  The circumstellar structure on 100 AU scales of the massive young stellar
object W33A is probed using the VLTI and the MIDI instrument. N-band
visibilities on 4 baselines are presented which are inconsistent with a
spherically symmetric geometry. The visibility spectra and SED are
simultaneously compared to 2D axi-symmetric dust radiative transfer models with
a geometry including a rotationally flattened envelope and outflow cavities. We
assume an O7.5 ZAMS star as the central source, consistent with the observed
bolometric luminosity. The observations are also compared to models with and
without (dusty and gaseous) accretion disks. A satisfactory model is
constructed which reproduces the visibility spectra for each (u,v) point. It
fits the silicate absorption, the mid-IR slope, the far-infrared peak, and the
(sub)mm of the SED. It produces a 350 micron morphology consistent with
observations. The 10 micron emission on 100 AU scales is dominated by the
irradiated walls of the cavity sculpted by the outflow. The visibilities rule
out the presence of dust disks with total (gas and dust) masses more than 0.01
Msun. However, optically thick accretion disks, interior to the dust
sublimation radius, are allowed to accrete at rates equalling the envelope's
mass infall rate (up to 10^(-3) Msun/yr) without substantially affecting the
visibilities due to the extinction by the extremely massive envelope of W33A.
