Hypofractionation of partial breast irradiation using radiobiological models.
OBJECTIVE
To reduce the fraction number in Partial Breast Irradiation (PBI) with initial prescription of 40 Gy in 10 fractions using radiobiological models with specific focus on risk of moderate/severe radiation-induced fibrosis (RIF) and report clinical results.


METHODS
68 patients (patient group A) were treated with 40 Gy in 10 fractions delivered by field-in-field, forward-planned IMRT. Isotoxic regimens with decreasing number of fractions were calculated using Biological Effective Dose (BED) to the breast. Risk for RIF in hypofractionated treatment was predicted by calculating NTCP from DVHs of group A rescaled to fractions and dose of novel regimens. Moderate/severe RIF was prospectively scored during follow-up. Various NTCP models, with and without incomplete repair correction, were assessed from difference to observed incidence of RIF. In order to verify the value for α/β of 3 Gy assumed for breast, we fitted α/β to observed incidences of moderate/severe RIF.


RESULTS
Treatments with 35 Gy/7f and 28 Gy/4f were selected for the fraction reduction protocol. 75 patients (group B) were treated in 35 Gy/7f. Incidence of moderate/severe RIF was 5.9% in group A, 5.3% in group B. The NTCP model with correction for incomplete repair had lowest difference from observed RIF. The α/β obtained from fitting was 2.8 (95%CIs 1.1-10.7) Gy.


CONCLUSIONS
The hypofractionated regimen was well tolerated. The model for NTCP corrected for incomplete repair was the most accurate and an assumed α/β value of 3 Gy is consistent with our patient data. The hypofractionation protocol is continuing with patients treated with 28 Gy/4f.
Medical Physics Department, CRO Aviano, 33081 Aviano, Italy. Electronic address: mavanzo@cro.it.
Michele
M
Avanzo
Radiation Oncology Department, CRO Aviano, 33081 Aviano, Italy.
Marco
M
Trovo
MRI Applications and Workflow, General Electric, 78533 Buc, France.
Joseph
J
Stancanello
Department of Oncology, University of Cambridge, Cambridge CB2 0QQ, UK.
Rajesh
R
Jena
Radiation Oncology Department, CRO Aviano, 33081 Aviano, Italy.
Mario
M
Roncadin
Institute of Diagnostic Radiology, Department of Medical and Biological Sciences, University of Udine, 33100 Udine, Italy.
Giulia
G
Toffoli
Institute of Diagnostic Radiology, Department of Medical and Biological Sciences, University of Udine, 33100 Udine, Italy.
Chiara
C
Zuiani
Medical Physics Department, CRO Aviano, 33081 Aviano, Italy.
Elvira
E
Capra
D001940:Breast / Q000528:radiation effects*; D001943:Breast Neoplasms / Q000532:radiotherapy; D006801:Humans; D008954:Models, Biological; D011336:Probability; D000069473:Radiation Dose Hypofractionation; D011853:Radiobiology; D050397:Radiotherapy, Intensity-Modulated
Breast; Fibrosis; Hypofractionation; NTCP; Partial breast irradiation; TCP