS |
20 | | 51/F | Left colon | NS | 30 | TVA | TVA with squamous morules | 2 | NS | – |
21 | | 73/M | Hepatic flexure | NS | NS | TVA | NS | NS | + | – |
Herein, we summarized the clinicopathologic findings of a total of 45 CIAM cases, including results from previous studies as well as our own. There were 24 men and 21 women, indicating a nearly equal sex ratio (male:female = 1.1:1). The patients’ ages ranged from 28 to 82 years, with a mean age of 62.6 years. Notably, the mean age among our 24 patients (62.0 years) did not significantly differ from the 21 patients in the previous studies (63.1 years, p = 0.767). Approximately two-thirds (n = 29, 64.5%) of the microcarcinoid components were accompanied by adenoma with LGD (17 tubular, 11 tubulovillous, and 1 unspecified), 12 by adenoma with HGD (4 tubular, 8 tubulovillous), 2 by intramucosal carcinoma arising in tubulovillous adenoma, and 2 by submucosal invasive adenocarcinoma. Therefore, we suggest that the terminology of “CIAM” might be misleading because microcarcinoids can be associated with glandular lesions that exhibit various histologic degrees of dysplasia.
Contrary to classic NET, CIAMs tended to occur in the colon according to data from previous studies as well as ours, which indicated that most lesions (38 of 44, 86.4%) were located in the colon except for 6 (13.6%) that occurred in the rectum and 1 that had an unknown location. In particular, more than half of the polyps were located in the proximal colon (27 of 44 cases, 61.4%), with the most frequent site being the ascending colon (9 of 44 cases, 20.5%). The size of the polyps ranged from 5 to 127 mm, with a mean size of 23.8 mm. There was no significant difference in polyp size between our study and previous findings (27.2 mm vs. 19.6 mm, respectively; p = 0.277). The microcarcinoid component was confined to the mucosa with (22 of 40 cases) or without (18 of 40 cases) a connection to the glandular component. The mean size of the microcarcinoid component was 4.7 mm, and did not differ significantly in our patients compared to those in previous studies (4.7 mm vs. 3.1 mm, respectively; p = 0.201).
In our study, all lesions were completely removed either by endoscopic procedures (n = 22) or surgical resection (n = 2), and no subsequent surgeries were required. This is inconsistent with the results of previous studies in which most patients underwent subsequent surgery (n = 7, 77.8%) because of incomplete or partial prior polypectomy (n = 9). Moreover, removal of the majority of samples (83.3%) by endoscopic mucosal resection (EMR, n = 7), endoscopic submucosal dissection (ESD, n = 11), and surgery (n = 2) in our study facilitated the procurement of well-oriented tissue sections perpendicular to the basal lamina and muscularis mucosae; this assisted us in collecting the largest series of CIAM samples. A microcarcinoid component cannot be detected in poorly-oriented tissue samples such as small polypectomy specimen because it is mainly located in the basal lamina propria. Nevertheless, the prevalence rate of CIAM appears to be extremely low based on our estimates. We prospectively collected 13 CIAM cases from among 40,939 patients who underwent endoscopic procedures including polypectomy, EMR, and ESD between January 2014 and March 2017.
So far, the natural history as well as the pathogenetic mechanism of colorectal microcarcinoids have not been fully elucidated because of their rarity, which in turn may partly be due to their under-recognition. As a result, microcarcinoids occurring in the colon and rectum have remained an ambiguous entity denoting small-sized NE lesions in many instances. According to the definition of microcarcinoid described by Pulitzer et al., most previously reported microcarcinoids occurring in ulcerative colitis patients appear to correspond to small-sized classic NETs [
1]. As for the stomach, most enterochromaffin-like cell NETs arise in patients with chronic atrophic gastritis or multiple endocrine neoplasia type 1-Zollinger-Ellison syndrome through a sequence of hyperplasia-dysplasia-neoplasia, where growth patterns as well as endocrine cell sizes are known to be important for the classification of such lesions [
2,
3,
11]. A gastric NE lesion is classified as a microcarcinoid when the nodule is greater than 0.5 mm but less than 5 mm in size, or if it invades the submucosa; lesions less than 0.5 mm and confined to the mucosa are designated as carcinoma in situ/dysplasia [
11]. However, to our knowledge, such size-based criteria have not yet been defined in colorectal microcarcinoids. Based on our findings, the clinical outcomes of colorectal microcarcinoids appear to be quite favorable regardless of their sizes, likely because none of the patients showed submucosal invasion and/or increased proliferative activity. Considering that none of the 45 CIAM patients showed recurrence or metastasis after endoscopic or surgical treatment, even in lesions larger than or equal to 5 mm (8 of 39 cases, 20.5%), the 5 mm-size cutoff appears to be meaningless. Therefore, we posit that the absence of submucosal invasiveness and/or proliferative activity of the NE cells, and not their size, explain the favorable biologic behavior of microcarcinoids.
It is also worth considering whether or not colorectal microcarcinoids are neoplastic lesions and how best to define them. We suggest that it is premature to define colorectal microcarcinoids as neoplastic lesions, particularly when these lesions are confined to the mucosa with no obvious signs of proliferative activity. Indeed, 7 of 9 patients (77.8%) in previous studies underwent additional surgery after polypectomy for fear of residual lesions and possible ominous outcome. We suggest that intramucosal WD NE lesions in the colorectum should be distinguished from classic small-sized or microscopic NETs.
From a pathologist’s perspective of view, awareness of microcarcinoids is critical; however, it is also important to avoid over-interpretation while not overlooking or under-recognizing such lesions. Pathologists can misinterpret microcarcinoids as high-grade lesions such as invasive components of associated glandular lesions, particularly when microcarcinoids show infiltrative or single-cell patterns, or else can consider them small-sized classic NET. In the former case, identifying desmoplastic reactions that appear as myofibroblastic proliferation as well as checking for NE differentiation are important. According to our analysis, the most common mistake was to overlook the microcarcinoid component (7 of 43 cases, 16.3%), followed by misdiagnosis of the microcarcinoid as a squamous metaplasia (squamous morules) (6 cases, 14.0%) or invasive glandular component (3 cases, 7.0%) [
1,
7,
8]. To that point, a CIAM reported by Lyda et al. was diagnosed as a composite adenoma-carcinoid tumor [
10].