Inclusion Criteria:

- Histologic proof of phyllodes tumor of borderline or malignant grade, as first
defined by Pietruszka and modified by Azzopardi and adopted by the World Health
Organization (1,2,17):

- Borderline malignant: 5-9 mitoses/10 HPF, pushing or infiltrating margins, 2+
(moderate) stromal cellularity and atypia.

- Malignant: 10 or more mitoses / 10 HPF, predominantly infiltrating margins,
usually 3+ (severe) stromal cellularity and atypia but occasionally 2+.

- The tumor has been excised with a breast-conserving resection and there is no tumor
seen at any of the margins of the resection.

- No prior breast carcinoma or ductal carcinoma in situ in the ipsilateral breast.
Patients with a local recurrence of a previously excised phyllodes tumor are eligible
if the recurrence is in the area of the previous excision.

- No history of irradiation of the ipsilateral breast.

- No evidence of other areas worrisome for cancer on physical examination and
mammography of the ipsilateral breast.

- Age >18 years.

- Informed consent.

- Documentation that either:

1. the patient's medical insurance company has certified that they will pay for the
cost of radiation therapy treatments, or

2. a letter from the patient indicating that they explicitly understand the costs
of radiation therapy and that the sponsor (Principal Investigator) of this
study will not be held responsible for these costs.

Exclusion Criteria:

- Histologically positive margins.

- Breast carcinoma or ductal carcinoma in situ in the ipsilateral breast.

- A history of irradiation to the ipsilateral breast.

- Pregnancy. A urine pregnancy test will be performed on each fertile premenopausal
female prior to entry into the study. Patients with childbearing potential must
employ effective contraception during the radiation therapy.

- A radiation planning CT scan which demonstrates a target lumpectomy cavity that is
not clearly delineated or a target lumpectomy cavity/whole breast reference volume >
30%.

- Unacceptable radiation therapy quality assurance parameters, as defined in Section 5
of the protocol.