The goal of this study is to develop an automated and objective method for determining the optimal tangential beam placement that cover the whole ipsilateral breast (WB) and minimize the dose to the organs at risk (OARs). IMRT for whole breast treatment based on direct aperture optimization (DAO) was analyzed and reported using either jaws-only or multileaf collimators (MLCs) in literature. However, for breast irradiation, tangential beams are generally preferred to minimize dose to OARs. In this project, we focus on tangential beams defined by jaws-only aperture but also show the ability of the method to optimize lung/heart blocking.
A support vector machine (SVM) based method is proposed to determine the optimal posterior plane of the tangential beams. Relative significances of including/avoiding the volumes of interests are incorporated into the cost function of the SVM. After finding the optimal 3-D plane that separates the whole breast (WB) and the included clinical target volumes (CTVs) from the OARs, the gantry angle, collimator angle, and posterior jaw size of the tangential beams are derived from the separating plane equation. Dosimetric measures of the treatment plans determined by the automated method are compared with those obtained by applying manual beam placement by the physicians. The method can be further extended to use multileaf collimator (MLC) blocking by optimizing posterior MLC positions.
The plans for 36 patients (23 prone- and 13 supine-treated) with left breast cancer were analyzed. Our algorithm reduced the volume of the heart that receives >500 cGy dose (V5) from 2.7 to 1.7 cm3 (p?0.058) on average and the volume of the ipsilateral lung that receives >1000 cGy dose (V10) from 55.2 to 40.7 cm3 (p?0.0013). The dose coverage as measured by volume receiving >95% of the prescription dose (V95%) of the WB without a 5mm superficial layer decreases by only 0.74% (p?0.0002) and the V95% for the tumor bed with 1.5 cm margin remains unchanged.
This project was supported in part by Varian project Grant no. 15-C6000-33560-33570.
Yao Wang1, Professor
Edward K. Wong1, Associate Professor
Gabor Jozsef2, Associate Professor
Jenghwa Chang3, Associate Professor
1Polytechnic Institute of NYU, Brooklyn, NY
2NYU Langone Medical Center, New York, NY
3Weill Medical College of Cornell University, New York, NY