Abstract Book

S323

ESTRO 37

OC-0612 Dedicated VMAT complexity metrics reduce patient QA workload D. Dechambre 1 , V. Baart 1 , M. Mathot 1 1 C.H.U. - Sart Tilman, Radiotherapy department, Liège, Belgium Purpose or Objective With the growing use of complex treatment delivery methods such as VMAT, physics workload has increased in regards to pre-treatment patient specific QA. In this work, we evaluated the ability of 8 RTPLAN-based metrics to predict robust treatment delivery. Material and Methods VMAT plans of ninety-three consecutive patients treated in our institution were delivered on the ArcCHECK geometry using global 2 %/2 mm (90 % passing points) and 3 %/3 mm (95 %) gamma index criteria. Using patient’s DICOM RTPLAN, accuracy and deliverability metrics were calculated using Python scripting. From Crowe et al (2014) we selected the following aperture-based metrics: MFA (Mean Field Area), MAD (mean Aperture Displacement), SAS (Small Aperture Score: threshold 10 mm), CLS (Closed Leaf Score) and CAS (Cross Axis Score), and from Mc Niven et al (2010) the MCS (Modulation Complexity Score) deliverability metric. In addition, we developed a novel metric based on TPS second dose calculation using a different leaf offset modelling. This metric called LOIC(PTV) (Leaf Offset Impact on Calculation) is defined as the percentage variation of PTV mean dose with respect to the leaf offset parameter in the model (from 0.5 to 0 mm) and aims to quantify the “global narrowness” of MLC aperture. Finally the total MU per Gy delivered was tested as a simple surrogate for beam complexity. Correlation between gamma passing rates (GPR) and metrics values was assessed using Pearson’s r-coefficient. Receiver-operating characteristic (ROC) analysis was performed to determine appropriate complexity threshold values above which a plan should be considered either for re-optimization (high specificity) or exempt from QA measurements (100 % sensitivity). Results Out of 93 VMAT plans, 77 and 41 passed the 3%/3mm and 2%/2mm gamma criteria, respectively. Table 1 shows absolute Pearson’s r coefficients, associated p-value and ROC Area Under the Curve (AUC) for the 8 metrics and 2 gamma criteria.

A LOIC threshold of 1.7 % allowed for the identification of robust delivery with a false positive rate of 6.5 % for a sensitivity of 0.69, while a CAS threshold of 0.602 presented a specificity of 0.91 and a sensitivity of 0.62. With priority given to sensibility (100 %), a LOIC threshold of 1.25 % and a CAS threshold of 0.485 provided no false negative, allowing for a workload reduction of 49 % and 54 %, respectively. Conclusion From the 8 metrics evaluated, LOIC and CAS were the most powerful tools in order to identify overly/sparsely modulated plans before time-consuming QA measurements are performed, allowing to halve the patient QA workload. OC-0613 The ACDS IMRT and VMAT audits: results from a two level approach J. Lye 1 , F. Gibbons 1,2 , M. Shaw 1 , A. Alves 1 , S. Keehan 1,3 , I. Williams 4 1 Australian Radiation Protection and Nuclear Safety Agency, Australian Clinical Dosimetry Service, Melbourne- Victoria, Australia 2 Sunshine Coast University Hospital, Physics, Queensland, Australia 3 PMCC, Physics, Melbourne, Australia 4 Australian Radiation Protection and Nuclear Safety Agency, Medical Radiation Branch, Melbourne- Victoria, Australia Purpose or Objective The ACDS began auditing IMRT and VMAT in 2016. The new modalities were incorporated into both our Level II and Level III audits. The Level II audit uses a 2D array and focuses on isolating commissioning errors in the TPS. The Level III is a full end-to-end test using ion chamber measurements in an anthropomorphic phantom that undergoes CT scanning, treatment planning and image guided treatment delivery. Material and Methods The plans used for the LIII audit are the “C-shape” target volume from AAPM TG119 (with and without lung inhomogeneities) and the “Complex case” with two adjacent target structures and a fully encapsulated low dose region. The dose is measured in a custom anthropomorphic phantom (CIRS) with removable lung inserts using 7 CC13 chambers and an eight channel TomoTherapy ® TomoElectrometer. The Level II plans also use the TG-119 C-shape” target, and add additional diagnostic tests; the Van Esch et al. Radiother Oncol 65 p53 “Chair” test and the nested “four L” test from Elekta commissioning tests. The plans are measured with the PTW Octavius 1500 array in a solid water and lung slab phantom. The array is stationary and measures at a single plane, and is flipped to measure the posterior beams. There have been 40 IMRT/VMAT Level II audits and 24 Level III audits. Results The ACDS level III IMRT and VMAT audits score the absolute dose difference in all six measured points in the target volume and use ± 5 % as the threshold for an out of

A strong correlation (p < 0.001) was observed between gamma passing rates and LOIC, CAS, MCS, SAS and MU/Gy. The highest Pearson’s r absolute value was obtained for LOIC (0.66 and 0.69), followed by CAS (0.53 and 0.57). ROC curves showed the best results for LOIC and CAS metrics versus GPR 3%/3mm, with AUC of 0.92 (Fig.1) and 0.87 respectively.