Abstract
BackgroundThe 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.MethodsThe 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.ResultsFor 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.ConclusionsTopologies 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s13014-015-0494-5) contains supplementary material, which is available to authorized users.
Highlights
The aim is to analyze characteristics and to study the potentials of non-coplanar intensity modulated radiation therapy (IMRT) techniques
Note that one organ at risk (OAR) or several OARs in a dense sequence distributed around a convex planning target volume (PTV) cannot in principle be perfectly spared in the coplanar beam arrangement: there are always beam orientations critical for the dose saturation in the PTV, which irradiate through the OARs
For non-coplanar techniques a, b and d substructures of the fluence distribution reflected the symmetry of the beam arrangement
Summary
The aim is to analyze characteristics and to study the potentials of non-coplanar intensity modulated radiation therapy (IMRT) techniques. The aim of this work is to analyze characteristics of non-coplanar IMRT for planning target volumes (PTVs) encompassing an organ at risk (OAR). Note that one OAR or several OARs in a dense sequence distributed around a convex PTV cannot in principle be perfectly spared in the coplanar beam arrangement: there are always beam orientations critical for the dose saturation in the PTV, which irradiate through the OARs (type III geometry, Fig. 1). The optimal number of beams, their directions and fluence distributions depends on the properties of the OARs. A sparse distribution of OARs around PTV, which allows beam directions which perfectly spare the OAR (type II geometry, Fig. 1) leads to a problem of beam angle optimization (BAO). This work is mainly concentrated on type I and type II problems, a PTV encompassing an OAR
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