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Background
Cytoreductive surgery (CRS) and hyperthermic intraperitoneal chemotherapy (HIPEC) are increasingly utilized therapies in patients with peritoneal metastasis from gastrointestinal and genitourinary malignancies. CRS is the surgical removal of macroscopic peritoneal tumor, and HIPEC is the bathing of the peritoneal cavity with concentrated, heated chemotherapeutic agents to eradicate microscopic tumor burden. Imaging, particularly abdominopelvic MRI, is the cornerstone of identifying patients suitable for CRS/HIPEC and can quantify tumor burden with the peritoneal carcinomatosis index (PCI). MRI also evaluates for post CRS/HIPEC response, complications, and recurrence. Despite advances in imaging, accurate interpretation of abdominopelvic MRI for peritoneal disease before and after CRS/HIPEC remains challenging.

Educational Goals / Teaching Points
Characteristic MRI appearance of peritoneal metastasis will be demonstrated, highlighting key sequences, sites anatomically prone to peritoneal disease, and commonly missed regions. A paradigm for evaluating peritoneal metastasis for CRS/HIPEC suitability will be discussed. Use of MRI as a surgical guide and predictor of successful cytoreduction will be illustrated. Expected MRI findings and complications after CRS/HIPEC will be shown. Finally, imaging of recurrence will be presented, with distinction between recurrence and postoperative changes.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
High b-value DWI and 5-minute delayed postcontrast T1 sequences are key. T2 sequences provide anatomic correlation and identify T2 intermediate nonmucinous and T2 hyperintense mucinous implants. Delayed contrast uptake distinguishes mucinous disease (occult on PET and cystic on MRI and CT) from fluid. Search for implants should focus on sites of stasis in the peritoneal fluid circulation: the pouch of Douglas/rectovesical space, ileocecal junction, right paracolic gutter, and sigmoid mesocolon. Focus is also directed to occult foci on the liver capsule, diaphragm, small bowel serosa/mesentery, peritoneal lining, bladder, and abdominal wall. MRI assesses nonresectability with attention to the hepatoduodenal ligament, percentage of small bowel involvement, and involvement of the pelvic sidewall, along with surgically blind areas of the retroperitoneum and organ parenchyma. PCI predicts completeness of cytoreduction (CC). Initial postoperative MR also estimates the CC score. MRI identifies expected postsurgical findings of bowel wall thickening, hyperenhancement, mesenteric fluid, and stranding. Complications include abscess, fistula, obstruction, and diaphragmatic perforation. Surveillance MRI accuracy hinges on comparison to baseline postoperative scan. New nodularity or progression of diffusion-restricting enhancement indicate recurrence, whereas nondiffusion restricting enhancement may be seen with postoperative fibrosis.

Conclusion
This exhibit aims to familiarize radiologists with key MRI findings before and afterCRS/HIPEC.