Room: Exhibit Hall
Purpose: To create a monthly test that analyzes the accuracy of photon field junctions using half beam matching to achieve better accuracy in clinical cases involving matched fields, such as a 3-field breast technique.
Methods: Half beam matched plans were generated in the Eclipse. Each jaw (X1, X2, Y1, Y2) is matched to itself via a collimator rotation of 180Â°, and to the other 3 jaws via an appropriate collimator rotation (0Â° and 90Â°). This results in a total of 10 jaw combinations. Each jaw combination is compared against an open jaws 10x10 field central axis image and analyzed in Portal Dosimetry. A tolerance value was obtained from a theoretical calculation in Eclipse by offsetting each matching jaw by 1 mm (TG-142 tolerance). In our clinic, the half beam matched plans are run monthly and jaws are recalibrated using EPID (Electronic Portal Imaging Dosimetry) and raw encoder counts if the result is greater than 25%. A lower tolerance value of 25%, rather than the theoretical calculation tolerance value of 28% is used as the action level for safety and the treatment sites treated in house.
Results: From October 2017 until February 2018, all of the half beam matching plans have been run monthly on three Varian Truebeam linear accelerators. After the first month, the jaws were recalibrated on two of the machines. Since implementation, the jaws were recalibrated again in December 2017 on one of the linear accelerators that had previously been recalibrated. The third linear accelerator recently had its jaws recalibrated in February of this year. After jaw recalibration, half beam matching results were consistently less than 8%.
Conclusion: This monthly test presents an alternative method for determining the photon field junction accuracy that is quantifiable, more objective, and more convenient than film analysis.