ISO+ & CROSS TEST OBJECTS

The OTP-ISO phantom, patented by QUALIFORMED, enables you to accurately perform the test proposed by Winston & Lutz [Int J Radiat Oncol Biol Phys (1988), 14(2):373-81] to determine in the treatment room, the location and the "size" of each 3 MV rotation axes (gantry, collimator and couch) of linacs used for radiation therapy. The position of each radiation isocenter is determined relative to the point of intersection of the laser plans used to materialize this isocenter in the treatment room, i.e. the center of the radio-opaque ball aligned on the laser planes. The alignment accuracy of the radio-opaque ball is ensured by the 3 perfectly orthogonal white equatorial grooves engraved on the surface of an acrylic sphere 100 mm in diameter, whose center corresponds to the center of the radio-opaque ball. The unit is suspended from an carbon rod connecting it to a large counterweight base which includes an accurate bull’s eye spirit level, 3 adjustable screws for levelling and 4 white pre-alignment etchings.

The acrylic sphere provides a zoom effect to better align the phantom on the lasers planes. The orthogonal white equatorial grooves set into the black background further facilitate alignment. The “2 to 2” coincidence, level and orthogonality of the laser planes may all be verified before carrying out the Winston & Lutz test.

A set of 3 pairs of peripheral balls, used to determine the gantry’s rotation angle, complete the phantom. The carbon holding rod completed with a inox ring enables the treatment couch’s rotation angle to be measured. it's important to note that these angles measured with MV images reflect the reality of the rotations used for treatments. 

The OTP-CROSS-MV test object is affixed to the treatment machine's collimator outlet. The OTP-CROSS-KV is the equivalent test objet-test for the X-ray tube. Both materialise in portal images a fixed point coming from of the radiation source. This is essential in the Winston-Lutz test in order to overcome the imager sag with the gantry rotation This reference fixed point is originally the centre of a small radiation field, however by using OTP-CROSS test object results are not impacted by sagging of the jaws due to the gantry rotation angle. The collimator angle may also be measured from the orientation of the OTP-CROSS on the images.

The test is carried out on high energy (MV) portal images in order to determine the treatment isocenter for the corresponding energy. By repeating this test on a low energy (KV) portal imager or/and CBCT imager, the alignment of the onboard imager onto the treatment isocenter can be verified.