The purpose of this study is to determine in a phase II clinical trial whether accelerated partial breast irradiation after breast conserving surgery using 3 dimensional external beam radiotherapy (3D-CRT) and intensity modulated radiotherapy (IMRT) for low-risk invasive breast cancer patients is safe without causing serious late radiation side-effects.

It is a sequential phase II trial. Eligible patients after breast conserving surgery (BCS) are treated with accelerated partial breast irradiation (APBI) using three-dimensional conformal radiotherapy (3D-CRT) for the first 45 patients or intensity modulated radiotherapy (IMRT) for the second 45 patients.

Primary end-point of the study: late radiation side-effects at 5 years. Early stopping criteria: Not used as primary end-point is late side-effects. Interim analysis: Not planned Secondary end-points: cosmetic results, quality of life, local recurrence-free survival, regional recurrence free survival, distant metastasis free survival, relapse free survival, overall survival, cancer specific survival.

Null-hypothesis for primary end-point: The incidence of severe (>G2) late radiation side-effects after APBI using 3D-CRT or IMRT will not exceed 10%.

Stratification:

• by treatment delivery: 3D-CRT - first 45 patients; IMRT - second 45 patients
• by bra capsize: Capsize A-B; C; and D-D+

Treatment intervention: 9 x 4.1 Gy APBI using 3D-CRT (first 45 patients) or IMRT (second 45 patients) delivered within 5 days, using twice-a-day fractionation.

Clinical target volume (CTV) and Planning target volume (PTV) definition:

CTV = excision cavity + 20 mm minus the actual pathological surgical margin in mm PTV = CTV + 5 mm in each direction

Follow-up:

• ECOG PS, physical examination (inspection and palpation), registration of side-effects (RTOG (Radiation Therapy Oncology Group) EORTC (European Organization for Research and Treatment of Cancer)) early and late radiation morbidity scoring scheme) and cosmetic results (using the Harvard scale) : 7-14 days after RT, than in every 3 months for 2 years, and every 6 months for 5 years.
• Mammography: at 6, 12, 18, 24 months, and annually thereafter
• Quality of life questionnaire (QLQ) (e.g. EORTC (European Organization for Research and Treatment of Cancer) QLQ-30 and Breast (BR)-23) before RT, at 7-14 days after RT, at 3, 6, and 12 months, and annually thereafter
• digital photography: before treatment, at 7-14 days after treatment, and yearly thereafter

Inclusion Criteria:

• previous breast conserving surgery within 12 weeks (in case of adjuvant chemotherapy within 28 days after completion of chemotherapy)
• patient age > 40 years
• ECOG performance status: 0-1
• life expectancy >5 years
• invasive ductal, papillary, mucinous, medullary or tubular carcinoma
• unifocal tumor
• pathological tumor size < or = 30 mm
• pathological negative axillary nodal status with axillary dissection or sentinel lymph node biopsy
• surgical clips in the tumor bed
• pathological surgical free margins of at least 2 mm
• written informed consent of patients

Exclusion Criteria:

• Stage III-IV
• multifocal tumor
• extensive intraductal component (EIC)
• ductal or lobular carcinoma in situ (DCIS or LCIS)
• invasive lobular cancer (ILC)
• lymph-vascular invasion (LVI)
• contralateral breast cancer
• history of treatment for previous breast cancer
• lactation or breast feeding women
• Paget-disease of the nipple
• psychiatric disorder
• increased radiosensitivity (e.g. collagen vascular disease)