Beam's-eye-view Dosimetrics-guided inverse planning for aperture-modulated arc therapy

Abstract

Purpose: To use angular beam's-eye-view dosimetrics (BEVD) information to improve the computational efficiency and plan quality of inverse planning of aperture-modulated arc therapy (AMAT).

Methods and materials: In BEVD-guided inverse planning, the angular space spanned by a rotational arc is represented by a large number of fixed-gantry beams with angular spacing of approximately 2.5 degrees. Each beam is assigned with an initial aperture shape determined by the beam's-eye-view (BEV) projection of the planning target volume (PTV) and an initial weight. Instead of setting the beam weights arbitrarily, which slows down the subsequent optimization process and may result in a suboptimal solution, a priori knowledge about the quality of the beam directions derived from a BEVD is adopted to initialize the weights. In the BEVD calculation, a higher score is assigned to directions that allow more dose to be delivered to the PTV without exceeding the dose tolerances of the organs at risk (OARs) and vice versa. Simulated annealing is then used to optimize the segment shapes and weights. The BEVD-guided inverse planning is demonstrated by using two clinical cases, and the results are compared with those of a conventional approach without BEVD guidance.

Results: An a priori knowledge-guided inverse planning scheme for AMAT is established. The inclusion of BEVD guidance significantly improves the convergence behavior of AMAT inverse planning and results in much better OAR sparing as compared with the conventional approach.

Conclusions: BEVD-guidance facilitates AMAT treatment planning and provides a comprehensive tool to maximally use the technical capacity of the new arc therapeutic modality.

Fig. 1.

Beam’s-eye-view dosimetrics (BEVD) score vs. beam orientation for the pancreatic (a) and prostate cases (b).

Fig. 2.

The objective function vs. iteration step for simulated annealing for the pancreas (a) and prostate cases (b).

Fig. 3.

DVHs of the BEVD-guided pancreas AMAT plan together with that of the conventional AMAT plan (a) and the uniform-weight AMAT plan (b).

Fig. 4.

DVHs of the BEVD-guided prostate AMAT plan together with that of the conventional AMAT plan (a), and the uniform-weight AMAT plan (b).

Fig. 5.

Dose distributions of three different AMAT plans for pancreas case (top) and prostate case (bottom). For each case, the results from BEVD-guided, conventional and uniform-weight optimization schemes are shown in the left, middle and right panels, respectively.

Fig. 6.

Beam weights (solid line) as a function of gantry angle for three AMAT plans for the pancreas case (left) and the prostate case (right). For each case, the results from the BEVD-guided optimization, the conventional optimization with a constant initialization of the beam weight, and optimization with uniform beam weight constraint are shown in the top, middle and bottom, respectively. For comparison, the initial beam weights (dash-dot line) are also shown in the plots.

Fig. 7.

Aperture shapes resulted from BEVD-guided (left) and conventional (right) AMAT optimization for beams located between beam angle 250° and beam angle 275° for the pancreas case.