Background

Immunohistochemical staining techniques are used to determine the most likely source of metastatic deposits when they are seen within surgical specimens. The choice of stains is made based on the most likely candidates for the primary malignancy. When unexpected metastases are discovered, a careful and detailed past medical history is essential to aid in the selection of appropriate, targeted tests. The aim of this case report is to illustrate how such techniques were used to identify unexpected oligometastatic prostate cancer within a surgical specimen following elective curative surgery for colorectal cancer.

Case presentation

An 89-year-old Caucasian man who lived independently with good exercise tolerance was referred to colorectal services with a computed tomography (CT) finding of an obstructing mass in the sigmoid colon with some mesenteric lymphadenopathy. There were features consistent with colorectal cancer on flexible sigmoidoscopy. There were no metastases seen on a staging CT of his chest, abdomen, and pelvis. No positron emission tomography-CT was undertaken. He had radical radiotherapy for prostate cancer 10 years earlier, with a recent biochemical relapse that was being managed with hormonal therapy. His latest prostate-specific antigen (PSA) was 1.8 ng/ml. After fully discussing all options for management of his sigmoid tumor, the patient opted for management with a colonic stent. However, this did not relieve his symptoms and the stent became occluded. He therefore underwent a sigmoid colectomy with end colostomy. The inferior mesenteric artery was ligated, and a colonic specimen from descending colon to the upper rectum was resected along with the corresponding mesocolon. There were no peritoneal deposits, and no evidence of metastatic disease intraoperatively. He was discharged on the fourth postoperative day without complications. The specimen was sent for histological examination.

When the specimen was examined, there was mucinous colonic adenocarcinoma in the mesenteric fat (Fig. 1a) with some further nodal metastases of the same in 1 out of 27 sampled lymph nodes (Fig. 1b). There were also separate, well-defined metastatic tumor deposits comprising smaller, cuboidal cells embedded within the pericolic fat of the mesocolon that differed from the tall columnar cells seen in colorectal cancer (Fig. 1c, d). Due to the patient’s previous history of prostate cancer, immunohistochemical staining for PSA and prostate-specific acid phosphatase (PSAP) was undertaken and was positive (Fig. 1e and f respectively). Furthermore, CDX2 and CK20 immunochemistry confirmed that these cells were not from the gastrointestinal tract (Fig. 1g and h, respectively). The colorectal margins were clear, and the final histological staging was T4 N1 M0 V1 R0, with the presence of incidental oligometastatic deposits of prostate cancer within the pericolonic fat. There was no evidence of any other disseminated prostatic metastases.

Fig. 1
figure 1

Histopathology from the colonic specimen depicting A mucinous colonic adenocarcinoma in mesenteric fat, B nodal metastases (mucinous colonic adenocarcinoma), C, D deposits of metastatic prostate adenocarcinoma, E PSA immunohistochemistry positive confirming prostate adenocarcinoma metastases, F PSAP immunohistochemistry positive confirming prostate adenocarcinoma metastases, G CDX2 immunohistochemistry negative, excluding gastrointestinal tract metastases, and H CK20 immunohistochemistry, very focally positive, excluding a gastrointestinal tract primary

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Discussion and conclusions

Prostate cancer most commonly metastasizes to bone, but may also metastasize to distant lymph nodes, liver, thorax, brain, kidneys and adrenals, the retroperitoneum, and digestive tract [1]. There are some reports in literature of colonic masses mimicking colorectal cancer and polyps that turned out to be prostatic metastases [2,3,4,5,6,7]. There have also been some reports of prostatic metastases to colorectal lymph nodes [8, 9]. However, to our knowledge, our patient is the first in the literature to have had a focus of prostate cancer metastasis found incidentally within the pericolic fat of a colorectal cancer specimen without involvement of the colonic or lymph node tissue. Since these were solitary metastatic deposits without further dissemination, this may have represented a successful resection of previously undetected oligometastatic disease entirely by chance. There is some evidence of improved patient outcomes after resection of oligometastatic prostate cancer [10], but this is controversial [11] and it is unknown whether our patient will benefit from this serendipitous event.

Patients who have had previous radiotherapy for prostate cancer are at higher risk of developing colonic or rectal cancers [12]. It is not possible to know whether our patient’s colorectal cancer was a consequence of radiotherapy a decade earlier, but the coincidental prostatic metastases adjacent to colorectal adenocarcinoma is intriguing. Immunohistochemical staining was selected according to the likelihood of prostatic disease based on the previous history of the patient. This illustrates the importance of a thorough knowledge of the past medical history of patients undergoing surgical resection within the framework of a multidisciplinary team discussion.

We report the presence of coincidental metastatic prostate cancer within the pericolic fat of a colorectal cancer resection in a patient who had radical radiotherapy for prostate cancer 10 years earlier. This case demonstrates the versatility of the biology of prostatic cancer, and the importance of having a detailed patient history, to select appropriate immunohistochemical staining for unexpected findings within surgical specimens.