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Strong gravitational lensing leads to an occurrence of multiple images, withdifferent magnifications, of a lensed source. Those magnifications can in turnbe modified by microlensing on smaller mass scales within the lens. Recently,measurements of the changes in the magnification ratio of the individual imageshave been proposed as a powerful tool for estimation of the size and velocityof the emission region in the lensed source. The changes of the magnificationratios in blazars PKS1830-211 and QSO B0218+357, if interpreted as caused by amicrolensing on individual stars, put strong constraints on those twovariables. These constraints are difficult to accommodate with the currentmodels of gamma-ray emission in blazars. In this paper we study if similarchanges in the magnification ratio can be caused by microlensing onintermediate size structures in the lensing galaxy. We investigate in detailsthree classes of possible lenses: globular clusters (GC), open clusters (OC)and giant molecular clouds (GMC). We apply this scenario to the case of QSOB0218+357. Our numerical simulations show that changes in magnifi- cations withsimilar time scales can be obtained for relativistically moving emissionregions with sizes up to 0.01 pc in the case of microlensing on the cores ofGCs or clumps in GMCs. From the density of such structures in spiral galaxieswe estimate however that lensing in giant molecular clouds would be morecommon.

Variability of GeV gamma-ray emission in QSO B0218+357 due to microlensing on intermediate size structures