Scripting a planning avoidance structure to perform simultaneous integrated boost breast on Halcyon linear accelerators

Abstract

When planning a simultaneous integrated boost in breast treatment with sliding window intensity-modulated radiotherapy, the use of an oblique anterior field can be advantageous. To limit the irradiation to the tumor bed on the oblique anterior field, we use the "fixed jaw" option during optimization in Eclipse (v15.6, Varian Medical System). However, this option is not available for Halcyon (Varian Medical System) linear accelerators because the beam-defining collimator is only composed of a dual-layer multileaf collimator (MLC) without any jaw collimator. Hence, the optimizer opens the leaves on all target volumes, and leaves facing the heart or medullary canal may be opened on the oblique anterior field. To avoid this unacceptable behavior, we created an avoidance structure using the Eclipse Scripting Application Programming Interface (ESAPI) to force the optimizer to close the leaves when facing critical organs at risk. The avoidance structure was the whole body of the patient subtracted from every beam path used: the tangential fields collimated to the breast planning target volume (PTV) and the oblique anterior field collimated to the tumor bed PTV. The ESAPI has a built-in method that returns a table of points, drawing the outline of a structure at the isocenter plane of a beam. We used this method to build the avoidance structure. We planned 6 breast cancer patients using this structure, and we were able to meet all dosimetric constraints. All MLC leaves were fully closed outside the tumor bed PTV for the oblique anterior field.

Keywords: Breast cancer; IMRT; Jaw-free linear accelerator; Scripting.