A dosimetric comparison of IORT techniques in limited-stage breast cancer.

BACKGROUND AND
PURPOSE: For intraoperative radiotherapy (IORT) during breast-conserving treatment four different techniques have been addressed: interstitial brachytherapy, an inflatable balloon with a central high-dose-rate source (MammoSite), a miniature orthovolt system (Intrabeam), and linac-based electron radiotherapy (IOERT). The dosimetric properties of these methods are compared.
MATERIAL AND METHODS: Planning target volumes (PTVs) of the same size but of different shapes are assumed, corresponding to the technique's specific situs. Dose distributions for the PTVs and for surrounding tissues are demonstrated by dose-volume histograms and a list of physical parameters. A dose inhomogeneity index (DII) is introduced to describe the deviation of a delivered from the prescribed dose, reaching its minimal value 0 in case of perfect homogeneity.
RESULTS: In terms of DII, IOERT reaches the lowest value followed by the MammoSite, the Intrabeam and interstitial implants. The surrounding tissues receive the smallest average dose with IOERT, closely followed by the orthovolt system.
CONCLUSION: When comparing simplified geometric figures, IOERT delivers the most homogeneous dose distributions. However, in clinical reality PTVs often present asymmetric shapes instead of ideal geometries. Due to a strictly centric dose fall-off, any system with a round central applicator will have technical limits. During IOERT margin-directed applicator guidance is possible and interstitial brachytherapy allows for polygonal dose shaping. These techniques seem to be superior for asymmetric PTV irradiation.