Rectosigmoid Adenocarcinoma Mimicking a Tubo-Ovarian Abscess: CT and Apple Core Sign
How pelvic abscess, entero-ovarian fistula, rectosigmoid stenosis, and Apple Core morphology can reveal an occult colorectal malignancy.
Executive Clinical Summary
A woman in her early 40s presented with approximately one month of progressively worsening abdominal pain, nausea, vomiting, and fever. Two months earlier, she had been treated for pelvic inflammatory disease associated with a tubo-ovarian abscess.
Contrast-enhanced CT initially appeared to support a gynecologic diagnosis. A multiloculated gas-fluid collection with peripheral rim enhancement was present in association with the right ovary, compatible with a tubo-ovarian abscess. However, the adjacent rectosigmoid colon was abnormal: bowel-wall thickening was present, and gas appeared to pass through a suspected defect in the bowel wall toward the pelvic collection.
The critical diagnostic question was therefore not simply whether an abscess was present, but why the abscess had developed.
Subsequent barium enema demonstrated severe narrowing at the rectosigmoid junction with sharp shouldering at both ends of the stenotic segment. This morphology raised strong concern for a circumferential colorectal malignancy. Endoscopic examination was limited by severe stenosis, but biopsy ultimately demonstrated adenocarcinoma. Surgical treatment subsequently demonstrated right ovarian invasion, supporting the suspected relationship between the rectosigmoid malignancy and the pelvic inflammatory process.
This case illustrates an important principle in abdominal imaging: a pelvic abscess may represent the consequence of an underlying bowel disease rather than the primary disease itself.
When gas-containing adnexal collections coexist with adjacent bowel-wall thickening, focal or severe stenosis, or evidence of enteric communication, the radiologist should reconsider an apparently straightforward tubo-ovarian abscess and actively evaluate the adjacent bowel.
Key Clinical Questions
Can colorectal cancer present as a tubo-ovarian abscess?
What CT findings suggest an entero-ovarian fistula?
Why can a rectosigmoid carcinoma be difficult to identify on initial CT?
What does the Apple Core or Napkin Ring sign represent?
How should severe rectosigmoid stenosis be differentiated from inflammatory disease?
What is the complementary role of CT, contrast studies, endoscopy, and pelvic MRI?
How can artificial intelligence support, but not replace, radiologic reasoning in complex pelvic disease?
Introduction
A gas-containing pelvic collection in a woman with fever and abdominal or pelvic pain naturally directs clinical attention toward pelvic inflammatory disease and tubo-ovarian abscess. That diagnostic pathway is often appropriate. The difficulty arises when the collection is not the primary disease but the downstream consequence of an adjacent gastrointestinal lesion.
The rectosigmoid region occupies a particularly important anatomical position in this setting. The sigmoid colon and rectum lie in close proximity to the uterus, adnexa, pelvic peritoneum, urinary bladder, and other pelvic structures. A locally advanced inflammatory or malignant bowel process can therefore extend beyond the intestinal wall and produce abscess formation, perforation, or fistulization.
In the present case, the initial CT impression was dominated by the pelvic collection. The underlying rectosigmoid abnormality was less conspicuous because the tumor did not present as a clearly defined exophytic mass. Instead, the disease manifested through bowel-wall thickening, severe luminal narrowing, inflammatory change, and suspected communication with the adjacent pelvic collection.
The diagnostic turning point came with the recognition of the stenotic morphology on contrast examination. The combination of severe narrowing and sharp shouldering at both ends created an apple-core or napkin-ring appearance, a classic morphological clue to circumferential colorectal carcinoma.
The broader lesson extends beyond this individual diagnosis. Radiologic diagnosis should explain the entire disease process, not merely identify the most visually dominant abnormality.
Clinical Hook: When the Abscess Is Not the Whole Diagnosis
The initial imaging question was apparently simple: Is this a tubo-ovarian abscess?
A more useful question was considerably harder:
What caused the abscess?
The CT demonstrated a multiloculated gas-fluid collection associated with the right ovary, accompanied by rim enhancement and surrounding inflammatory change. These findings readily support an abscess.
However, gas within an adnexal collection deserves particular attention. The ovary does not normally contain gas. When gas is present within a complex adnexal collection, an enteric source should be considered, particularly when an adjacent bowel abnormality is present.
In this case, the rectosigmoid bowel wall was thickened, and gas appeared to extend through a suspected wall defect toward the collection. The spatial relationship between the abnormal bowel and the pelvic collection therefore became diagnostically important.
The abscess was real. The mistake would have been to assume that the abscess represented the entire disease.
Learning Objectives
By the end of this article, readers should be able to:
Recognize imaging clues suggesting colorectal malignancy in a patient presenting with a pelvic abscess.
Understand the anatomical relationship between the rectosigmoid colon and female pelvic organs.
Identify CT features that should raise suspicion for an entero-ovarian fistula.
Recognize the Apple Core or Napkin Ring appearance of circumferential colorectal carcinoma.
Understand the complementary roles of CT, contrast examination, endoscopy, and pelvic MRI.
Recognize how AI-assisted imaging could support complex colorectal and pelvic imaging workflows while retaining radiologist oversight.
Anatomy: Why the Rectosigmoid Region Matters
The rectosigmoid region represents the transition between the sigmoid colon and rectum. Its deep pelvic location places it in close anatomical relationship with multiple structures, including the uterus, ovaries, fallopian tubes, urinary bladder, pelvic peritoneum, small bowel, and pelvic fat.
This anatomy becomes clinically important when a lesion extends beyond the bowel wall.
A colorectal malignancy can begin in the mucosa and progressively invade the submucosa, muscularis propria, subserosal tissues, and surrounding structures. Once the process extends beyond the bowel, inflammatory reaction, necrosis, localized perforation, abscess formation, and fistulization can occur.
In the female pelvis, direct extension toward an ovary can create an abnormal communication between the gastrointestinal tract and adnexal structures. The resulting pelvic collection may then resemble a primary gynecologic infection.
The anatomical relationship therefore provides the framework for interpreting the imaging findings.
Case Presentation
Patient Profile
A woman in her early 40s presented with approximately one month of persistent and progressively worsening abdominal pain, nausea, vomiting, and fever.
Approximately two months before this presentation, she had been treated for pelvic inflammatory disease associated with a tubo-ovarian abscess.
Initial CT
Contrast-enhanced abdominal and pelvic CT demonstrated a multiloculated gas-fluid collection associated with the right ovary. The collection showed peripheral rim enhancement and surrounding inflammatory change.
At first glance, these findings were compatible with a tubo-ovarian abscess.
However, the rectosigmoid bowel wall was also thickened. A suspected defect in the bowel wall was associated with abnormal gas passage toward the pelvic collection.
This raised the possibility of an entero-ovarian fistula.
Importantly, no definite large mass was apparent on the initial CT examination.
Subsequent Contrast Examination
Barium enema subsequently demonstrated marked narrowing at the rectosigmoid junction. Sharp shouldering was present at both ends of the stenotic segment.
The morphology was substantially more suspicious for an annular malignant stricture than for nonspecific inflammatory wall thickening.
Endoscopy and Pathology
Sigmoidoscopy could not traverse the severe stenosis. Although the suspected fistula was not directly visualized within the limited examination, tissue sampling demonstrated adenocarcinoma.
Surgical Findings
Low anterior resection was subsequently performed. Right ovarian invasion was identified during surgery, supporting the suspected local relationship between the rectosigmoid malignancy and pelvic inflammatory process.
Axial Contrast-Enhanced CT
Figure 1. Axial contrast-enhanced CT demonstrating a complex right pelvic gas-fluid collection associated with the ovary and adjacent rectosigmoid abnormality.
Radiologist Interpretation:
A multiloculated rim-enhancing gas-fluid collection is present in the right adnexal region. Adjacent rectosigmoid wall thickening and a suspected bowel-wall defect raise concern for communication between the bowel and pelvic collection. The absence of an obvious discrete mass should not exclude an infiltrative circumferential tumor.
Clinical Significance:
The dominant abnormality may initially appear gynecologic, but the adjacent bowel abnormality changes the diagnostic framework. A gas-containing adnexal abscess adjacent to abnormal bowel should prompt evaluation for an enteric source.
ALT Text:
Axial contrast-enhanced CT showing a gas-fluid pelvic abscess adjacent to thickened rectosigmoid bowel.
Coronal CT Reconstruction
Figure 2. Coronal contrast-enhanced CT demonstrating the spatial relationship between the rectosigmoid colon and right pelvic inflammatory collection.
Radiologist Interpretation:
The coronal plane clarifies the anatomical relationship between the abnormal rectosigmoid segment and the right adnexal collection. Extensive inflammatory change can obscure the margins of an underlying bowel lesion.
Clinical Significance:
Multiplanar review is particularly valuable in complex pelvic disease because the relationship between bowel, adnexa, and abscess may not be adequately appreciated on axial images alone.
ALT Text:
Coronal contrast-enhanced CT demonstrating a complex right pelvic collection adjacent to the rectosigmoid colon.
Sagittal CT Reconstruction
Figure 3. Sagittal contrast-enhanced CT showing the rectosigmoid region and adjacent pelvic inflammatory abnormalities.
Radiologist Interpretation:
The sagittal reconstruction demonstrates the deep pelvic location of the abnormal process and its relationship to adjacent pelvic structures. A small amount of gas within the uterine cavity was also described.
Clinical Significance:
Abnormal gas distribution within the female pelvis may provide an additional clue to communication between the gastrointestinal and reproductive tracts, particularly when accompanied by an adjacent bowel abnormality.
ALT Text:
Sagittal pelvic CT showing abnormal gas and inflammatory changes around the rectosigmoid region.
CT Findings That Should Not Be Missed
The most important CT lesson is straightforward:
A visible mass is not required for colorectal cancer to be present.
Circumferential colorectal carcinoma may grow along the bowel wall and produce a relatively long segment of mural thickening and stenosis. When severe inflammation or abscess formation accompanies the tumor, the tumor margin can become poorly defined.
The following findings should therefore be evaluated systematically:
| CT Finding | Potential Significance |
|---|---|
| Focal or asymmetric bowel-wall thickening | Raises concern for neoplasm |
| Irregular mural thickening | Suspicious for infiltrative disease |
| Severe luminal narrowing | May indicate advanced circumferential tumor |
| Sharp transition at stenotic margins | Supports malignant morphology |
| Adjacent fat stranding | May reflect inflammation or tumor extension |
| Localized perforation | May occur with complicated malignancy |
| Adjacent abscess | Can result from perforation or fistulization |
| Gas between bowel and adnexal collection | Suggests enteric communication |
| Regional lymphadenopathy | Relevant to staging |
| Liver or lung lesions | May indicate distant metastatic disease |
| Peritoneal nodules | Raise concern for peritoneal involvement |
The important interpretive step is to integrate these findings rather than evaluating each abnormality independently.
A pelvic abscess adjacent to abnormal bowel should not automatically be labeled as uncomplicated pelvic inflammatory disease.
Why the Initial CT Could Miss the Tumor
There are several reasons a rectosigmoid carcinoma may be difficult to recognize when accompanied by a pelvic abscess.
First, circumferential tumors may not form a large discrete mass.
Second, inflammatory edema can produce substantial bowel-wall thickening that overlaps morphologically with neoplastic thickening.
Third, an abscess can become the visually dominant lesion.
Fourth, extensive inflammatory change may obscure the interface between tumor, bowel, and adjacent organs.
Fifth, clinical anchoring on a previously diagnosed tubo-ovarian abscess may narrow the subsequent diagnostic search.
This is a classic example of a radiologic cognitive trap: the most conspicuous lesion can dominate interpretation even when it is a secondary manifestation of a deeper disease process.
Barium Enema and Stenotic Morphology
Figure 4
Figure 4. Axial CT demonstrating the pelvic drainage catheter and the relationship between the abscess and rectosigmoid region.
The drainage catheter is positioned within the pelvic collection. The adjacent rectosigmoid segment remains relevant because the location of the catheter and collection provides an anatomical framework for evaluating possible enteric communication.
ALT Text:
Axial CT showing pelvic drainage catheter within a collection adjacent to the rectosigmoid colon.
Figure 5
Figure 5. Barium enema scout lateral image obtained before contrast instillation.
The scout image documents the pelvic drainage catheter and establishes the baseline anatomy before contrast administration.
ALT Text:
Lateral scout radiograph before barium enema showing pelvic drainage catheter.
Figure 6
Figure 6. Lateral image after contrast instillation demonstrating severe rectosigmoid narrowing.
Marked luminal narrowing is present at the rectosigmoid junction. Sharp shouldering at the margins of the stenotic segment provides an important morphological clue to an annular obstructing lesion.
ALT Text:
Barium enema lateral image demonstrating severe rectosigmoid stenosis with sharp shouldering.
Figure 7
Figure 7. Post-evacuation PA image demonstrating persistent rectosigmoid narrowing.
The stenotic morphology remains apparent after evacuation, while the pelvic drainage catheter remains visible in the region of the abscess.
ALT Text:
Post-evacuation abdominal radiograph showing persistent rectosigmoid stenosis and pelvic drainage catheter.
Figure 8
Figure 8. Post-evacuation lateral image demonstrating the morphology of the rectosigmoid stricture.
The stenosis is clearly demonstrated. No definite direct fistulous communication with the right adnexal abscess was demonstrated on the fluoroscopic examination.
ALT Text:
Post-evacuation lateral barium enema image showing severe rectosigmoid narrowing without definite visible fistula.
The Apple Core Sign: A Morphologic Turning Point
Figure 9. Apple Core or Napkin Ring appearance at the rectosigmoid junction.
The rectosigmoid segment demonstrates a long, severe stenosis with sharp shouldering at both ends. This creates the classic Apple Core or Napkin Ring morphology associated with circumferential colorectal carcinoma.
The Apple Core sign reflects circumferential growth of a lesion around the bowel lumen. As the tumor progressively narrows the lumen, the transition from relatively preserved bowel to abnormally narrowed bowel can appear abrupt.
Sharp shouldering is particularly useful because it provides a morphological distinction from some forms of smooth inflammatory narrowing.
However, the Apple Core sign is not pathognomonic for cancer. Inflammatory bowel disease and other processes can produce strictures. Therefore, the sign should be interpreted in the context of the patient's symptoms, CT findings, endoscopic findings, and pathology.
In this case, the combination of severe stenosis, sharp shouldering, adjacent pelvic abscess, suspected fistulization, and subsequent adenocarcinoma on biopsy established a coherent disease process.
ALT Text:
Barium enema demonstrating Apple Core or Napkin Ring morphology at the rectosigmoid junction.
Entero-Ovarian Fistula: Why the Negative Study Does Not End the Question
The suspected entero-ovarian fistula was not directly demonstrated on barium enema.
That does not necessarily invalidate the CT suspicion.
Fistulas may be very small, transiently sealed, or functionally closed during a particular examination. Drainage of an abscess can also change the pressure relationship between the bowel and collection, reducing the likelihood that contrast will traverse the communication.
In this case, the drainage material contained fecal material, providing an important clinical clue to communication between the gastrointestinal tract and pelvic collection.
Therefore, fistula assessment should be regarded as a multimodal diagnostic problem.
| Evidence Source | Potential Contribution |
|---|---|
| Contrast-enhanced CT | Demonstrates anatomical relationship, gas, inflammation, and possible tract |
| Drainage material | May provide evidence of enteric contents |
| Fluoroscopy | May directly demonstrate luminal communication |
| Endoscopy | Evaluates mucosal disease and permits biopsy |
| Surgery | Directly assesses organ invasion and fistulous relationships |
| Pathology | Establishes tissue diagnosis |
The absence of a visible fistula on one examination should therefore be interpreted within the complete clinical and imaging context.
Differential Diagnosis
| Diagnosis | Key Imaging Finding | Clinical Clue | Differentiating Point |
|---|---|---|---|
| Tubo-ovarian abscess | Rim-enhancing adnexal collection, often complex | Fever and pelvic pain | Does not by itself explain marked adjacent bowel stenosis |
| Crohn disease with fistulization | Bowel thickening, stricture, fistula, abscess | Chronic or recurrent inflammatory symptoms | Usually supported by broader inflammatory bowel findings |
| Colorectal neoplasm with fistulization | Irregular wall thickening, severe stricture, shouldering, adjacent abscess | Persistent symptoms, obstruction, recurrent pelvic infection | Apple Core morphology and pathology support malignancy |
| Ovarian torsion | Enlarged ovary and altered vascularity | Acute pelvic pain | Gas-containing rim-enhancing abscess is atypical |
| Complicated pelvic infection | Complex adnexal collection | Fever, pelvic inflammatory symptoms | Should be reconsidered when adjacent bowel is distinctly abnormal |
The central diagnostic challenge is not distinguishing an abscess from cancer in isolation. It is recognizing when both processes may coexist because one has caused the other.
Multimodal Imaging: Complementary Questions, Not Competing Tests
Different imaging modalities answer different clinical questions.
| Modality | Primary Role | Important Strength | Important Limitation |
|---|---|---|---|
| Contrast-enhanced CT | Whole abdomen and pelvis | Detects complications, abscess, invasion, obstruction, and distant disease | Local rectal tumor staging is limited compared with dedicated MRI |
| Barium enema | Luminal morphology | Demonstrates stenosis and shouldering | Limited soft-tissue characterization |
| Colonoscopy/Sigmoidoscopy | Mucosal evaluation and biopsy | Histologic diagnosis can be obtained | Severe stenosis may prevent passage |
| CT Colonography | Luminal assessment | Three-dimensional colonic evaluation | Does not provide tissue diagnosis |
| Pelvic MRI | Local rectal staging | T category, mesorectal fascia, EMVI, sphincter and adjacent structures | Not primarily a substitute for endoscopic biopsy |
| PET/CT | Selected systemic/metabolic assessment | Metabolic-anatomic correlation in selected situations | Not routinely required for every rectal cancer patient |
The present case demonstrates why sequential imaging can be more informative than expecting one modality to answer every question.
CT identified the pelvic complication and raised suspicion for bowel communication. The contrast examination clarified the stenotic morphology. Endoscopy provided tissue sampling. Pathology established adenocarcinoma. MRI has an important role in subsequent local staging and treatment planning for rectal cancer.
Why Pelvic MRI Matters in Rectal Cancer
Once rectal cancer is established, the diagnostic question changes.
The issue is no longer simply whether a tumor exists. The clinical team needs to understand its local extent and relationship to structures that influence treatment planning.
High-resolution pelvic MRI can evaluate:
Tumor location
Longitudinal tumor extent
T category
Nodal disease
Mesorectal fascia
Extramural vascular invasion
Sphincter complex
Adjacent organ involvement
Treatment response
Mesorectal fascia involvement and extramural vascular invasion are particularly relevant because they can influence risk assessment and treatment strategy.
MRI therefore complements rather than replaces CT and endoscopy.
CT is valuable for evaluating the broader abdomen and pelvis, including complications and distant disease. Endoscopy remains essential for direct mucosal assessment and biopsy. MRI provides detailed local staging information for rectal cancer.
Treatment Considerations
The initial management of a tubo-ovarian abscess may include antimicrobial therapy and drainage when clinically indicated. However, when an underlying bowel malignancy is responsible for the pelvic collection, treating the abscess alone cannot address the primary disease.
In this case, image-guided drainage was performed, and fecal material in the drainage raised further suspicion of enteric communication. Endoscopy was limited by severe stenosis, but biopsy demonstrated adenocarcinoma.
Surgical management subsequently included low anterior resection, with right ovarian invasion identified intraoperatively.
Contemporary rectal cancer management is not determined by surgery alone. Treatment planning depends on local stage, MRI findings, nodal status, mesorectal fascia involvement, extramural vascular invasion, tumor biology, and other clinical factors.
The supplied case material cites the 2024 ASCO guideline for locally advanced rectal cancer and the 2025 ESMO guideline for localized rectal cancer. It also notes the role of total neoadjuvant therapy in selected locally advanced cases and the relevance of MSI-H/dMMR status to immunotherapy considerations.
The specific treatment decision for an individual patient requires multidisciplinary assessment and cannot be determined from imaging alone.
Prognostic Factors
The prognosis of rectosigmoid or rectal adenocarcinoma depends on multiple factors rather than on the presence of a tumor alone.
Relevant factors include:
TNM stage
Regional lymph-node involvement
Distant metastasis
Adjacent-organ invasion
Resection margin status
Extramural vascular invasion
Tumor biology
MSI/MMR status
Treatment response
Recurrence
For rectal cancer, high-quality MRI staging is particularly important because local anatomical relationships can affect treatment planning and risk assessment.
Imaging Physics: Why Multiplanar CT Reconstruction Matters
CT generates volumetric data that can be reconstructed in multiple planes. In a complex pelvic inflammatory process, the diagnostic value of this capability is substantial.
On axial images, the relationship between bowel and adnexal structures may appear fragmented. Coronal and sagittal reconstructions can clarify whether an abnormal bowel segment directly contacts a pelvic collection and whether the abnormal gas distribution follows an anatomically plausible route.
The underlying physics is based on differences in X-ray attenuation among tissues. Intravenous contrast increases attenuation within vascularized tissues and helps distinguish enhancing bowel wall from low-attenuation fluid collections.
However, inflammation can reduce the visual separation between normal and abnormal tissues. Edema, hyperemia, fluid, and adjacent inflammatory change can obscure the margins of a tumor.
This is one reason why image interpretation should not depend exclusively on the visibility of a discrete mass.
Artificial Intelligence Perspective
Complex cases such as this present an interesting opportunity for clinical AI because the relevant information is distributed across multiple imaging studies and clinical events.
An AI system designed only to detect large colorectal masses may have limited value when the dominant abnormality is an abscess and the underlying tumor is infiltrative.
A more useful clinical AI strategy would integrate multiple signals:
Bowel-wall thickening
Focal or asymmetric mural abnormality
Luminal narrowing
Gas-containing adjacent collection
Abnormal spatial relationships
Suspected fistulous tract
Recurrent pelvic infection
Obstructive features
Endoscopic evidence of stenosis
Computer vision models could potentially assist with lesion localization and segmentation. Classification models could estimate the likelihood that a stenotic segment is malignant. Multimodal systems could potentially integrate imaging findings with structured clinical information.
However, these applications should be regarded as decision-support functions rather than autonomous diagnosis.
AI Development and Validation Pipeline
A clinically responsible AI system for complex colorectal imaging would require a lifecycle extending beyond model training.
Training → Internal Validation → External Validation → Deployment → Monitoring → Drift Detection → Revalidation → Clinical Governance
Several technical problems require particular attention.
Dataset Diversity
Training data should represent differences in scanner vendors, acquisition protocols, reconstruction parameters, patient populations, disease prevalence, and hospital environments.
Label Quality
Pathology-confirmed diagnoses are particularly valuable, but the temporal relationship between imaging and pathology must be carefully defined.
Class Imbalance
Large numbers of uncomplicated CT examinations may coexist with relatively few cases of colorectal cancer presenting with pelvic abscess and fistulization. This can create substantial class-imbalance problems.
Domain Shift
An algorithm developed in one institution may encounter substantially different patient populations and imaging protocols elsewhere.
Calibration
A model's probability output should not automatically be interpreted as a clinically meaningful risk unless appropriate calibration has been demonstrated.
Model Drift
Changes in CT protocols, referral patterns, patient populations, or disease prevalence can affect performance after deployment.
Enterprise Clinical AI Workflow
A practical hospital architecture could be represented as:
DICOM CT → PACS → AI Orchestration Layer → Colorectal Imaging AI → Results → PACS Visualization → Radiologist → RIS/EMR → Clinical Decision Support
In a larger enterprise environment, the orchestration layer becomes particularly important.
Rather than sending every examination to every algorithm, an orchestration platform could route examinations according to modality, body region, clinical indication, urgency, and available models.
For example, a pelvic CT containing a complex adnexal collection could trigger a pathway that evaluates both pelvic inflammatory disease and adjacent bowel abnormalities.
The AI result should appear within the radiologist's existing workflow rather than becoming a separate destination.
Interoperability may involve DICOM for imaging, HL7 and FHIR for clinical data exchange, PACS/RIS/EMR integration, and enterprise imaging infrastructure.
Where AI Could Fail
AI should not be treated as an independent authority in a case of this complexity.
Potential failure modes include:
False reassurance from the absence of a discrete mass.
Failure to recognize an infiltrative circumferential tumor.
Misclassification of tumor-associated abscess as uncomplicated pelvic infection.
Incorrect localization of a suspected fistulous tract.
Poor performance with unusual pelvic anatomy.
Domain shift between institutions.
Dependence on incomplete clinical information.
Overinterpretation of nonspecific inflammatory changes.
False-positive cancer alerts in severe inflammatory disease.
Automation bias when the radiologist accepts an AI interpretation without reviewing the underlying images.
Explainability mechanisms such as heat maps or localization overlays may help the radiologist understand where an algorithm is focusing. They do not, however, prove that the algorithm's conclusion is correct.
The radiologist remains responsible for evaluating the complete examination.
Ten Expert Insights
Expert Insight 1 — Radiologist Perspective
The visually dominant lesion may not be the primary disease. In a complex pelvic CT, the abscess may attract immediate attention while the adjacent bowel abnormality provides the actual diagnostic clue.
Expert Insight 2 — Emergency Medicine Perspective
Persistent or recurrent abdominal pain and fever should be interpreted in the context of the imaging trajectory. A prior diagnosis of pelvic infection should not prevent reconsideration when the clinical course remains unexplained.
Expert Insight 3 — Gastrointestinal Imaging Perspective
Circumferential colorectal carcinoma may manifest predominantly as mural thickening and stenosis rather than as a discrete mass. Absence of an obvious mass is therefore not equivalent to absence of malignancy.
Expert Insight 4 — Pelvic Imaging Perspective
Gas within an adnexal collection should trigger an anatomical search for a gastrointestinal communication, particularly when adjacent bowel is abnormal.
Expert Insight 5 — Contrast Imaging Perspective
The Apple Core sign is valuable because it describes morphology rather than merely density. The abrupt transition and sharp shouldering of a stenosis can change the diagnostic probability substantially.
Expert Insight 6 — Multimodality Perspective
When one test answers only part of the clinical question, a complementary examination can reveal the missing component. CT, contrast study, endoscopy, pathology, and MRI each provide different information.
Expert Insight 7 — Surgical Perspective
Imaging suspicion of adjacent-organ invasion should be communicated clearly because the relationship between the tumor and neighboring structures can affect operative planning.
Expert Insight 8 — AI Deployment Perspective
A model designed to detect colorectal cancer should not be evaluated only on classic mass-forming cancers. Rare presentations involving abscess, fistula, and severe inflammation represent important real-world stress tests.
Expert Insight 9 — Enterprise Workflow Perspective
AI is most useful when its output is integrated into PACS/RIS workflow and presented at the moment of interpretation rather than delivered through a disconnected application.
Expert Insight 10 — Patient Journey Perspective
The ultimate purpose of detecting the underlying cause of an abscess is not simply diagnostic accuracy. Correctly identifying the primary disease can redirect the entire clinical pathway from repeated treatment of complications toward definitive management of the underlying pathology.
Clinical Pearls
A pelvic abscess may be a consequence rather than the primary disease.
Gas within an adnexal collection should prompt consideration of enteric communication.
Adjacent bowel-wall thickening deserves deliberate evaluation.
Circumferential colorectal cancer may not form a conspicuous mass.
Severe focal stenosis is an important warning sign.
Sharp shouldering increases concern for malignant stenosis.
Apple Core and Napkin Ring are morphological descriptions, not absolute diagnoses.
A negative fistula study does not necessarily exclude a small or transient fistula.
Drainage material containing enteric contents can provide an important diagnostic clue.
Multiplanar CT reconstruction is particularly useful in complex pelvic anatomy.
Endoscopic biopsy remains essential for tissue diagnosis.
Pelvic MRI is particularly important for local rectal cancer staging.
CT and MRI provide complementary information rather than interchangeable information.
Treatment of an abscess does not necessarily treat the disease that caused it.
AI should support comprehensive image interpretation rather than replace radiologic reasoning.
Common Diagnostic Pitfalls
Pitfall 1: Anchoring on the Initial Diagnosis
Once a tubo-ovarian abscess has been diagnosed, subsequent imaging may be interpreted within that framework.
Pitfall 2: Searching Only for a Mass
A circumferential tumor may manifest primarily through stenosis and mural thickening.
Pitfall 3: Ignoring Gas
Gas in an adnexal collection is an important clue to possible enteric communication.
Pitfall 4: Evaluating the Abscess Without the Adjacent Bowel
The bowel should be examined systematically whenever a pelvic collection lies immediately adjacent to it.
Pitfall 5: Overinterpreting a Negative Fistulogram
A fistula may not be patent at the time of contrast examination.
Pitfall 6: Treating Inflammation as the Complete Diagnosis
Inflammatory change may obscure an underlying neoplasm.
Pitfall 7: Assuming Young Age Excludes Colorectal Cancer
Age modifies probability but does not eliminate the diagnosis.
Pitfall 8: Treating the Apple Core Sign as Pathognomonic
Inflammatory strictures can mimic malignant narrowing.
Pitfall 9: Separating Imaging Studies Instead of Integrating Them
The most useful diagnosis may emerge only when CT, contrast study, endoscopy, pathology, and surgical findings are considered together.
Pitfall 10: Overreliance on AI
AI can miss atypical disease and should never substitute for review of the complete clinical and imaging context.
A Practical Diagnostic Algorithm
This algorithm emphasizes an important principle: the diagnostic pathway should follow the disease process rather than the initial label.
What Is the Apple Core Sign?
The Apple Core sign, also called the Napkin Ring sign, describes a circumferential bowel-wall lesion that produces a relatively long segment of luminal narrowing with abrupt or shouldered margins. It is classically associated with colorectal carcinoma.
The sign is highly useful as a morphological clue but is not independently diagnostic of malignancy. Inflammatory strictures and other processes can produce similar narrowing, so the finding must be correlated with CT features, clinical presentation, endoscopy, and pathology.
What Is the Key CT Finding in This Case?
The key CT pattern is the combination of a complex gas-containing pelvic collection, adjacent rectosigmoid wall thickening, and suspected communication between the bowel and the collection.
The absence of a clearly defined mass should not be interpreted as evidence against malignancy when the bowel demonstrates severe or irregular mural abnormality.
When Is Pelvic MRI Important?
Pelvic MRI is particularly important after rectal cancer has been established or strongly suspected and local staging is required. It provides detailed assessment of tumor extent, T category, mesorectal fascia, extramural vascular invasion, sphincter involvement, and adjacent structures.
It complements rather than replaces CT and endoscopic assessment.
FAQ
Can colorectal cancer cause a tubo-ovarian abscess?
Yes. A locally advanced colorectal tumor can cause perforation, inflammation, abscess formation, or fistulization into adjacent pelvic structures. When a tubo-ovarian abscess occurs alongside abnormal bowel, an enteric source should be considered.
Why is gas in an ovarian-region abscess important?
Gas within an adnexal collection can indicate infection, but when the collection directly contacts abnormal bowel, enteric communication should be considered because the ovary itself does not normally contain gas.
Does the absence of a visible mass exclude colorectal cancer?
No. Circumferential colorectal carcinoma can manifest primarily as bowel-wall thickening and severe luminal stenosis without a conspicuous exophytic mass.
Is the Apple Core sign specific for cancer?
No. It is a classic morphological appearance of colorectal carcinoma but can be mimicked by inflammatory and other stenosing processes.
Can a fistula be present even if it is not seen on contrast examination?
Yes. A small or intermittently patent fistula may not be demonstrated during a particular examination.
Why was the barium enema useful?
It demonstrated the morphology of the rectosigmoid stenosis, including severe narrowing and sharp shouldering, which substantially increased suspicion for a circumferential malignant lesion.
Why was endoscopy important?
Endoscopy allowed direct evaluation of the stenotic bowel segment and, where feasible, tissue sampling. In this case, biopsy demonstrated adenocarcinoma.
Why is MRI important after rectal cancer diagnosis?
MRI provides detailed local staging information that can influence treatment planning, including assessment of tumor extent, mesorectal fascia, extramural vascular invasion, sphincter complex, and adjacent structures.
Can drainage alone cure a tumor-associated pelvic abscess?
Drainage can address the collection, but if the abscess is caused by an underlying malignancy, definitive management must also address the primary disease.
Can AI diagnose this condition independently?
AI may assist with detection, localization, prioritization, and workflow support, but atypical presentations involving inflammation, fistula, and infiltrative disease require expert clinical and radiologic interpretation.
Clinical Reasoning Quiz
Question 1
A woman in her early 40s has fever and abdominal pain. CT shows a right adnexal gas-fluid collection and adjacent rectosigmoid wall thickening. What additional process should be actively considered?
① Ovarian cyst
② Renal infarction
③ Enteric fistulous communication
④ Uterine fibroid
⑤ Uncomplicated nephrolithiasis
Correct Answer: ③
Explanation:
The combination of a gas-containing adnexal collection and adjacent abnormal bowel raises concern for communication between the gastrointestinal tract and the pelvic collection.
Question 2
A rectosigmoid stricture demonstrates severe narrowing with sharp shouldering at both ends. Which interpretation is most appropriate?
① Functional bowel disease
② Normal anatomical variation
③ Malignant stenosis should be considered
④ Isolated ovarian torsion
⑤ Simple pelvic fluid
Correct Answer: ③
Explanation:
Sharp shouldering and severe focal stenosis create an apple-core or napkin-ring morphology that is classically associated with circumferential colorectal carcinoma, although inflammatory strictures remain in the differential.
Question 3
Which modality is particularly important for local staging of rectal cancer?
① Bone densitometry
② High-resolution pelvic MRI
③ Routine chest radiography
④ Cranial CT
⑤ Renal ultrasound
Correct Answer: ②
Explanation:
Pelvic MRI provides detailed information about local tumor extent, mesorectal fascia, extramural vascular invasion, sphincter complex, and adjacent structures.
Question 4
A fistula is suspected on CT but is not demonstrated on a subsequent contrast study. What is the most appropriate conclusion?
① The fistula is definitively absent
② CT must be incorrect
③ The diagnosis of cancer is excluded
④ The fistula may still be present but not demonstrable during that examination
⑤ The pelvic collection must be ovarian torsion
Correct Answer: ④
Explanation:
Small or intermittently patent fistulas may not opacify during a particular contrast examination, especially after drainage changes the pressure relationship.
Question 5
What is the most important general lesson from this case?
① Every pelvic abscess is caused by cancer
② A visible abscess never requires further evaluation
③ The most visually prominent lesion is always the primary disease
④ The cause of a complication should be actively sought when imaging findings do not form a coherent single process
⑤ AI should replace radiologist review
Correct Answer: ④
Explanation:
The central diagnostic lesson is to identify the underlying disease process rather than stopping at recognition of the most obvious complication.
Expert Takeaway
This case demonstrates why advanced imaging interpretation is fundamentally a reasoning process.
The first CT abnormality was a pelvic abscess. That diagnosis was not wrong. The problem was that it was incomplete.
The more important question was whether the abscess could be explained by an adjacent bowel abnormality.
Once the rectosigmoid wall thickening, abnormal gas distribution, severe stenosis, and pelvic collection were interpreted together, the clinical picture became substantially more coherent. The barium enema then provided a morphological clue that was difficult to appreciate on the initial CT: the severe stenosis and sharp shouldering of the rectosigmoid segment.
The subsequent biopsy established adenocarcinoma, while surgery demonstrated ovarian invasion.
The lesson is therefore not simply “recognize the Apple Core sign.”
The deeper lesson is:
When imaging findings appear to represent separate diseases, ask whether they are actually different manifestations of one disease process.
Key Takeaways
A tubo-ovarian abscess can occasionally represent a complication of an underlying colorectal malignancy.
Gas within a complex adnexal collection should prompt consideration of enteric communication.
Rectosigmoid wall thickening adjacent to a pelvic abscess deserves deliberate evaluation.
Circumferential colorectal cancer may not appear as a discrete mass on CT.
Severe stenosis with sharp shouldering is an important morphological clue to malignant narrowing.
Apple Core and Napkin Ring morphology should be interpreted in clinical and pathological context.
A negative fistula study does not necessarily exclude a small or intermittently patent communication.
CT, contrast examination, endoscopy, pathology, surgery, and MRI provide complementary information.
Pelvic MRI is important for local staging of rectal cancer.
Treating a pelvic abscess is not equivalent to treating the disease that caused it.
AI can assist complex imaging workflows but must remain subject to radiologist verification and clinical governance.
Continue Learning
The broader topic can be developed into a MediAI colorectal imaging cluster covering:
CT Imaging of Rectal Cancer: What Radiologists Should Evaluate: Related Link
MRI Staging of Rectal Cancer: Mesorectal Fascia and EMVI
Apple Core Sign: Classic and Mimicking Conditions: Related Link
Colorectal Cancer Presenting as Pelvic Abscess: Related Link
Entero-Ovarian Fistula on CT
Early-Onset Colorectal Cancer: Imaging Red Flags
Rectal Cancer and Multimodal Imaging
AI-Assisted Detection of Colorectal Cancer on CT: Related Link
AI for Fistula and Abscess Detection in Abdominal Imaging: Related Link
PACS-Based Clinical AI for Complex Pelvic Imaging: Related Link
References
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[9] A. J. Scott et al., “Management of Locally Advanced Rectal Cancer: ASCO Guideline,” Journal of Clinical Oncology, vol. 42, no. 28, pp. 3355–3375, 2024. DOI: 10.1200/JCO.24.01160.
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Medical Disclaimer
This article is intended for professional medical education and imaging discussion. It does not provide individualized medical advice, diagnosis, or treatment recommendations. Clinical decisions should be based on the complete patient history, examination, laboratory findings, imaging, pathology, multidisciplinary assessment, and applicable clinical guidelines.
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