Introduction

Adenocarcinoma, not otherwise specified is the most common colorectal cancer (CRC) accounting for 95% of all tumours. The term ‘mucinous adenocarcinoma’ is used to describe tumours with >50% of extracellular mucin while the term ‘signet ring cell carcinoma’ (SRCC) is preferred when the tumour contains >50% of signet ring cells, with intracytoplasmic mucin vacuoles and peripherally placed nucleus. ^[1] If signet ring cells constitute < 50% of tumor cells, then, it is designated as tumor having a component of signet ring cells (CSRC). ^[2] Signet ring cell carcinoma constitute 1.1% of colorectal carcinoma. Secondary involvement of the colo-rectum is a known but rare occurrence. Carcinomas metastasizing to the colon include those from stomach, lung, prostate, breast and ovaries. ^[2] However, diagnosis of metastatic adenocarcinoma on a colonoscopic biopsy is challenging due to various reasons, an important one being that the endoscopic biopsies may not be representative as the deposits may be deep seated or even serosal without producing any mucosal changes. ^[3] We describe 5 cases of metastatic deposits in the colo-rectum, in which relevant history was not always available. Immunohistochemistry (IHC) was quite helpful in identifying these cases as metastatic signet ring cell adenocarcinoma. One last case of rectal carcinoma mimicked metastatic signet ring cell carcinoma, on biopsy. However, IHC was useful to clearly established the primary nature of the tumour.

Subjects and Methods:

Data from the laboratory information system was retrieved and biopsies/ resection specimen, taken from colon or rectum, with a diagnosis of metastatic adenocarcinoma from stomach. Only cases with immunohistochemical studies supporting the same were included. One case which did not have IHC was excluded from the study, though there was relevant history suggesting its metastatic nature. Since many patients were lost to follow-up, institutional ethical committee clearance was obtained to access the case files (IEC:500/2018)

Results

Clinical features and radiological findings of all the cases are summarized in Table 1. Four of the 5 cases were endoscopic biopsies, while the 5th case (case number 1) was a right hemicolectomy specimen. Endoscopy findings ranged from polypoid mass to flattened mucosa similar to linitis plastica of stomach. Computed tomography (CT) finding varied and ranged from lesion with loss of mucosal folds (Figure 1) to a polypoid lesion (Figure 2). Gross examination of a single case of resected right hemicolon showed multiple areas of wall thickening with loss of mucosal folds.

Figure 1: Computed tomography (CT) showing circumferential growth in rectum infiltrating the bladder and uterus anteriorly in Case 2	Figure 2: Computed tomography (CT) showing thickening of transverse colon (red star) in Case 3

All the cases on microscopy showed intact colonic mucosa with the lamina showing diffuse infiltration by sheets of signet ring cells with moderate to abundant eosinophilic to vacuolated cytoplasm, eccentric nuclei, anisonucleosis, some with prominent nucleoli (Figure 3) and mitoses, including atypical ones. Two cases, in addition, showed focal glandular pattern (Figure 4). Colonic glands were normal in all cases. One case showed an occasional gland with regenerative change, adjacent to an ulcerated area. IHC panel of Cytokeratin (CK) 7, CK20, Epithelial membrane antigen (EMA) and CDX2 was employed in 4 of the 5 cases. All tumours showed consistent positivity with CK7 and EMA and were CK20 and CDX2 negative (Figure 5). Thus, a diagnosis of metastatic adenocarcinoma to colon from a gastric primary was suggested. Table 2 summarizes the IHC findings of all the cases.

Figure 3: Infiltration of lamina by signet ring and histiocytoid cells with bland nuclei (Hematoxylin and eosin, 20x)	Figure 4: Infiltrating closely packed glands (Hematoxylin and eosin, 20x) with intra-cytoplasmic mucicarmine positivity (inset, Mucicarmine, 20x)
Figure 5: Immunohistochemistry a. and c. CK7 and epithelial membrane antigen positive tumour cells (DAB, 10x) b. and d. CK20 and CDX2 negative tumor cells in contrast to positive colonic epithelial cells (DAB, 10x)

The last case was that of 46 year old lady with features of intestinal obstruction. Sigmoidoscopy showed an ulceroproliferative growth in the rectum, biopsy from which revealed an intact colonic mucosa overlying lamina infiltrated by nests, cords and sheets of signet ring cells with intracytoplasmic vacuolation and eccentric nucleus with few lymphatic tumour emboli. Based on these features, the authors wanted to rule out metastatic signet ring cell carcinoma and proceeded with IHC consisting of CK7, CK20, CDX2 and EMA. Except CK7, all the other three markers turned positive. Hence a diagnosis of primary signet ring cell carcinoma of rectum was rendered. During the hospital stay, the patient developed severe pain abdomen with pneumoperitoneum with perforation in transverse colon, covering ileostomy/ colostomy was performed. Liver nodule biopsied showed metastatic adenocarcinoma. In view of explained poor prognosis, the patient opted for discharge against medical advice and had no further follow-up.

Discussion

Primary colorectal carcinoma is the 4th and 3rd most common malignancies in males and female, respectively, worldwide. Occurrence of metastases in colon is less frequent than in the small intestine. The primary sites include breast, lung, ovary and prostate. The most common mode of spread is direct invasion along fasciae and mesenteric attachment, followed by peritoneal seeding, intraluminal or intramural dissemination. ^[2]

The presenting features are usually non-specific and hence may suggest a colonic primary. In 2 of our cases, primary colorectal carcinoma was the clinical diagnosis, without any suggestion of metastases. In a study on imaging by Jang et al, ^[3] common finding of metastases to colo-rectum was multiple long segments, target-like bowel wall thickening. Peritoneal carcinomatoses and intestinal obstruction were other common features. Even though imaging might suggest the possibility of metastases, there is a high percentage of false negative results on colonoscopic biopsies. ^[2] The pale staining signet ring cells may be mistaken for histiocytes and therefore, may be ignored as they blend in with the inflammatory infiltrate. Thus, an awareness of past history of malignancy is very useful to pick up these cells on biopsies. Furthermore, the tumour infiltrate may cause widening of lamina, especially inter-crypt distance. However, as seen in the last case in the current series, tumour involving the lamina with an intact mucosa and lymphatic emboli does not necessarily imply a secondary. Though histological clues are pertinent, IHC plays an important role in arriving at the right diagnosis.

Primary signet ring cell carcinoma of colon (SRCC) are infrequent (or uncommon). Hence, when a malignancy with signet ring cells or histiocytoid features is identified in the colon, possibility of metastases should be a diagnostic differential. The utility of IHC is well known in the evaluation of metastases of unknown origin. The most useful markers are CK7 and CK20, the latter being positive in up to 85% of cases of colonic malignancies. Goldstein [4] et al found that the staining pattern of colonic SRCC using CK7 and CK20 was similar to glandular adenocarcinoma of colon. They concluded that signet ring carcinomas of stomach and large intestine had an identical immunophenotype as their gland-forming counterparts. In other words, primary colonic SRCC retain characteristic CK 7 (-) and CK 20 (+) staining pattern, while a gastric primary is confirmed by CK 7 (+) and CK 20 (-) staining profile. ^[4] Further, positive staining with CDX2 confirms a colorectal origin. However, Bayrak ^[5] et al found that CK7-/CK20+ phenotype is superior in its specificity and positive predictive value when compared to CDX2. Terada et al, ^[6] in a comprehensive IHC study on 42 cases of primary SRCC of stomach and colorectum, showed a statistically significant difference in the expression of EMA (gastric SRCC 57% vs colorectal SRCC 25%) and CDX2 (43% vs 93%). Hence the use of a battery of 4 markers as employed in 4 of our cases is useful in distinguishing metastases to colo-rectum from stomach malignancy. This may be useful in scenarios where there may be a remote history of carcinoma stomach or in occult gastric cancer.

A word on the new kid on the block – SATB2 (Special AT-rich sequence-binding protein 2), which is a highly specific marker for colorectal carcinoma. After the initial excitement about its specificity, there have been more studies of late which have shown that this novel marker is positive in a small percentage of other carcinomas, including gastric carcinoma though it is focal and weak staining. ^[7,8] Zhang et al concluded that a three IHC markers of SATB2, CK20 and CDX2 used on liver biopsy samples is most sensitive and specific to determine a colorectal origin for adenocarcinomas. ^[9] In a detailed review, Bellizzi ^[10] emphasizes the utility of quantifying CK7 and CK20 expression and correlating it with CDX2 expression to determine whether it is from upper or lower GI tract.

Conclusion

In a colonic biopsy with a relatively normal mucosa and signet ring cells in the lamina, the possibility of metastatic signet ring cell carcinoma, especially from stomach should also be considered. Clinical history and imaging can be useful to corroborate findings. IHC panel with a minimum of 4 markers (CK7/20, CDX2 and EMA) serves as a valuable adjunct to confirm the same.

Acknowledgement

We would like to thank the technical team, especially IHC team of Mr. Raghavendra Bhat and Mrs. Reshma, Kasturba Medical College, Manipal for their constant support and help.

Source of funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not for- profit sectors

Conflict of interest

We have no conflict of interest to declare.

Ethical approval

Obtained from Kasturba Medical College and Kasturba Hospital Institutional ethics committee, (Registration No. ECR/146/lnsUKA/2013/RR-16) IEC – numbered 500/2018

References

1. Pande R, Sunga A, Levea C, Wilding GE, Bshara W, Reid M, et al. Significance of Signet-Ring Cells in Patients with Colorectal Cancer. Dis Colon Rectum. 2008;51(1):50-5.
2. Mourra N, Jouret-Mourin A, Lazure T, Audard V, Albiges L, Malbois M, et al. Metastatic tumors to the colon and rectum: a multi-institutional study. Arch Pathol Lab Med. 2012;136:1397-401.
3. Jang HJ, Lim HK, Kim HS, Cho EY, Lee SJ, Kim KA, Choi D: Intestinal metastases from gastric adenocarcinoma: helical CT findings. J Comput Assist Tomogr. 2001;25:61–67.
4. Goldstein NS, Long A, Kuan SF, Hart J. Colon signet ring cell adenocarcinoma: immunohistochemical characterization and comparison with gastric and typical colon adenocarcinomas. Appl Immunohistochem Mol Morphol. 2000;8:183-8.
5. Bayrak R, Haltas H and Yenidunya S. The value of CDX2 and cytokeratins 7 and 20 expression in differentiating colorectal adenocarcinomas from extraintestinal gastrointestinal adenocarcinomas: cytokeratin 7-/20+ phenotype is more specific than CDX2 antibody. Diagnostic Pathology. 2012;7:9
6. Terada T. An immunohistochemical study of primary signet-ring cell carcinoma of the stomach and colorectum: III. expressions of EMA, CEA, CA19-9, CDX-2, p53, Ki-67 antigen, TTF-1, vimentin, and p63 in normal mucosa and in 42 cases. Int J Clin Exp Pathol. 2013;6: 630-7.
7. Kyra B. Berg, David F. Schaeffer; SATB2 as an Immunohistochemical Marker for Colorectal Adenocarcinoma: A Concise Review of Benefits and Pitfalls. Arch Pathol Lab Med. 1 October 2017;141:1428–33.
8. Robertson S, Patil DT. An Update on the Role of Immunohistochemistry in the Evaluation of Gastrointestinal Tract Disorders. Adv Anat Pathol. 2020 May;27(3):193-205.
9. Zhang YJ, Chen JW, He XS, Zhang HZ, Ling YH, Wen JH, et al. SATB2 is a Promising Biomarker for Identifying a Colorectal Origin for Liver Metastatic Adenocarcinomas. EBioMedicine. 2018;28:62-69.
10. Bellizzi AM. An Algorithmic Immunohistochemical Approach to Define Tumor Type and Assign Site of Origin. Adv Anat Pathol. 2020 May; 27(3): 114–163.