Implementation of the isocenter‐shift technique for smoothing MLC field edge on a 3D treatment planning system

Abstract

Stepped leaf edges are the major limitation of conforming to the prescribed treatment contour defined by the conventional multileaf collimator (MLC), which produces a scalloped dose pattern. The commercial HD‐270 MLC™ (HDI) technique provides a software solution of the conventional MLC to achieve smoothed edge and optimal penumbra of the MLC shaped field. We implemented the HDI functionality on a 3D treatment planning system and compared the dosimetric effects of the HDI delivery in simulation with those in experiment for a number of the MLC fields. The fields from the contour of varied shapes with different sizes of the leaf stepping were tested for the HDI delivery. There is a good agreement of the dose distribution between the calculation as implemented in the planning system and the measurement performed on the treatment machine. It has been shown that the HDI delivery significantly smoothes the stepped field edge with the reduced isodose undulation and effective penumbra. A problem may be present when the HDI is applied for the treatment of the circular contour of smaller diameter, and the conformity of the MLC shaping may not be achievable satisfactorily with the existing system. The optimization of leaf configuration is suggested to improve the conformity of the HDI technique. The HDI planning then can be used to assist in the decision making of applying the HDI treatment delivery.

Target-tracking deliveries on an Elekta linac: a feasibility study

“…The square edges of the leaves are no longer visible and the delivery has achieved the dose conformality that may be expected of an MLC system with a smaller leaf width. This effect replicates the result of Xue et al (2002) where an isocentre shift was deliberately induced by moving the treatment couch halfway through the delivery in order to improve the conformality of the dose from the MLC defined field to a circular target projection in the beam's-eye view.…”

Virtual micro MLC commissioning

“…These equations reduce to those published previously (7) when the isocentric and eccentric couch rotations are combined as a single angle. The HD‐270 takes both the isocentric and eccentric couch rotations into account.…”

“…We investigated the performance of the system using films of a 45° field edge generated by a stepped MLC, the angle most commonly used in the literature (3, 4, 7, 9) . The films were taken at the isocenter of a 6‐MV beam with 4 cm of solid water.…”

Virtual Micro MLC Commissioning

“…We investigated the performance of the system using films of a 45° field edge generated by a stepped MLC, the angle most commonly used in the literature. (3,4,7,9) The films were taken at the isocenter of a 6-MV beam with 4 cm of solid water. When the collimator, gantry, and couch…”