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Establishment of Classification and Early Warning Threshold for IMRT Dose Verification in Breast Cancer

X Lin1*, Y Yin2 , Q Qiu3 , (1) Shandong Cancer Hospital, Affiliated to Shandong University, Jinan, 37, (2) Shandong Cancer Hospital Affiliated to Shandong University, Jinan, Shandong, (3) Shandong Cancer Hospital Affiliated to Shandong University, Jinan, Shandong


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: To refine the IMRT QA early warning threshold to find the optimal pass rate threshold for specific intensity modulation verification equipment, and to improve the ability to detect unacceptable plans.

Methods: Fifty-four breast cancer patients with intensity-modulated radiotherapy (IMRT) were selected for pre-treatment dose verification. The gamma passing rates of these plans were measured with different validation devices (Mapcheck and EPID) and compared under different gamma standards (3mm/3%; 3mm/2%; 2mm/3%; 2mm/2%). Dose verification software tools based on anatomic structures are used for dose reconstruction(Mobius3D). Plans with DVH metrics dose difference < 5% or with gamma passing > 90% under 3%/3 mm criteria were accepted. By plotting the receiver operating characteristic (ROC) curve and calculating the Jordan index, the optimal gamma acceptance rate of the two kinds of verification equipment was studied, which were based on the standards of 3mm/3%, 3mm/2%, 2mm/3% and 2mm/2%.

Results: The passing rates of Gamma verified by Mapcheck were (97.60 ±2.30)% , (90.24 ±3.05)%, (91.56 ±2.07)% and (80.21 ±4.17)%; and those verified by EPID were (98.21 1.31) %, (93.66 1.35) %, (93.03 2.90) % and (81.89 2.70) %. The optimal thresholds of different gamma standards (3mm/3%; 3mm/2%; 2mm/3%; 2mm/2%) calculated were 94.72% 91.46 92.15 79.43 (Mapcheck),95.28% 94.12% 94.28% 81.10%(EPID), respectively.

Conclusion: There are statistical differences between the measurement results of different verification devices, so it is necessary to establish the standard gamma threshold for different devices.Compared with the conventional threshold (90%), the sensitivity of the optimal threshold at the baseline of 3%/ 3mm is significantly improved, the detection ability of the potential dose deviation scheme is improved, and a stricter gamma analysis standard (2mm/2%) is meaningful.


Absolute Dosimetry, Quality Control, Quality Assurance


TH- External beam- photons: portal dosimetry

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