The investigators are doing this study to find out whether lowering the dose of postoperative radiation therapy and targeting a smaller area of tissue for treatment is as effective as the standard dose and volume of radiation therapy to control soft tissue sarcoma after surgery. They also want to find out whether the study approach causes fewer and less severe long-term side effects than the standard approach.

Inclusion Criteria:

• Patients must be diagnosed with a primary soft tissue sarcoma of the extremity, confirmed by MSKCC pathologic review

• Age at the time of enrollment of ≥18 years

• Patients must have undergone margin-negative oncologic resection of their primary tumor, as confirmed by MSKCC pathologic review

• Patients must be able to start radiation within 3 months from time of surgery

  °If patient is receiving adjuvant chemotherapy, patient must start RT no longer than 3 months past their chemo treatment.

• Female patients of childbearing potential must have a negative serum pregnancy test within 14 days of radiation start (or if urine test, within 24 hours of radiation start)

• Sexually active patients of childbearing potential must agree to use effective contraception.

• The use of chemotherapy will not be dictated by this trial. Patients are allowed to receive chemotherapy at the discretion of the disease management team (as is standard practice to make individualized decisions for each patient regarding the use of chemotherapy).

Exclusion Criteria:

• Patients with positive margins after surgical resection as indicated by MSKCC pathologic review
• Patients with diagnosis of superficial myxofibrosarcoma with indistinct/infiltrative borders on preoperative MRI16 (see Appendix Figure 1 for reference figure)
• Patients with multifocal disease in the extremity
• Patients who have received prior radiotherapy at or adjacent to the primary tumor bed
• Patients with a differentinvasive cancer requiring active treatment at the time of enrollment.
• Patients with any concurrent medical or psychiatric condition or disease which, in the investigator's judgment, would make them inappropriate candidates for entry into this study

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.