A 60-year-old female underwent colonoscopy as part of a national bowel screening programme, and was noted to have a high grade dysplastic polyp of the distal sigmoid colon. She subsequently underwent a laparoscopic high anterior resection, removing the moderately differentiated adenocarcinoma (pT1, N0, MX).
6 month computed tomography scan follow-up showed two post-operative peritoneal lesions. Lesion A) was an oval 2x1cm diameter, arising immediately anterior to the pancreatic tail. Lesion B) was a rounded 3cm lesion contained within the gastrosplenic ligament (Fig 1a). At this time, both lesions were of cystic appearance. Fine needle aspiration showed the peri-pancreatic lesion (A) to be benign.
At 2 year CT follow-up, lesion A) was no longer visible. However, lesion B) was unchanged in size and was now showing a solid appearance (Fig 1a). At this point the patient was referred for PET/CT scanning to rule out recurrent peritoneal tumour.
The PET/CT scan showed lesion B) to be moderately FDG avid and presumed suggestive of malignancy (Fig 2). The patient then underwent CT guided biopsy. The subsequent histological report concluded that the lesion was xanthogranulomatous (XG) inflammation, and thus malignancy was finally ruled out. Another CT at 6 months post-biopsy showed that lesion B) had considerably reduced in size and the patient returned to standard follow-up.
Fig 1a) Cystic lesion in gastro-splenic ligament post laparascopic high anterior resection for a moderately differentiated pT1 Dukes's A sigmoid cancer. Second lesion adjacent to pancreatic tail not shown.
Fig 1b) 18 months later same lesion becoming more solid. Peri-pancreatic lesion has resolved (not shown).
Fig 2) Concurrent PET/CT shows significant FDG avidity.
Fig 3) 4 months after benign histology from CT guided biopsy: lesion has spontaneously regressed.
Fig 4) Photomicrograph showing a nodular collection of foamy histiocytes containing lipofuscin pigment.
Use of PET/CT scanning is becoming more frequent in the staging and follow-up of colorectal cancer. There is good evidence and support for its use, such as 25% of hepatic metastases being missed by standard CT imaging.
However, despite this increase in use there are still numerous pitfalls. FDG uptake only indicates a high metabolic rate of tissue and thus is not always specific to malignancy. This means that inflammation (1-3), infection and certain tissue types (4, 5) can also increase avidity and thus lead to false positive results. This in turn can lead to unnecessary and potentially harmful interventions.
In this case, increased FDG avidity was caused by an XG inflammatory process. This is characterised by foamy, vacuolated histiocytes. The most common sites of occurrence are the gall bladder (6) and kidneys, both of which are already documented extensively in the literature.
However, there are other atypical cases of XG inflammation. These include the pancreas, appendix, eyes, ovary, fallopian tubes, epididymis, adrenal gland, colon and urachus.
To our knowledge, this is the first reported case of XG inflammation causing a false positive finding on FDG-PET/CT within a peritoneal nodule in the context of curative surgery for colorectal cancer.
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Friday, 11 September 2015
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