The purpose of this study is to closely observe people with desmoid-type fibromatosis over 1 months.

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 \

In most cases, patients will be identified by primary care providers in sarcoma medical and surgical oncology clinics.

Memorial Sloan Kettering Cancer Center supports the international committee of medical journal editors (ICMJE) and the ethical obligation of responsible sharing of data from clinical trials. The protocol summary, a statistical summary, and informed consent form will be made available on clinicaltrials.gov when required as a condition of Federal awards, other agreements supporting the research and/or as otherwise required. Requests for deidentified individual participant data can be made beginning 12 months after publication and for up to 36 months post publication. Deidentified individual participant data reported in the manuscript will be shared under the terms of a Data Use Agreement and may only be used for approved proposals. Requests may be made to: crdatashare@mskcc.org.