An unusual delayed rectal metastasis from prostate cancer masquerading as primary rectal cancer

Introduction and importance Rectal metastasis of prostate cancer origin is exceedingly rare. Its clinical presentation, endoscopic morphology, and histopathology are similar to primary rectal cancer. Misdiagnosis may lead to inappropriate treatment. Case presentation We report a case of a gentleman in his 80's with a history of treated prostate cancer T3aN0M0 with radical prostatectomy sixteen years ago. He presented with one-year complaints of altered bowel habits and weight loss. Physical and rectal examination was unremarkable. Colonoscopy manifested some inflammatory changes in the rectum. The pelvis magnetic resonance imaging (MRI) showed an abnormal posterior rectal wall thickening 2 cm above the anal canal. Biopsy confirmed poorly differentiated adenocarcinoma of prostate origin. The staging workup was negative for other distant metastasis. After a multidisciplinary decision, the patient was started on androgen deprivation therapy and given palliative radiotherapy to the rectum. Six weeks later, the patient was stable with mild radiation proctitis. Clinical discussion Management of rectal metastasis varies depending on the patient's choice, the extent of metastatic burden, symptoms, age, life expectancy, quality of life and comorbidities. While surgery remains the standard of care, other option including radiotherapy, hormonal therapy and chemotherapy has been documented in the literature with survival of few weeks to 2 years. Conclusion Delayed rectal metastasis of prostate cancer after radical prostatectomy is a rare entity. Its clinical presentation and endoscopic and histopathological findings of rectal metastasis are similar to primary colorectal cancer, making diagnosis more demanding.


Introduction
Prostate cancer is the second most predominant malignancy diagnosed in men globally, after lung cancer, with 52,300 cases every year (2016-2018) [1]. Metastatic disease is the leading cause of prostate cancer-associated deaths accounting for 13 % of all cancer deaths in men in the UK (2018) [2]. Regional lymph nodes are often the first site of metastasis, followed by metastases to the liver, lungs, and bones; however, the metastatic involvement of the rectum deriving from prostate cancer is a rare manifestation [3]. Due to anatomical adjacency, the rectum is predisposed to locoregional penetration through Denonvilliers' fascia by locally advanced prostate cancer in 1-12 % of cases [3]. Other postulated routes are lymphatic spread through the pelvic lymph node basins; and implantation along a trans-rectal needle biopsy tract in the rectum [4]. Prevailing lower gastrointestinal (GI) symptoms, the demonstration of a rectal mass on colonoscopy in the absence of prostate-specific antigen (PSA) rise in many cases, rectal metastases are mis-diagnosed as primary rectal cancers resulting in erroneous management [5]. Lane Z et al. who studied 23 patients with colorectal metastasis, reported that 19 of them had a prostate cancer diagnosis 2-18 years before colorectal metastasis, in contrast the remaining four patients had initially presented with lower GI symptoms [6].
We present a case of biopsy-proven rectal metastasis of prostate cancer origin in an elderly gentleman who underwent radical prostatectomy sixteen years ago for localized prostate cancer. This case has been reported in line with the SCARE 2020 criteria and PROCESS guidelines [7,8].

Case presentation
A gentleman in his 80's underwent radical prostatectomy and lymphadenectomy in 2005 due to prostate adenocarcinoma pT3aN0M0. In 2007, his prostate-specific antigen (PSA) relapsed and was treated with salvage prostate radiotherapy and androgen deprivation therapy (ADT). Since then, he became incontinent and underwent implantation of an artificial sphincter in 2009. Sixteen years after radical prostatectomy, in September 2021, he presented with one-year history of altered bowel habits and rectal bleeding. He had also lost a notable amount of weight, approximately two stones over a year. On physical examination, Eastern Cooperative Oncology Group (ECOG) performance status was 1-2; his abdomen was soft and non-tender with no palpable masses or visceromegaly. A digital rectal examination (DRE) revealed a smooth feeling rectal mucosa but no perceptible craggy pelvic mass.

Investigations
PSA was 4.7 microgram per litre (μg/L), and carcinoembryonic antigen (CEA) was normal (2 μg/L). Patient underwent a Faecal Immunochemical Test (FIT) test which was >400 microgram of Haemoglobin per gram of faeces (normal <10 μg Hb/g), and went on to have a screening colonoscopy, which showed some inflammatory changes within his rectum, which were thought to be secondary to radiation proctitis but showed no other colorectal pathology (Fig. 1). Biopsies taken from the inflamed region within his rectum showed poorly differentiated adenocarcinoma. Immunohistochemical stains of CK7 and PSA were positive suggestive of prostate adenocarcinoma as the primary origin ( Fig. 2). Pelvic magnetic resonance imaging (MRI) showed abnormal posterior wall thickening within the lower rectum, around 2 cm above the anal canal; there were irregular outlines with some adjacent soft tissue (Fig. 3). Staging computed tomography (CT) of thorax, abdomen and pelvis was negative for any distant metastasis.

Treatment
After a multidisciplinary meeting, ADT was commenced and had prescribed palliative radiotherapy 20 Gy in five fractions over a week due to his age for the symptom control.

Outcome and follow-up
At the time of submission, patient was reviewed in the clinic six weeks after the completion of radiotherapy, complained of grade 2 radiation proctitis for which symptomatic medications were given and planned for follow-up at 3 months.
The lower gastrointestinal tract is a rare site of metastasis. The incidence of metastasis to the lower gastrointestinal tract is 0.05 % of all metastases [11,12]. Rectal Metastasis are particularly scarce, and its true incidence is unrevealed. The most common origins of rectal metastasis are breast, stomach, gynecologic cancers and prostate [11][12][13].
In conclusion, Rectal metastasis of prostate cancer origin is a rare entity. Its clinical presentation and endoscopic and histopathological findings of rectal metastasis are similar to primary colorectal cancer, making diagnosis more demanding. Thus patients with a history of prostate cancer with emergent rectal symptoms, a multidisciplinary approach is crucial to reach an accurate diagnosis and prompt treatment.

Consent
Written informed consent was obtained from the patient for publication of this case report and accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal on request.   Synch = synchronous; Metach = metachronous; NM = not mentioned; AR = anterior resection; APR = abdominoperineal resection; ADT = androgen deprivation therapy; CRT = chemoradiation; RFA = radiofrequency ablation; BSO = bilateral subcapsular orchiectomy.

Ethical approval
Nil.

Research registration number
Not applicable.

Declaration of competing interest
None to declare.