• Clinical diagnosis of desmoid fibromatosis, either new or newly recurrent. Both patients with sporadic and FAP-associated disease will be eligible for inclusion. Patients with clinical diagnosis of desmoid (as assessed by MSK physician) but without pathologic diagnosis may be enrolled on the study to facilitate acquisition of appropriately protocoled baseline scans and allow for biospecimen collection; these patients will be replaced for purposes of analysis if pathology does not confirm clinical diagnosis.

• At least 18 years of age.

• Able to undergo cross-sectional imaging by either MRI or CT with intravenous contrast

• Disease detectable on cross-sectional imaging with target lesion measuring \>/= 1 cm

• Eligible for management by active observation as assessed by primary MSK care provider

• Willing and able to sign an informed consent document