Durante, M.

Graeff, C.

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Robust 4D-optimized treatment plans in scanned carbon ion beam therapy forintrafractionally moving lung cancerC. Graeff1 and M. Durante11GSI, Darmstadt, GermanyIntroductionThe treatment of moving tumors with a scanned car-bon ion beam is challenging due to interplay effects andrange changes. This problem is even more acute for mod-ern hypo- or single fractionated treatment, where averag-ing effects between fractions are diminished or missing. Atechnique for conformal treatment is tracking, where theion beam is deflected to follow the target. This permits tocompensate translatory motion components only and needssophisticated hardware to cope with range changes online.Plans optimized on an entire 4DCT depicting the tumormotion over a breathing cycle inherently include the char-acteristic motion and also range information. Here, wepresent a modality that uses this 4DCT information to de-liver homogeneous, conformal doses to each motion phase,and as such includes a form of rescanning.Material and MethodsThe treatment plans are generated separately for eachmotion phase, using the same set of beam energies, thoughnot each plan necessarily has to use all energies. This issimilar to single field uniform dose (SFUD) plans com-monly generated for static tumors. In contrast to othermotion mitigation techniques, a deformable registration be-tween motion phases is not needed during optimization ex-cept to propagate the target contours. For delivery, the re-sulting independently optimized plans are then joined to a4D plan, which can be irradiated using a 4D control systemthat was experimentally validated at GSI in 2012 [1]. Planswere computed for a lung cancer patient using 4 fields fora single fraction of 17.7 Gy(RBE). The patient had a com-plex tumor geometry with a motion amplitude of 12.7mmover the breathing cycle. The fields were both optimizedseparately (SFUD) and simultaneously (IMPT). DVH re-sults were compared to a static (no motion) and interplay(no motion mitigation) plan on the same patient. We re-port dose coverage (V95) and the conformity number (CN).Interplay and 4D-optimized plans were calculated on the4DCT and did not consider residual motion within the dis-crete motion phases.ResultsExemplary dose cuts of the patient are shown in figure1. For SFUD, the static case resulted in V95 of 98.5% andCN of 70.8%, which degraded to V95 of 72.5% and CNof 52.5% for interplay. 4D-optimization could restore V95to 99.4% and CN to 75.6%. For IMPT, the results of thestatic case were V95 of 99.3% and CN 83.4% comparedto V95 of 76.4% and CN of 56.4% for interplay. Again4D-optimization maintained excellent coverage with V95of 100.0% and CN of 82.1%. The IMPT plans indicate thatthe additional degrees of freedom from 4D-optimization on10 motion phases as well as from the simultaneous opti-mization of fields leads to nominally better plan results.Figure 1: Axial dose cut of the sample patient for static(left panels) and 4D-optimized (right). The top row showsindividually optimized fields, the bottom row simultaneousoptimization. The static dose coverage could be completelyrestored.DiscussionThe presented 4D-optimizaton method leads to highlyconformal irradiation of moving tumors, restoring or ex-ceeding the results of an irradiation without motion. A ma-jor benefit of the proposed method with respect to robust-ness is the lack of dose gradients between motion phases.This is partially negated through the use of IMPT, which in-troduces gradient between fields, though not between mo-tion phases. The results presented here will be experimen-tally tested in the GSI beam times of 2014.References[1] C. Graeff, R. Lu¨chtenborg, J.G. Eley, M. Durante, C. Bert,“A 4D-optimization concept for scanned ion beam therapy”,Radiother Oncol. (2013) 109(3):419-24BIOPHYSICS-36 GSI SCIENTIFIC REPORT 2013238 doi:10.15120/GR-2014-1-BIOPHYSICS-36

Robust 4D-optimized treatment plans in scanned carbon ion beam therapy for intrafractionally moving lung cancer

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Robust 4D-optimized treatment plans in scanned carbon ion beam therapy for intrafractionally moving lung cancer

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