This will be a walkthrough on how to perform an electron cutout calibration, i.e, calculating the MU necessary to treat to a depth using a specific cutout. For this walkthrough, I will be using an example on the Elekta Synergy of a cutout meant for the following: 12 MeV electron beam, 20 x 20 CM cone, prescribe to 90% isodose line, 100 CM SSD, and 250 cGy/fx. For an electron treatment, patients are simulated on the treatment machine, and the therapists alongside the physicians will determine the gantry, collimator, and couch positions for treatment. The therapists will outline the treatment area onto a piece of paper and then create a corresponding electron cutout. An electron cutout calibration is not necessary if there is an cutout with a similar shape existing in the library.
Assuming the therapists have been treating on the machine prior that day, enter into ‘stand-by’ mode on Mosaiq which will allow you to operate the machine without needing to mode up a field using a fake patient. On the console screen, you will also need to log out of ‘clinical e’ and log into ‘service mode’ to manually set the electron beam parameters. In service mode, go to ‘service function’ and then ‘deliver quick beam’.
The D_max of a 12 MeV electron beam for the Elekta Synergy machine is 2.8 CM. Begin by placing two 5 CM slabs of water phantom onto the couch which will serve to provide the necessary back scatter. Place the slab of water phantom on top which has an insert for the farmer chamber to slide into. This slab is 2 CM thick, and the distance from the chamber, when fully inserted, to the surface of the phantom is 1 CM. There is also a cross on the slab surface which will be the location of the chamber that is in the phantom, and align that cross to the crosshairs of the light field. Place an additional 1.8 CM of water phantom on top to total to 2.8 CM depth (D_max for a 12 MeV electron beam) from the surface to the center of the chamber. Raise the couch such that the surface of the water phantom is at 100 SSD by using either the ODI and/or room lasers. Mount the applicator onto the gantry head for a 10 x 10 CM cone, and place an open 10 x 10 CM cutout. Connect the chamber that is now in the phantom to an electrometer in the console room using a wire.
Exit the treatment room, and input the correct beam parameters into the console screen. Prior to taking the readings, you will need to saturate or warm up the chamber first by exposing the chamber using 500 MUs. Once the chamber is saturated, dial in 100 MUs and record two readings. Re-enter the treatment room, and swap out the applicator for a cone size of 20 x 20 CM and the open cutout for the molded cutout to be used for treatment. If the electron field was simulated for an extended SSD, adjust the couch vertical position to that SSD. An alternative method is to leave the SSD at 100 CM and apply the inverse square factor into the hand calculation. Record two readings with the cutout for treatment. Take the average for each pair of readings, and then calculate for the RFF by dividing the treatment cutout average by the open cutout average. As in a regular MU hand calculation, take the dose and divide it by your output factors which, in this case, will be the RFF and also the isodose line prescription. The MUs calculated for will be entered into Mosaiq to be moded up for treatment.
Log out of service mode and log back into clinical mode. Go to ‘receive external prescription’ and mode up a field in ‘QA mode’ for a random patient in Mosaiq to check that the console screen and Mosaiq are in communication. If there is no communication, re-logging into Mosaiq should fix the issue.
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