Study Performed:

MRI system: 3.0T magnet (MAGNETOM Skyra, Siemens Medical Technologies, Malvern, PA)

Pre-contrast Sequences (Pelvis):

• Sagittal T2W images (TR 3400ms, TE 97ms, 3mm slice thickness)
• Transverse T2W images (TR 4700 ms, TE 117ms, 3mm slice thickness)
• Sagittal T1W images (TR 700ms, TE 10ms, 3mm slice thickness)
• Transverse T1W images (TR 600ms, TE 11ms, 3mm slice thickness)

Post-contrast Sequences (Pelvis) obtained after intravenous injection of 0.2 ml/kg (0.1 mmol/kg) gadopentetate dimeglumine (Magnevist®):

• Sagittal T1W DIXON images (TR 500ms, TE 13ms, 3mm slice thickness)
• Transverse T1W DIXON images (TR 800ms, TE 13ms, 3mm slice thickness)
• Dorsal T1W DIXON images (TR 700ms, TE 13ms, 3mm slice thickness)

Findings:

• In the terminal 8.8cm of the large bowel, filling the rectum and extending to the anus, there is severe concentric and circumferential thickening of the mucosal layer, 10mm thick. There is multifocal infolding of the submucosa and muscularis into the mass on the left and dorsally. The abnormal tissues are mildly T2w hyperintense, T1w isointense to the muscularis layer, with a distinct villous pattern. Contrast enhancement is homogenous and moderate. There are multiple regions where distinction between mass and submucosa is effaced, consistent with wall invasion, worst 5.6cm cranial from the anus where the mass breaks through the wall focally and blends in with the right m. levator ani and 4 cm cranial from the anus where dorsally, abnormal tissues extend on the left to involve the m. rectococcygeus and the fascial planes.
• The caudal abdominal lymph nodes are normal.
• There is no evidence of periosteal reaction of the adjacent osseous structures.
• In the right vastus lateralis muscle there is a focal region of T2 hyperintensity (as compared to the adjacent musculature) which is strongly contrastenhancing and likely represents a site for intramuscular injection.

Conclusions:

• Invasive rectal wall mass, regional metastasis. This finding is consistent with the reported carcinoma.

Carcinoma in-situ was diagnosed on biopsy; however, MRI findings demonstrate a progression to an invasive carcinoma.

Despite the extent of the mass and poor prognosis for local disease control, the owners elected to proceed with treatment. Based on the size and location of the mass a caudal laparotomy, pubic osteotomy and colostomy were performed the next day to marginally excise and debulk the mass. The patient recovered well and was walking the next morning. The following evening the patient became dyspneic and was diagnosed with aspiration pneumonia, rapidly declined despite aggressive treatment and sustained cardiorespiratory arrest.

In dogs, primary colorectal tumors are uncommon although approximately fifty percent are malignant, with adenocarcinoma being the most common.^1,2 Malignant transformation of adenomatous polyps to carcinoma in-situ has been documented in veterinary patients.³ Invasive carcinoma is diagnosed when dysplastic changes penetrate the submucosa, vasculature or lymphatic vessels.⁴ In people, where colorectal cancer is the third most common cancer diagnosed, five-year survival is 90% if the disease is diagnosed while still localized (i.e. confined to the bowel wall).⁵ The intestinal layering in MRI follows a pattern similar to ultrasound, with signal intensities alternating between T2-w hypointense mucosa, hyperintense submucosa, and hypointense muscularis⁶, facilitating a tight evaluation of these processes’ behavior.

Of particular interest to surgeons is the relation of the tumor to the peritoneal reflection (para-rectal fossa dorsally and the rectogenital pouch, ventrally) as this may alter the approach and technique used. In this case the tumor extended cranially to the most aborad portion of the descending colon and a pull-through technique could not be performed. In humans, particular attention is paid to the mesorectal fascia, the involvement of which is associated with a higher grade in regards to tumor staging. Complete removal of the mesorectum along the mesorectal fascia results in overall local recurrence rates inferior to 10% without the addition of radiotherapy.⁷ In this case, extent of the tumor was underestimated on CT due to the intrinsically reduced contrast resolution provided by this modality.

In humans, MRI-based staging is increasingly recommended in such cases.⁸ Interestingly, most sources rely on standard MR imaging sequences, and recommend no bowel preparation, colonic contrast agents or insufflation with air⁹, a clear advantage over CT colonoscopy. Spasmolytic agents (butylscopolamine) have been administered to prevent intestinal motion during image acquisition and improve image quality. The core MRI sequence used in humans is a T2-w FSE perpendicular to the long axis of the tumor or the intestinal lumen. Additional sequences include T2-w FSE in a dorsal plane to evaluate infiltration of the anal sphincter, and a T1-w to PD 2D TSE in an axial (transverse) plane or T1-w 3D sequences for evaluation of local or distant lymph nodes.¹⁰ Intravenous contrast is reported not to improve MRI staging.¹¹

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