beamIx = 2;
stf(beamIx).isoCenter(1) = stf(beamIx).isoCenter(1) + 3;

resultGUI_isoShift = matRad_calcDoseDirect(ct,stf,pln,cst,w);

slice = round(stf.isoCenter(3)./ct.resolution.z);

DoseDiff = resultGUI(:,:,slice) - resultGUI_isoShift(:,:,slice);

figure, imagesc(DoseDiff);colorbar