Accelerated Partial Breast Irradiation in Clinical Practice

Abstract

Accelerated partial breast irradiation (APBI) has been under clinical investigation for more than 15 years. There are several technical approaches that are clinically established, e.g. brachytherapy, intraoperative radiotherapy (IORT), or external-beam radiotherapy. The understanding of the underlying biology, optimal technical procedures, patient selection criteria, and imaging changes during follow-up has increased enormously. After completion of several phase III trials using brachytherapy or IORT, APBI is currently increasingly used either in phase IV studies, registries, or in selected patients outside of clinical studies. Consensus statements about suitable patients are available from several international and national societies like ASTRO, ESTRO, and DEGRO. One may expect that 15-25% of patients undergoing breast-conserving surgery may qualify for APBI, i.e. patients with small invasive ductal breast cancer without clinical lymph node involvement.

Keywords: Breast cancer; Intraoperative radiotherapy; Partial breast irradiation; Radiotherapy.

Fig. 1

Cell survival curves in vitro. An increasing radiation dose leads to lower cell survival. Depending on the cell type, the survival curve may have a different shape, which is mathematically characterized by the parameter α/β given in gray. A low value of α/β implies a larger curvature (narrow shoulder), while a higher α/β value is associated with less curvature (broad shoulder, more linear slope at low doses). It is well established that a reduction in the daily fraction size must be compensated by an increase of the total dose to achieve the same biological effect. The way different tumor and normal tissues react to changes in the daily fraction size is usually characterized by the α/β ratio. The difference in α/β values implies that late reacting tissue is spared relative to tumor cells when the daily fraction size is reduced and the total dose is increased by the amount that will give the same rate of local tumor control. Conversely, increasing the size of the daily dose fraction would increase damage to the late reacting normal tissue if the total dose were adjusted to keep the tumor control rate constant.

Fig. 2

IORT during BCS using the Intrabeam® system (Carl Zeiss Meditec, Oberkochen, Germany).

Fig. 3

Flow chart of the ongoing TARGIT-C trial corresponding to the experimental arm of the TARGIT-A study. The TARGIT-E trial used basically the same approach but different entry criteria (70 years and older).

Fig. 4

Outcome 3 years after BCS and IORT. Imaging changes like calcifications are typically more pronounced after APBI as compared to standard WBRT without compromising cosmesis.