Abstract
To suggest a definition of dose deposition anisotropy for the purpose of ad hoc adaptation of intensity modulated arc therapy (IMRT) and volumetric arc therapy (VMAT), particularly in the vicinity of important organs at risk (OAR), also for large deformations. Beam's-eye-view (BEV) based fluence warping is a standard adaptation method with disadvantages for strongly varying OAR shapes. 2-Step-adaptation overcomes these difficulties by a deeper analysis of the 3D properties of adaptation processes, but requires separate arcs for every OAR to spare, which makes it impractical for cases with multiple OARs. The authors aim to extend the 2-Step method to arbitrary intensity modulated plan by analyzing the anisotropy of dose contributions. Anisotropy was defined as a second term of Fourier transformation of gantry angle dependent dose contributions. For a cylindrical planning target volume (PTV) surrounding an OAR of varying diameter, the anisotropy and the dose-normalized anisotropy were analyzed for several scenarios of optimized fluence distributions. 2-Step adaptation to decreasing and increasing OAR diameter was performed, and compared to a usual fluence based adaptation method. For two clinical cases, prostate and neck, the VMAT was generated and the behavior of anisotropy was qualitatively explored for deformed organs at risk. Dose contribution anisotropy in the PTV peaks around nearby OARs. The thickness of the "anisotropy wall" around OAR increases for increasing OAR radius, as also does the width of 2-Step dose saturating fluence peak adjacent to the OAR [K. Bratengeier et al., "A comparison between 2-Step IMRT and conventional IMRT planning," Radiother. Oncol. 84, 298-306 (2007)]. Different optimized beam fluence profiles resulted in comparable radial dependence of normalized anisotropy. As predicted, even for patient cases, anisotropy was inflated even more than increasing diameters of OAR. For cylindrically symmetric cases, the dose distribution anisotropy defined in the present work implicitly contains adaptation-relevant information about 3D relationships between PTV and OAR and degree of OAR sparing. For more complex realistic cases, it shows the predicted behavior qualitatively. The authors claim to have found a first component for advancing a 2-Step adaptation to a universal adaptation algorithm based on the BEV projection of the dose anisotropy. Further planning studies to explore the potential of anisotropy for adaptation algorithms using phantoms and clinical cases of differing complexity will follow.
Highlights
Real time or “ad hoc” adaptation of volumetric arc therapy (VMAT) or intensity modulated arc therapy (IMRT) are topics of increasing interest, as image guided radiotherapy (IGRT) is available on the radiation table
Its capabilities are demonstrated for IMRT (Refs. 11 and 12) and an intensity modulated arc technique (IMAT) (Ref. 13) similar to VMAT, 5826 Med
The first results suggest that distribution of anisotropy defined in Eq (1) behaves under changed patient geometry similar to the segments of 2-Step IMRT and VMAT plans, which saturate the dose in the planning target volumes (PTV) areas adjacent to organs at risk (OAR)
Summary
Real time or “ad hoc” adaptation of volumetric arc therapy (VMAT) or intensity modulated arc therapy (IMRT) are topics of increasing interest, as image guided radiotherapy (IGRT) is available on the radiation table. Several methods are developed to provide smooth adaptation to deformed planning target volumes (PTV) and organs at risk (OAR): Fast optimization (i.e., Men et al.) using parallel computing, re-optimization (Wu et al.), dose or fluence deforming algorithms (i.e., Mohan et al.).. Several methods are developed to provide smooth adaptation to deformed planning target volumes (PTV) and organs at risk (OAR): Fast optimization (i.e., Men et al.) using parallel computing, re-optimization (Wu et al.), dose or fluence deforming algorithms (i.e., Mohan et al.).6 The latter are employed for prostate patients using markers (Crijns et al.).. 2-Step constructs segments that on the one hand allow fully adequate IMRT (Ref. 1) and an intensity modulated arc technique (IMAT)..
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