Session Item

Although pre-treatment patient-specific QA is the main tool to validate the dose delivery during VMAT treatment, there is still a lack of consensus on using universal tolerance limits recommended by AAPM TG-218. The aim of this work is to determine locally based Control Limits (CL) and Action Limits (AL) and to show how limits can change due to adopted technical solution and anatomical treatment site.

QA measurements of VMAT plans were performed for three anatomical sites with detectors array PTW Octavius1500 (46 Head&Neck, 35 Lung, 32 Breast) and with detectors array PTW Octavius729 (87 Head&Neck, 61 Lung, 26 Breast) inserted in the same PTW Octavius4D phantom. 50 of the total sample of patients were validated with both detector arrays. Measurements were compared with TPS Elekta Monaco 5.1 computed doses via 3D absolute dose γ-analysis by using PTW VeriSoft software 6.1. γ passing rate and γ-mean were calculated under several criteria (3%/3mm, 3%/2mm, 2%/2mm). Both global (90% of maximum dose) and local normalizations were adopted. According to the AAPM Report TG-218, the procedure to set locally based AL (eq. 1, 2) and CL (eq. 4) has been implemented by using the Statistical Process Control (SPC) analysis.

In eq. 1-4,  β=6.0 (suggested value), x-mean and σ² are the process mean and variance respectively (i.e., the results of QA on an initial set of measurements), T is the process target (typically 100% in γ passing rate analysis; T is set equal to x-mean when a target is not defined), mR is the moving range (i.e., the absolute difference between two consecutive QA verifications in time sequence, eq. 4), n is the total number of measurements on which SPC analysis is computed. SPC analysis is represented in an i-chart, showing CL and AL, as well as Central line (i.e., x-mean).

CL and AL of Octavius1500 and Octavius729 were assessed on an initial set of measurements (n=20) for the three sites separately and compared. Performance were compared and statistically significance (p<.05) was assessed with paired t-Test.

Octavius1500 always allowed to obtain better concordance between calculated and measured dose distributions with respect to Octavius729 (p<.001 for all sites). This led SPC analysis to define stricter CL and AL for Octavius1500 detectors array. CL and AL were significantly different among treatment sites, showing for both detectors the strictest CL for Breast treatment and the widest ones for Head&Neck. Process mean for γ-mean evaluation resulted lower than 0.5 for Octavius1500 while always higher for Octavius 729. In example, a comparison between γ(2%/3mm) passing rate i-charts is shown in Fig. 1.

Locally based CL and AL limits are generally lower than TG218 universal tolerance limits. Nevertless, the SPC analysis indicated that the process is under control. CL and AL must be set specifically for technology and treatment site.