Gnatkovich Aleksey

Automatic beam angle selection in IMRT planning

Intensity-modulated radiation therapy (IMRT) has gained more and more attention because of its advantage of producing a highly three dimensional conformal dose distribution to the target, while sparing organs at risk (OARs) and normal tissues. Compared with traditional threedimensional conformal radiation therapy (3D-CRT) that conforms only two dimensionally to the target projection in the plane perpendicular to the beam orientation, IMRT can significantly improve the outcome of radiation therapy. Many efforts have been made to make IMRT an easy implementation through increasing the automatization of beam set-up, shortening the optimization time of inverse planning, enhancing the ability of dose verification. The selection of suitable beam angles in external beam radiotherapy is at present generally based upon the experience of the human planner. Normally several trial-and-error attempts are needed in order to find a group of adequate beam angles. Although this leads to good treatment plans, they are not necessarily optimal. Computer optimization of the beam directions has the potential to achieve optimal plans that are customized for each individual patient. The requirement to automatically select beam angles is particularly highlighted in IMRT, in which a smaller number of modulated beams is hoped to be used, in comparison with conformal radiotherapy, in order to shorten the treatment time. It has been proved bymany researchers that the selection of suitable beam angles is most valuable for a plan with a small number of beams. The ultimate goal of the beam angle optimization is to achieve the best possible dose distribution by using the minimum number of beams.

The problem of selection of suitable beam directions in external beam radiation therapy was studied as early as 1992 by Soderstrom and Brahme. They proposed entropy and Fourier transform measures for the selection of suitable beam orientations. Bortfeld and Schlegel (1993) used simulated annealing to optimize the beam directions in the frequency domain. Their general conclusion is that the optimal beam configuration for multiple-beam irradiations (with more than three beams) tends to be an even distribution over an angular range of 0 to 2?, whereas it is very important to optimize the beam directions when the number of beams is only two or three. They also concluded that it is not in general useful to avoid beam orientations through organs at risk for modulated beams.

The genetic algorithm (GA), as a powerful global optimization approach, has been introduced by many researchers to solve radiation optimization problems. In this paper, a new efficient technique is developed to implement automatic beam angle selection (ABAS). InABAS a specified number of angle candidates are selected from a discrete angle candidates pool using a GA, and then the intensity maps of these angles are optimized using a conjugate gradient (CG) method under the guidance of a dose-based objective function. After a great number of iterations (generations), the angle combination with the highest fitness value is regarded as giving the optimal beam angles for the case. A final refined beam intensity map optimization is performed using these optimal beam angles, under the guidance of a more complicated dose–volume-based objective function. The results show that ABAS is valid and efficient and can improve the dose distributions within a clinically acceptable computation time.

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