ESTRO 2020 Abstract book

S823 ESTRO 2020

PO-1451 Robotic MLC-based plans: a study of modulation complexity L. Masi 1 , R. Doro 1 , S. Calusi 1 , I. Bonucci 2 , S. Cipressi 2 , V. Di Cataldo 2 , G. Francolini 2 , L. Livi 3 1 IFCA, Medical Physics, Firenze, Italy ; 2 IFCA, Radiation Oncology, Firenze, Italy ; 3 University of Florence, Clinical and Experimental Biomedical Sciences "Mario Serio", Firenze, Italy Purpose or Objective Analysis of modulation complexity has never been performed for Robotic MLC-based plans. In this study several complexity metrics, mostly used for IMRT and VMAT plans, were computed adapting definitions to CyberKnife (CK) plans. The purposes were i) to compare the complexity of plans by two optimization algorithms, ii) to analyse relationships between metrics and iii) correlations between metrics and patient-specific quality assurance (PSQA) results. Material and Methods An in-house program was developed in R to compute 5 metrics for CK MLC plans exported in xml format: Modulation Complexity Score (MCS), Edge Metrics (EM), Plan Modulation (PM), Plan Irregularity (PI) and weighted Leaf Gap (LG). MCS and PM definitions were adapted to CK plan characteristics, i.e. several non-coplanar beams and mostly 1-2 segments per beams. For MCS, two different solutions (a-b) were examined. By definition MCS and LG values decrease with increasing modulation, whereas the relation is reversed for EM, PI, and PM. All Modulation indices were computed over a total of 80 clinically acceptable plans, created for Liver, Pancreas, Prostate and Spine SBRT. Among these plans, 24 Sequential and 24 VOLO plans, created for the same cases using the same clinical protocols, were compared in terms of modulation complexity. Pearson’s r was used to explore dependencies between modulation indices over the totality of VOLO plans (56). Relationships of each metric with PSQA gamma passing rates for 32 plans were also analysed. Correlation was regarded as weak for absolute r values in the range 0.2 - 0.39, moderate 0.4 - 0.59, strong 0.6 -0.79 and very strong 0.8-1. Results Average beam number per plan and segment number per beam were respectively 44.0 and 1.68. Mean values and standard deviations of modulation metrics over all treatment sites and for both optimizers are shown in table1. When compared to VOLO, Sequential plans exhibited a higher complexity showing lower MCS(a,b) and LG values and higher EM, PM and PI. Differences were significant for 4/5 metrics (Wilcoxon p<0.01). Among treatment sites, the lowest modulation degree was found for liver plans whereas the highest complexity was found for spinal plans. A very strong significant negative correlation (p<0.01) was observed between MCS(a,b) and PM, as well as between EM and LG (table2). (2%,1mm) gamma pass-rate (range 83%-99.1%) did not correlate significantly with any metrics and Pearson’s r absolute values were below 0.4.

the prescribed dose (PD) and the D95% ≥ 95% of PD. Time required by EZFluence to generate the fluences plus final dose calculation in the Eclipse was also registered for each case. For each FiF plan, a new sliding window plan (EZF plan) was generated in the Eclipse after importing the EFZ- based optimal fluences. For each patient, the original FiF plan was compared (paired two-tailed Student's t-test) to the corresponding EZF plan. The following dosimetric parameters were compared: D98%, D2% and homogeneity index (HI) for the planning target volume (PTV); V16 and V4 of the ipsilateral lung; V5 and mean dose (Dmean) of the heart; and maximum dose at the whole patient's volume (Dmax). Dx% is the minimum dose at the x% of volume, and Vx is the volume receiving at least x Gy. In addition, each EZF plan was simulated in the PRIMO Monte Carlo software (v. 0.3.1.1772). PRIMO was validated for dose calculations and dose accuracy within 2.8% was found [Radiat Oncol. 2018 Aug 7;13(1):144] for the same set of simulation parameters that was also used in the present study. The PRIMO-based plan was compared with the original EZF plan using the 3D gamma analysis tool of PRIMO. Gamma analysis was performed for the PTV, the ipsilateral lung and the heart with 5%/2 mm and 3%/2 mm criteria.

Results Table 1 shows that EZF plans were significantly better than FiF plans for any metric. Table 2 shows the Gamma passing rates (GPRs) of the EZF plan verifications using the PRIMO software. GPRs ≥ 90% were obtained for the 5%/2 mm criteria. Mean time required to design and calculate an EZF plan was 2.9 min (SD: 0.3 min), once the tangential fields were added to the patient. Manual FiF plans takes at least 15 min according to our experience. Conclusion 1. EZF plans resulted in better dose distributions than the manual FiF plans. 2. Accuracy of EZF plans was within 5%/2 mm according to a Monte Carlo-based verification. It is more advisable to use 5%/2 mm instead of 3%/2 mm for dose evaluation as the PRIMO software used in this study has an accuracy ~ 3%. Definitively, scripted EZF software allows a clear speed-up of the breast planning process producing clinically acceptable plans. 3.