Unusual Presentation of Colonic Adenocarcinoma at Third Trimester with Possible Underlying Familial Adenomatous Polyposis Detected on (¹⁸F)-FDG PET-CT

INTRODUCTION

Colorectal cancer (CRC) in pregnancy is rare, with an incidence of 0.002%, often found in the second trimester, presenting with nonspecific symptoms attributed to pregnancy-related physiological changes.^[1]

CASE REPORT

A 30-year-old female with progressive abdominal distension, initially attributed to pregnancy, presented in the early third trimester with vaginal bleeding and upper abdominal pain, culminating in spontaneous preterm delivery. Postpartum ultrasound abdomen revealed hepatomegaly with multiple heterogeneous hypoechoic lesions in the liver parenchyma. ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (¹⁸F-FDG PET/CT) was performed to further characterize these lesions and disease staging The maximum intensity projection (MIP) image demonstrated intense tracer uptake in liver lesions, multifocal tracer uptake in the colon and reduced physiological brain uptake [Fig 1a]. Axial fused PET/CT images revealed intense multifocal tracer uptake corresponding to oral contrast filling defects in the large bowel, sigmoid colon (maximum standardized uptake value SUVmax:24.43) [Fig 1b, 1c and 1d], gross hepatomegaly with multiple FDG avid hypodense lesions involving bilateral liver parenchyma (SUVmax: 15.89) [Fig 1e, 1f and 1g].The tumor markers CEA, CA 19-9 and CA-125 were markedly elevated. CECT of abdomen and pelvis was performed to assess the extent of polyposis and to accurately define the liver lesions [Fig 1h, 1i and 1j]. A liver biopsy was performed to establish a definitive diagnosis, which confirmed metastatic adenocarcinoma. Immunohistochemistry was positive for SATB2, indicating colonic origin. The tumor cells retained expression of MSH2, MSH6, MLH1, and PMS2, confirming mismatch repair protein proficiency and ruling out microsatellite instability-high status and Lynch syndrome. Colonoscopy revealed multiple colonic polyps, raising suspicion for familial adenomatous polyposis (FAP) and metastatic colorectal carcinoma. Genetic screening for confirmation of FAP was not possible as the patient’s condition deteriorated rapidly despite supportive care, and she ultimately succumbed to her illness within 1 month of diagnosis.

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Fig 1:

MIP (maximum intensity projection) image of ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) (a) demonstrated intense tracer uptake in liver lesions, multifocal tracer uptake in colon and reduced physiological brain parenchymal uptake (tumor sink effect). (b) The second column of images showing axial PET, (c) CT and (d) fused PET/CT demonstrating multifocal FDG uptake in soft tissue thickening involving the sigmoid colon with corresponding oral contrast filling defects. (e) Third column of images showing coronal PET, (f) CT, and (g) fused coronal PET/CT (h) demonstrating hepatomegaly with multiple FDG concentrating hypodense lesions in liver parenchyma. Fourth column of images showing axial and (i) sagittal contrast-enhanced CT (CECT) demonstrating enhancing uterine lesions (yellow arrow in (h)) and enhancing endophytic polypoidal growths in the sigmoid colon and rectum (red arrow in i), and coronal CECT (j) revealing multiple ill-defined hypodense lesions involving liver parenchyma. Enhancing lesions within the uterine cavity appeared to arise from the uterine walls, with differential considerations including retained products of conception or placental polyps, or endometrial polyps.

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DISCUSSION

CRC in pregnancy is rare, with an incidence of 0.002%, often found in the second trimester, presenting with nonspecific symptoms attributed to pregnancy associated physiological changes. The nonspecific presentation complicates diagnosis, leading to advanced disease and poor prognosis,^[1] with an increased risk (25%) of ovarian metastasis.^[2] Approximately 23% of patients with CRC during pregnancy have an underlying genetic cause, such as Lynch syndrome, FAP, Gardner syndrome, Peutz–Jeghers syndrome, or chronic inflammatory bowel disease.^[1] FAP is the most common gastrointestinal polyposis syndrome, with autosomal dominant inheritance,^[3] characterized by the presence of more than 100 synchronous colorectal polyps by the second or third decade of life with a 100% lifetime risk of CRC.^[4]

CECT is not the primary diagnostic tool for FAP but it helps in assessing disease severity and complications. Colonoscopy remains the gold standard for screening,^[5] but due to the limited field of view inherent to colonoscopy, ¹⁸F-FDG PET/CT can play a crucial complementary role not only in evaluating the extent of disease but also in identifying potential metastatic lesions and simultaneously provide disease staging in the event of malignancy.^{[6] 18}F-FDG PET/CT has 100% sensitivity and 90% specificity for the detection of carcinoma in FAP patients.^[7]

Thus, it is important to be aware of such atypical presentations of colorectal carcinoma during pregnancy. In our case, ¹⁸F-FDG PET/CT played an important role in establishing the diagnosis and staging the disease to initiate appropriate treatment. However, due to an unfavorable disease prognosis, the patient succumbed within a month duration of treatment.

CONCLUSION

This case highlights the atypical presentation of colorectal carcinoma during pregnancy and emphasizes the pivotal role of ¹⁸F-FDG PET/CT in delineating disease extent and identifying metastatic sites.