Classification of space-occupying masses
All retrorectal masses are divided into three major groups based on their origin and the localization of the tissue of origin.
- Developmental cysts (Developmental cysts). Arise from intrauterine development disorders.
- Epidermoid cyst (Epidermoid);
- Dermoid cyst (Dermoid);
- Enteric cyst (Enteric) — may represent a cystic hamartoma or abnormal rectal duplication.
Tailgut cysts (tailgut cysts) also belong to this group.
- Sacral lesions (Sacral lesions). Associated with the structures of the sacrum and spinal canal.
- Teratoma (Teratoma);
- Chordoma (Chordoma);
- Anterior sacral meningocele (Anterior sacral meningocele);
- Lymphangioma (Lymphangioma).
- Anorectal lesions (Anorectal lesions). Tumors growing directly from the tissues of the anorectal zone.
- Lipoma (Lipoma);
- GIST (GIST — gastrointestinal stromal tumor);
- Anal gland cyst (Anal gland cyst).
Role of MRI in diagnosis
Magnetic resonance imaging is the gold standard and the most comprehensive imaging modality for evaluating retrorectal masses.
Main objectives of MRI:
- Clarifying the nature of the pathology: allows differentiation of cystic and tissue structures, and detailed assessment of the signal characteristics of fluid within cysts.
- Detecting signs of malignancy: identifying a solid (mural) tissue component is an alarming symptom. For example, in tailgut cysts, the presence of such a component often indicates the development of a carcinoid tumor.
- Assessing spread: determining local invasion (extension) into adjacent organs and tissues.
- Ruling out spinal pathologies: MRI reliably excludes or confirms the presence of sacral meningoceles and other sacral lesions.
Differential diagnosis of functional disorders
In addition to space-occupying masses, functional disorders such as Hirschsprung disease (Hirschsprung disease) and megarectum (Megarectum) occur in the anorectal zone. Although typically seen in infants and children, both conditions may first present in young adults as severe chronic constipation (intractable constipation).
Imaging methods are used to distinguish them:
- Plain radiography (Plain radiographs): shows a massive fecal impaction and a gigantically dilated rectosigmoid colon.
- Water-soluble contrast enema (Water-soluble contrast enema): performed strictly without preliminary bowel preparation (cleansing).
Signs of megarectum: lateral radiographs show a dilated distal segment (greater than 6.5 cm in diameter) that extends down to the pelvic floor, filling the pelvic cavity. Proximally, there is an abrupt transition to a normal-diameter colon. Signs of Hirschsprung disease: a short abnormal segment that is pathologically narrowed is revealed.
Clinical cases and associated pathologies
Pelvic examination may also reveal other critical conditions with prominent radiographic features:
- Intussusception (Intussusception). On contrast-enhanced CT, it presents with the classic "target appearance" (target appearance). Contrast is visible within the lumen of the intussuscepted segment, surrounded by drawn-in mesenteric fat. Adenocarcinoma is frequently the cause (lead point, lead-point) of colocolorectal intussusception.
- Endometriotic lesion. On T2-weighted MRI scans, it is visualized as a low-signal nodular lesion. It can cause obliteration of the pouch of Douglas (pouch of Douglas), angular deformity, and tethering (tethering) of the rectal wall. Important criterion: the absence of submucosal edema indicates that total infiltration of all layers of the bowel wall is not yet present.
Briefly: answers to topical questions
What do sacral chordomas look like on radiographs and CT?
On radiographs, sacral chordoma manifests as bone destruction, sometimes accompanied by amorphous calcification. CT readily reveals a soft-tissue component extending anteriorly into the pelvic cavity. Changes can be easily missed on plain radiographs, except for large pelvic or lumbosacral tumors. Differential diagnosis is facilitated by the midline origin of the tumor.
What pathologies, other than adenocarcinoma, can act as a lead point for intestinal intussusception in adults?
In adults, lead points for intestinal intussusception, aside from adenocarcinoma, can include submucosal lipomas, lymphomas, and melanoma metastases.
How to determine the depth of invasion of an endometriotic lesion into the layers of the rectal wall using MRI data?
Based on sagittal T2-weighted MRI, the absence of submucosal edema of the anterior rectal wall indicates that full-thickness wall infiltration is currently absent. Additionally, low-signal nodularity, obliteration of the pouch of Douglas, tethering, and angular deformity of the adjacent rectal wall toward the deep endometriotic rectovaginal nodule may be visualized.
The groups of retrorectal masses are easy to remember by the acronym DAS: D — Developmental (cysts), A — Anorectal (tumors), S — Sacral (sacral lesions).
Class and exam questions
What on MRI suggests possible malignant transformation of a tailgut cyst?
The main sign of malignant transformation (e.g., development of a carcinoid) is the appearance of an intramural (solid) tissue component within the cyst.
Why perform an unprepared contrast enema when megarectum is suspected?
The absence of preparation preserves the natural distribution pattern of fecal masses and allows accurate visualization of the abrupt transition zone from the dilated colon to the normal or narrowed colon.
How to differentiate megarectum from short-segment Hirschsprung's disease?
In megarectum, the dilated part of the colon reaches the pelvic floor, whereas in Hirschsprung disease, a short pathologically narrowed distal segment is identified.
What does intestinal intussusception look like on computed tomography?
It has a characteristic "target" appearance, where dense contrast within the bowel lumen is surrounded by layers of the pulled-in wall and mesenteric adipose tissue.
What else to study on this topic
- MRI semiotics of various developmental cysts (epidermoid, dermoid, enteric)
- Diagnostic imaging of sacral teratomas and anterior meningocele
- Hirschsprung disease: pathogenesis and radiological criteria
- The "target" sign in intussusception: differential diagnosis
- Assessment of depth of invasion in deep infiltrating pelvic endometriosis using MRI
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