TY - JOUR A1 - Bratengeier, Klaus A1 - Holubyev, Kostyantyn T1 - Anisotropy of dose contributions-an instrument to upgrade real time IMRT and VMAT adaptation? JF - Medical Physics N2 - Purpose: 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. Methods: 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. # Results: 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. Conclusions: 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. KW - modulated arc therapy KW - 2-step IMRT KW - radiation-therapy KW - online adaption KW - prostate-cancer KW - plans KW - IMAT KW - tracking KW - radiotherapy KW - adaption KW - IMRT KW - VMAT Y1 - 2016 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-186833 VL - 43 IS - 11 ER - TY - JOUR A1 - Bratengeier, Klaus A1 - Holubyev, Kostyantyn A1 - Wegener, Sonja T1 - Steeper dose gradients resulting from reduced source to target distance—a planning system independent study JF - Journal of Applied Clinical Medical Physics N2 - Purpose: To quantify the contribution of penumbra in the improvement of healthy tissue sparing at reduced source‐to‐axis distance (SAD) for simple spherical target and different prescription isodoses (PI). Method: A TPS‐independent method was used to estimate three‐dimensional (3D) dose distribution for stereotactic treatment of spherical targets of 0.5 cm radius based on single beam two‐dimensional (2D) film dosimetry measurements. 1 cm target constitutes the worst case for the conformation with standard Multi‐Leaf Collimator (MLC) with 0.5 cm leaf width. The measured 2D transverse dose cross‐sections and the profiles in leaf and jaw directions were used to calculate radial dose distribution from isotropic beam arrangement, for both quadratic and circular beam openings, respectively. The results were compared for standard (100 cm) and reduced SAD 70 and 55 cm for different PI. Results: For practical reduction of SAD using quadratic openings, the improvement of healthy tissue sparing (HTS) at distances up to 3 times the PTV radius was at least 6%–12%; gradient indices (GI) were reduced by 3–39% for PI between 40% and 90%. Except for PI of 80% and 90%, quadratic apertures at SAD 70 cm improved the HTS by up to 20% compared to circular openings at 100 cm or were at least equivalent; GI were 3%–33% lower for reduced SAD in the PI range 40%–70%. For PI = 80% and 90% the results depend on the circular collimator model. Conclusion: Stereotactic treatments of spherical targets delivered at reduced SAD of 70 or 55 cm using MLC spare healthy tissue around the target at least as good as treatments at SAD 100 cm using circular collimators. The steeper beam penumbra at reduced SAD seems to be as important as perfect target conformity. The authors argue therefore that the beam penumbra width should be addressed in the stereotactic studies. KW - radiotherapy KW - stereotactic irradiation KW - penumbra KW - leaf width KW - virtual isocenter Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-177424 VL - 20 IS - 1 ER - TY - JOUR A1 - Bratengeier, Klaus A1 - Herzog, Barbara A1 - Wegener, Sonja A1 - Holubyev, Kostyantyn T1 - Finer leaf resolution and steeper beam edges using a virtual isocentre in concurrence to PTV-shaped collimators in standard distance – a planning study JF - Radiation Oncology N2 - Purpose: Investigation of a reduced source to target distance to improve organ at risk sparing during stereotactic irradiation (STX). Methods: The authors present a planning study with perfectly target-volume adapted collimator compared with multi-leaf collimator (MLC) at reduced source to virtual isocentre distance (SVID) in contrast to normal source to isocentre distance (SID) for stereotactic applications. The role of MLC leaf width and 20–80% penumbra was examined concerning the healthy tissue sparing. Several prescription schemes and target diameters are considered. Results: Paddick’s gradient index (GI) as well as comparison of the mean doses to spherical shells at several distances to the target is evaluated. Both emphasize the same results: the healthy tissue sparing in the high dose area around the planning target volume (PTV) is improved at reduced SVID ≤ 70 cm. The effect can be attributed more to steeper penumbra than to finer leaf resolution. Comparing circular collimators at different SVID just as MLC-shaped collimators, always the GI was reduced. Even MLC-shaped collimator at SVID 70 cm had better healthy tissue sparing than an optimal shaped circular collimator at SID 100 cm. Regarding penumbra changes due to varying SVID, the results of the planning study are underlined by film dosimetry measurements with Agility™ MLC. Conclusion: Penumbra requires more attention in comparing studies, especially studies using different planning systems. Reduced SVID probably allows usage of conventional MLC for STX-like irradiations. KW - radiotherapy KW - multi-leaf collimator KW - stereotactic irradiation KW - robotic table motion KW - planning study KW - virtual isocentre Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-157543 VL - 12 IS - 88 ER - TY - JOUR A1 - Bratengeier, Klaus A1 - Holubyev, Kostyantyn T1 - Characteristics of non-coplanar IMRT in the presence of target-embedded organs at risk JF - Radiation Oncology N2 - Background The aim is to analyze characteristics and to study the potentials of non-coplanar intensity modulated radiation therapy (IMRT) techniques. The planning study applies to generalized organ at risk (OAR) – planning target volume (PTV) geometries. Methods The authors focus on OARs embedded in the PTV. The OAR shapes are spherically symmetric (A), cylindrical (B), and bended (C). Several IMRT techniques are used for the planning study: a) non-coplanar quasi-isotropic; b) two sets of equidistant coplanar beams, half of beams incident in a plane perpendicular to the principal plane; c) coplanar equidistant (reference); d) coplanar plus one orthogonal beam. The number of beam directions varies from 9 to 16. The orientation of the beam sets is systematically changed; dose distributions resulting from optimal fluence are explored. A selection of plans is optimized with direct machine parameter optimization (DMPO) allowing 120 and 64 segments. The overall plan quality, PTV coverage, and OAR sparing are evaluated. Results For all fluence based techniques in cases A and C, plan quality increased considerably if more irradiation directions were used. For the cylindrically symmetric case B, however, only a weak beam number dependence was observed for the best beam set orientation, for which non-coplanar directions could be found where OAR- and PTV-projections did not overlap. IMRT plans using quasi-isotropical distributed non-coplanar beams showed stable results for all topologies A, B, C, as long as 16 beams were chosen; also the most unfavorable beam arrangement created results of similar quality as the optimally oriented coplanar configuration. For smaller number of beams or application in the trunk, a coplanar technique with additional orthogonal beam could be recommended. Techniques using 120 segments created by DMPO could qualitatively reproduce the fluence based results. However, for a reduced number of segments the beam number dependence declined or even reversed for the used planning system and the plan quality degraded substantially. Conclusions Topologies with targets encompassing sensitive OAR require sufficient number of beams of 15 or more. For the subgroup of topologies where beam incidences are possible which cover the whole PTV without direct OAR irradiation, the quality dependence on the number of beams is much less pronounced above 9 beams. However, these special non-coplanar beam directions have to be found. On the basis of this work the non-coplanar IMRT techniques can be chosen for further clinical planning studies. Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-125292 VL - 10 IS - 207 ER -