Several studies discussing imaging polarimetry observations of protoplanetary disks use the so-called radial Stokes parameters Q _ { \phi } and U _ { \phi } to discuss the results . This approach has the advantage of providing a direct measure of the noise in the polarized images under the assumption that the polarization is azimuthal only , i.e. , perpendicular to the direction towards the illuminating source . However , a detailed study of the validity of this assumption is currently missing . We aim to test whether departures from azimuthal polarization can naturally be produced by scattering processes in optically thick protoplanetary disks at near infrared wavelengths . We use the radiative transfer code MCFOST to create a generic model of a transition disk using different grain size distributions and dust masses . From these models we generate synthetic polarized images at 2.2 \mu m. We find that even for moderate inclinations ( e.g. , i = 40 \degr ) , multiple scattering alone can produce significant ( up to \sim 4.5 \% of the Q _ { \phi } image , peak-to-peak ) non-azimuthal polarization reflected in the U _ { \phi } images . We also find that different grain populations can naturally produce radial polarization ( i.e. , negative values in the Q _ { \phi } images ) . Despite the simplifications of the models , our results suggest that caution is recommended when interpreting polarized images by only analyzing the Q _ { \phi } and U _ { \phi } images . We find that there can be astrophysical signal in the U _ { \phi } images and negative values in the Q _ { \phi } images , which indicate departures from azimuthal polarization . If significant signal is detected in the U _ { \phi } images , we recommend to check the standard Q and U images to look for departures from azimuthal polarization . On the positive side , signal in the U _ { \phi } images once all instrumental and data-reduction artifacts have been corrected for means that there is more information to be extracted regarding the dust population and particle density .