The documentation of sliding window treatments is not restricted to PortalVision, DynaLog analysis or VARiS.
If a video camera is placed inside the treatment room and the light field is switched on before beam-on, the sliding window can be seen on the patient surface. In this example, Gantry was at 50°, the camera (Sony PC-110) was placed 4 meters away. The Clinac light field is quite faint, so contrast had to be enhanced. That's why the images look so noisy.
The video (55sec, 1033kB) requires the DivX codec which can be downloaded here.
If you do not want to download the video, here are four screenshots (but you miss the buzzing modulator noise):
Some clinical notes: the patient was a 65 year old female with a primary tumor in os sphenoidale and os ethmoidale with an infiltration into sinus cavernosus and the left orbita. Due to the proximity of critical structures, it was decided to give primary IMRT. The best plan was a 3field plan with gantry angles 0, 50 and 90°.
Here are the BEV images from CadPlan:
