Are we on the safe side? Action and tolerance limits for verification of IMRT and VMAT plans
PO-1354
Abstract
Are we on the safe side? Action and tolerance limits for verification of IMRT and VMAT plans
Authors: Borda-Ziemacka|, Anna(1);Zelechowska-Matysiak|, Kinga(2);Gabor|, Marta(3);Rygielska|, Anna(1);Pruska-Pich|, Dorota(3);WALEWSKA|, Agnieszka(4)*[sypcia@poczta.fm];
(1)Military Institute od Medicine, Radiotherapy Department- Laboratory of Medical Physics, Warsaw, Poland;(2)University of Warsaw- Faculty of Physics, Department of Biomedical Physics, Warsaw, Poland;(3)Military Institute od Medicine, Radiotherapy Department- Laboratory of Medical Physics, Warsaw, Poland;(4)Military Institute od Medicine, Radiotherapy Department, Warsaw, Poland;
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Purpose or Objective
There are two ways to define action limits for pre-treatment verification: accept commonly defined values or determined own values based on collected statistics. The optimal solution is to use action levels adapted to the conditions of treatment. The work aimed to determine the action and tolerance limits based on the AAPM TG 218 recommendations [1] for the results of dynamic plan verification (IMRT and VMAT). Action limit has been described as gamma passing rate (GPR) limits, exceeding of which may be associated with an increased risk of negative effects of patient treatment on a given dynamic plan. Tolerance limits determine the scope of gamma passing rate (GPR), in which the results of the quality control process of dynamic plans are considered to be unchanged. affected only by random errors.
Material and Methods
The results of verification measurements of 42 IMRT and 127 VMAT plans for 6 MV, collected during 6 months of 2018 were analyzed The pre-treatment measurements were performed with the Octavius 1500 (PTW) matrix placed in the Octavius 4D (PTW) modular phantom system. For each plan, measurements from individual fields/arcs were added and after reconstruction, the cumulative (composite) dose distribution in the phantom volume was compared (volume analysis option) in Verisoft v.7.1 (PTW) with the TPS Monaco v.5.1 (Elekta) calculations. Gamma analysis was performed for 3mm /3% parameters (max. and local dose) with 5%.threshold. Based on the results of the gamma passing rate (GPR) and AAPM TG 218 guidelines, tolerance and action limits for the pre-treatment verification of IMRT and VMAT performed in our department were determined.
Results
The tolerance limits determined based on measurement data for IMRT and VMAT plans expressed in GPR are higher than the action level set analogously, It confirms the stability of the verification process (Tab. 1). Both tolerance and action limits for the VMAT reach higher values than for the IMRT (Fig. 1).
Conclusion
The determined values of action and tolerance limits will allow better monitoring of the verification process. In the event of a change in the system/verification method or gamma analysis parameters or cut-off threshold, tolerance and action limits should be re-determined based on the new data collected after the changed method/parameters have been implemented.
[1] Miften et al.: Tolerance limits and methodologies for IMRT measurement-based verification QA: Recommendations of AAPM Task Group No. 218, Med.Phys. 45(4) April 2018
http://dx.doi.org/10.1002/mp.12810