Observing People With Desmoid-Type Fibromatosis

Inclusion Criteria: * 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 Exclusion Criteria: * Symptomatic desmoid fibromatosis requiring narcotic pain control, resulting in intestinal fistulization, or requiring inpatient admission during the 3 months prior to enrollment * Patients with intraabdominal desmoids greater than 7cm in diameter or localized within 1cm of the central mesenteric vessels and those with tumors in the head and neck * KPS performance status \</= 70 if performance status is limited due to tumor. Patients with low KPS performance status related to co-morbidities but without symptoms related to the tumor will be eligible for enrollment. * Age less than 18 years old * Strict contraindication for both cross-sectional MRI and CT imaging with intravenous contrast (e.g., as per standard institutional protocols regarding renal insufficiency or MRI incompatible implanted devices; patients requiring pre-medication to prevent contrast reactions will be eligible for study) * Unable to grant reliable informed consent * Ongoing systemic treatment for desmoid fibromatosis