PurposeInvestigation of a reduced source to target distance to improve organ at risk sparing during stereotactic irradiation (STX).MethodsThe 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.ResultsPaddick’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.ConclusionPenumbra requires more attention in comparing studies, especially studies using different planning systems. Reduced SVID probably allows usage of conventional MLC for STX-like irradiations.Electronic supplementary materialThe online version of this article (doi:10.1186/s13014-017-0826-8) contains supplementary material, which is available to authorized users.

The purpose of this study was the mathematical analysis of IMRT with many non-coplanar fields for planning target volumes (PTV) surrounding nearly spherical organs at risk (OAR). Our approach is partially analogous to the well known inverse planning for a cylindrically symmetric (CS) case (Brahme et al 1982 Phys. Med. Biol. 27 1221-9) and leads to a spherically symmetric (SS) solution. For the planning study we approximated isotropic 4 Pi irradiation by a quasi-isotropic non-coplanar IMRT technique with 16 fields which we compared to a coplanar IMRT technique with 15 equidistant fields. A virtual spherical phantom contained a spherical central organ at risk which was surrounded by a PTV shaped like a spherical shell with a gap towards the spherical OAR. We compared three types of plans: (1) non-segmented inversely planned fluence distributions prior to sequencing, (2) plans obtained by direct machine parameter optimization (DMPO) with up to 120 segments (good approximation of non-segmented fluence) and (3) more practical DMPO plans with up to 64 segments. In this study we sought an analytical SS solution for the non-segmented fluence distribution in 4 Pi-geometry. For the CS case Brahme et al found that a special narrow fluence peak ('Brahme peak') has to be applied to improve dose uniformity in PTV areas adjacent to the OAR. We showed that in the SS case the peak was steeper but the area under the peak was smaller. The relevance of the peak decreased for increasing gap between the OAR and the PTV. The plan quality of the non-segmented SS plans was higher albeit the fluence distributions were less uniform. The plan quality of the segmented plans degraded if the allowed number of segments was reduced; the degradation was quicker for the SS beam arrangement than for the CS beam arrangement. For 64 segments, the SS plans delivered less uniform and more conformal dose distributions than the CS plans, ensuring better sparing of the healthy tissue. Also, the SS plans always needed less monitor units than the CS plans. In conclusion, due to substructures or steeper fluence gradients, the improved potential of quasi-isotropic SS-plan quality can only be exploited, if many segments are allowed. SS plans seem to spare normal tissue better. Further analysis of non-coplanar beam arrangements with less degree of symmetry is planned, followed by a study on non-coplanar intensity modulated arc techniques.

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.

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.