S1800 A Case of Metastatic Prostate Cancer Masquerading as Radiation Proctitis on Colonoscopy

Abstract

Introduction: CDX2 is a homeobox transcription factor that regulates development, and differentiation of epithelial cells. It is traditionally associated with benign or malignant tumors of intestinal origin but has also been reported in cases of prostate tumors. Therefore, without a known primary cancer, a CDX2 positive biopsy does not exclude extra-intestinal primary malignancy especially in patients with non-specific clinical findings. Case Description/Methods: Here, we report a case of an 84-year-old male with long-standing history of low-grade, recurrent, prostate cancer who presented to the clinic with obstipation and rising prostate-specific antigen (PSA) levels. He was previously treated with radiation therapy 20 years ago followed by brachytherapy and Bicalutamide, and was on biannual Leuprolide at the time of exam. Due to a firm rectal mass palpated on digital rectal exam, a CT scan was obtained and showed circumferential thickening of the distal rectum. Colonoscopy revealed friable and edematous mucosa, whose appearance was suggestive of acute radiation proctitis but was inconsistent with the chronicity of radiation therapy. Therefore, rectal biopsies were taken and showed positive immunostaining for AE1, AE3, NKX3.1, CDX2, PSA, and prostatic specific acid phosphatase (PSAP). CK7 and CK20 were negative. Patient was subsequently started on Enzalutamide which led to a decline in PSA. Due to worsening obstipation and partial rectal obstruction, the patient had to undergo diverting colostomy placement. Discussion: Metastatic tumors to the large intestine are relatively rare compared to primary intestinal neoplasms. The prostate gland is one of the most common sites of origin for secondary colorectal adenocarcinoma. The presence of CDX2, NKX3.1, PSA, and PSAP, and the absence of CK7 and CK20 leads us towards the origin of the secondary tumor as prostate. In medical literature, besides primary gastrointestinal tumors, CDX2 has been infrequently shown to be positive in both benign and malignant prostate specimens. In the latter, it has been more frequently found to be from primary rather than metastatic disease which is contrary to our case. This case represents an exception where the presence of CDX2 was unable to exclude a non-gastrointestinal primary tumor. This case also provides clinicians with insight to arrive at the correct diagnosis and treatment, and help differentiate between similar presentations such as primary colorectal carcinoma, secondary neoplasm of colon, and radiation proctitis.Figure 1.: Left: Endoscopic image of colonoscopy showed an edematous area concerning for radiation proctitis prior to biopsies. Center: Medium power view of hematoxylin and eosin stained section showing neoplastic cells expanding the lamina propria. Right: The neoplastic cells are positive for pancytokeratin (AE1/AE3), NKX3.1 and PSAP, confirming metastatic carcinoma consistent with prostate primary.