Improvement of dose distributions in abutment regions of intensity modulated radiation therapy and electron fields.

In recent years, intensity modulated radiation therapy (IMRT) is used to radiate tumors that are in close proximity to vital organs. Targets consisting of a deep-seated region followed by a superficial one may be treated with abutting photon and electron fields. However, no systematic study regarding matching of IMRT and electron beams was reported. In this work, a study of dose distributions in the abutment region between tomographic and step-and-shoot IMRT and electron fields was carried out. A method that significantly improves dose homogeneity between abutting tomographic IMRT and electron fields was developed and tested. In this method, a target region that is covered by IMRT was extended into the superficial target area by approximately 2.0 cm. The length and shape of IMRT target extension was chosen such that high isodose lines bent away from the region treated by the electrons. This reduced the magnitude of hot spots caused by the "bulging effect" of electron field penumbra. To account for the uncertainties in positioning of the IMRT and electron fields, electron field penumbra was modified using conventional (photon) multileaf collimator (MLC). The electron beam was delivered in two steps: half of the dose delivered with MLCs in retracted position and another half with MLCs extended to the edge of electron field that abuts tomographic IMRT field. The experimental testing of this method using film dosimetry has demonstrated that the magnitude of the hot spots was reduced from approximately 45% to approximately 5% of the prescription dose. When an error of +/- 1.5 mm in field positioning was introduced, the dose inhomogeneity in the abutment region did not exceed +/- 15% of the prescription dose. With step-and-shoot IMRT, the most homogeneous dose distribution was achieved when there was a 3 mm gap between the IMRT and electron fields.