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New LINAC Acceptance & Commissioning Experience: Should We Be Accepting An Accelerator Overlooking a Risk of ±31% Dose Inhomogeneity at the Junctions?

N Paudel1*, (1) UPMC Susquehanna, Hillman Cancer Center, Williamsport, PA


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: To shout out loud about the inadequacy of vendor’s acceptance tests for a LINAC and warn clinical physicists of potential cold-spots or hot-spots at the junctions in zero jaw position split-field (ZJPSF) treatments.

Methods: Varian’s acceptance test procedures were run and a new Edge accelerator was accepted. Eclipse was commissioned for the accelerator. One treatment plan for each 6MV and 15MV energy was created in Eclipse with four 10cm × 10cm asymmetric fields each covering a Cartesian quadrant (X1, X2, Y1 and Y2 jaws for 4 fields: 0,10,-10,0; 0,10,0,10; -10,0,0,10 & -10,0,-10,0). Dose was calculated on a phantom image set with 100 MU for each field. One film at 100cm SAD for each energy was exposed at the planning setup.

Results: At the plane of maximum dose, a uniform dose distribution (within 3%) was formed in the Eclipse generated plan. However, on exposed films (analyzed with Fiji-ImageJ), 27% and 31% cold-strips for 6MV, and 25% and 27% cold-strips for 15MV were found along X and Y junctions respectively. With a hard-fought debate with the vendor, the case was escalated to their Product Support Engineering and jaws were re-calibrated to get uniformity within 2.2%, based on a compromise between the accelerator’s softest (6MV-FFF) and hardest (15MV) clinical energies. It was found very alarming that a large fraction of physicists (out of a sample) don’t run any such test for their ATP or QA.

Conclusion: Even though the accelerator met all the acceptance criteria designed by the vendor, hair-raising cold-strips were found along the Cartesian axes junctions for split jaw fields. Apart from cold spots, there are similar chances of overlapping radiation fields creating hot spots at the junctions. Therefore, dose uniformity for ZJPSF should be guaranteed during a LINAC acceptance and be checked during routine QA of a linear accelerator.


Acceptance Testing, Commissioning, Linear Accelerator


TH- External beam- photons: Quality Assurance - Linear accelerator

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