Background: Small field sizes used for stereotactic radiosurgery shouldn’t require the use of large unwieldy three-dimensional water tanks for commissioning. A study was undertaken to determine whether a smaller volume of water generates significant scatter radiation to yield beam data measurements similar to those obtained using a large full-sized water tank. In addition, the use of a light, small volume tank, would allow for movement of the whole tank in space so that direct SAD beam measurements can be obtained in a much shorter time. Methods: A novel new idea in scanner design was developed where a relatively small volume of water is moved in 3D space, keeping the detector centered in the tank in the horizontal X,Y plane and allowing for detector motion in the vertical Z direction. A separate vertical drive in the tank can move the probe detector independently of the external 3D (X,Y,Z) drives. This separate vertical Z drive is synchronized with the external Z drive to keep the detector at a fixed, constant distance from the linac radiation source (constant SAD). For TPR data collection, the tank vertical drive moves the detector down at the same speed as the external drive raises the tank yielding a direct TPR measurement. For radial, transverse, and diagonal profiles, the whole tank is moved across beam to accumulate cross profiles at various depths determined by the vertical drive in the tank. For percent depth dose measurements, the external drives remain stationary and only the in tank drive is used to move the detector down. Results: The final design (20 cm cylinder, 40 cm height) required three gallons of water weighing about 24 lbs. A robust support system and strong drive motors were required to negate gravity effects. TPR measurements obtained in the down and up directions were compared and found to be nearly identical below depth of maximum build-up. TPR’s, cross profiles at five depths, and percent depth doses were measured and compared to those obtained in a large tank. In all cases, the results were nearly identical. The greatest variation found was for the 6 cm field size profiles at 300 mm depth where the small tank showed values in the toe region, outside the primary beam, of about 0.5% less. In addition to direct SAD measurements, one of the greatest benefits of this system is the fast time of data acquisition. The total scanning time for one cone to obtain TPR and 10 cross profiles at five depths was under 10 minutes at the slowest, highest resolution, settings. For comparison, direct 12 point TPR measurements and SAD cross profile measurements using a standard large tank required significant operator interaction and took almost one hour to complete one cone. Conclusions: A small volume scanning system was found to yield beam scanning results almost identical to those obtained in a large tank. In addition, direct SAD TPR and profile beam data for 12 cones and 12 IRIS field sizes can be obtained in less than one day.