Abstract
We propose an approach to determining optimal beam weights in breast/chest wall IMRT treatment plans. The goal is to decrease breathing effect and to maximize skin dose if the skin is included in the target or, otherwise, to minimize the skin dose. Two points in the target are utilized to calculate the optimal weights. The optimal plan (i.e., the plan with optimal beam weights) consists of high energy unblocked beams, low energy unblocked beams, and IMRT beams. Six breast and five chest wall cases were retrospectively planned with this scheme in Eclipse, including one breast case where CTV was contoured by the physician. Compared with 3D CRT plans composed of unblocked and field‐in‐field beams, the optimal plans demonstrated comparable or better dose uniformity, homogeneity, and conformity to the target, especially at beam junction when supraclavicular nodes are involved. Compared with nonoptimal plans (i.e., plans with nonoptimized weights), the optimal plans had better dose distributions at shallow depths close to the skin, especially in cases where breathing effect was taken into account. This was verified with experiments using a MapCHECK device attached to a motion simulation table (to mimic motion caused by breathing).PACS number: 87.55 de
Highlights
Intensity-modulated radiation therapy (IMRT) utilizes dynamic MLC motion to spare surrounding normal tissues and achieve better dose conformity and uniformity within the target
We propose a treatment planning approach to assigning optimal weights to low- and high-energy unblocked beams, as well as to IMRT beams, in order to effectively decrease the breathing effect and to keep skin dose as much as possible when the skin is included in the target, or otherwise to keep skin dose as little as possible if the skin is excluded from the target
We have proposed a two-point approach to calculating optimal weights of beams in hybrid breast plans consisting of tangential low- and high-energy unblocked beams and low-energy IMRT beams
Summary
Intensity-modulated radiation therapy (IMRT) utilizes dynamic MLC motion to spare surrounding normal tissues and achieve better dose conformity and uniformity within the target. Breathing may be hard to control and not periodic,(9) and may interplay with MLC leaf motion when the sliding window technique is used to deliver dose.[10,11] phantom studies where doses delivered with and without breathing were compared, have shown that dose distribution discrepancy caused by breathing is similar between 3D CRT and IMRT, based on the statistical analyses such as gamma test, and mean and standard deviation.[12] The main discrepancy is located near the skin, where breathing effect is more pronounced in IMRT than in 3D CRT plans.[12] This is mainly because in 3D CRT plans, usually more than 90% of the dose is delivered via unblocked beams (including at least 2 cm air gap from the skin), while in IMRT plans, because objective-based inverse optimization algorithms are used, radiation fluence is restricted to be within the body. We refer skin flash as the technique to generate fluence in the air gap from the skin
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