We hypothesize that for large pendulous breasts, breast irradiation in supine and prone position are equivalent with regard to coverage of the PTV, but with a better dose homogeneity in prone position and a smaller radiation exposure of lung and heart.The purpose of this study is to compare the 3-D dose distribution in PTV(Planning Target Volume) and normal tissues in prone position versus supine position.

Local excision followed by adjuvant radiotherapy has become standard treatment for women with early-stage breast cancer. Adjuvant radiotherapy may encounter technical difficulties in patients with pendulous breasts when treated in supine position.

Based upon literature and our own preliminary experiences, we hypothesize that for large pendulous breasts, breast irradiation in supine and prone position are equivalent with regard to coverage of the PTV, but with a better dose homogeneity in prone position and a smaller radiation exposure of lung and heart. However, no quantitative data are yet available to test this hypothesis. Therefore, the aim of our study is to compare the 3-D dose distribution in PTV and normal tissues in prone position versus supine position. For treatment in prone-position an in-house developed device, the "mammaboard" was used.

Inclusion Criteria:

• Patients should have had breast-conserving surgery for breast cancer or DCIS (Ductal Carcinoma in Situ)
• No indication for radiotherapy of regional nodes
• Large, pendulous breasts (bra size D and over)

Exclusion Criteria:

• Regional radiotherapy is indicated
• Unable to lie in prone position

during treatment