Imaging assessment of desmoid tumours in familial adenomatous polyposis: is state-of-the-art 1.5 T MRI better than 64-MDCT?

OBJECTIVE: Desmoid tumour is a common extraintestinal manifestation of patients with familial adenomatous polyposis (FAP) who have undergone prophylactic colectomy. We aimed to determine whether MRI provides equivalent or better assessment of desmoid tumours than CT, the current first-line investigation.
METHODS: Following ethics approval and informed consent, FAP patients with known desmoid tumour underwent contrast-enhanced 64-slice multidetector CT (MDCT) and 1.5 T MRI (incorporating T(1) weighted, T(2) weighted, short tau inversion-recovery and T(1) weighted with contrast, axial, sagittal and coronal sequences). The number, site, size, local extent, tumour signal intensity and desmoid-to-aorta enhancement ratio were analysed.
RESULTS: MRI identified 23 desmoid tumours in 9 patients: 9 intra-abdominal desmoid (IAD) tumours, 10 abdominal wall desmoid (AWD) tumours and 4 extra-abdominal desmoid (EAD) tumours. CT identified only 21 desmoids; 1 EAD and 1 AWD were not identified. The two modalities were equivalent in terms of defining local extent of desmoid. Five IAD tumours involved the bowel, six caused ureteric compression and none compromised the proximal superior mesenteric artery. There was no difference in median desmoid size: 56.7 cm(2) (range 2-215 cm(2)) on MDCT and 56.3 cm(2) (3-215 cm(2)) on MRI (p=0.985). The mean MRI enhancement ratio, at 1.12 (standard deviation 0.43), was greater than the CT enhancement ratio, which was 0.48 (0.16) (p<0.0001). High signal intensity on T(2) MRI was associated with increased MRI enhancement ratio (p=0.006).
CONCLUSIONS: MRI is at least equivalent (and may be superior) to MDCT for the detection of desmoid tumours in FAP. Coupled with the advantage of avoiding radiation, it should be considered as the primary imaging modality for young FAP patients.