ESTRO 37 Abstract book

ESTRO 37

S470

Consortium DKTK partner site Tübingen, Heidelberg, Germany

Purpose or Objective A MR-linac provides high soft-tissue contrast imaging during radiotherapy treatments: magnetic resonance guided radiotherapy (MRgRT). However, trajectories of secondary electrons are influenced by the magnetic field and may deposit dose outside of the primary beam. This work investigated changes of dose depositions at water-air and tissue-lung interfaces due to a magnetic field (B-field) transverse to the beam direction by means of Monte Carlo (MC) simulations and Simulations were performed using the MC code EGSnrc (2016). Two phantoms were used: a 10x10x10cm³ phantom with a 4-2-4cm water-air-water (P1) and a 4-2- 4cm tissue-lung-tissue (P2) cross section along the beam axis. Both were irradiated with two opposing parallel 4x4cm² beams to compensate electron return effects. Dose depositions were scored for B-field values of 0, 0.35, 1, 1.5 and 3T with a statistical accuracy of <1%. Beams were based on a precalculated photon spectrum from an earlier simulated Elekta 6MV-FFF accelerator. Depth dose profiles were analysed on the central beam axis, lateral dose profiles were evaluated next to an interface, both were normalised to 0T data. In-field asymmetry S in was described by the relative difference of the integral dose in two areas separated by the central axis. Out-of-field dose change D out was quantified by the integral out-of-field dose normalised to B=0T. 80-20%- penumbras were evaluated at left and right beam edges, independently. Results Depth-dose curves in P1 showed local increases close to both interfaces of 2.7% and 1.4% for 3T and 1.5T, respectively (cf. figure 1). A shallower dose fall-off in air was found for 0.35T. In P2 dose changes of +6.1/-4.4, +4.6/-6.7 and -9.9/+1.4% were scored for 1, 1.5 and 3T, respectively. All lateral profiles showed a B-field dependent Lorentz force driven shift and asymmetry (cf. figure 2). Beams in P1 showed maximum penumbras of 0.9cm left and 1.4cm right for 0.35T. Calculated electron trajectory radii in air derived from an energy spectrum in a depth of 4cm water agree with these values. In P2, widest penumbras were scored for 1T measuring 0.5cm left and 0.7cm right. Strongest asymmetries were detected for 1T in P1/P2 resulting in S in =9.9/5.1%. Highest out-of-field doses were detected for 0.35T in P1 and 1T in P2 with D out =56.3/39.0%, respectively (cf. table 1). opposing beams setups. Material and Methods

Conclusion In this study, we observed B-field and density dependent dose effects at water-air and tissue-lung interfaces, which affect the resulting dose distribution, particularly out-of-field. As the relative contribution of the individual effects varies, MRgRT requires patient individual MC dose calculations. PO-0888 Comparison of x-ray CT and proton based CT planning in the presence of titanium dental implants C. Oancea 1,2 , K. Shipulin 1 , G. Mytsin 1 , M. Gao 3 , M. Pankuch 3 , G. Coutrakon 4 , C. Ordonez 4 , R.P. Johnson 5 , V. Bashkirov 6 , R. Schulte 6 1 Joint Institute for Nuclear Research, DLNP, Dubna, Russian Federation 2 University of Bucharest/NPI/ IFIN-HH, Medical Physics, Magurele/ Prague, Romania 3 Northwestern Medicine Chicago Proton Center, Medical

Table 1: Opposing beams data from P1 and P2.

P1: D out

P2: D out

P2:S in

P1:pright/ cm

P2:pright/ cm

/

/

B/ T

P1:S in

/

/

%

%

%

%

0 0

0

0

0

0.4

0.4

0.3 5

7.1

56.3 -0.4

16.7 1.4

0.3

1 9.9

-1.8 5.1

39.0 0.8

0.7

1.5 7.1

-26.7 4.8

24.0 0.6

0.6

3 2.7

-52.3 2.2

-12.7 0.4

0.4