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Session Item

Friday
May 07
14:15 - 15:30
21st century brachytherapy: is it available, affordable and relevant?
There are several reports of declining use of brachytherapy worldwide despite its well-documented advantages. Brachytherapy is often perceived to be resource-intensive and expensive and its role in the era of increasingly sophisticated external beam radiotherapy techniques has been questioned. This session discusses some of the issues that influence availability and utilisation of brachytherapy. Topics include current status of cervix cancer brachytherapy availability worldwide and global initiatives to improve access, socio-economic factors impacting brachytherapy treatment trends in the United States, a literature review of time activity based costing studies of brachytherapy, and brachytherapy training and interest amongst European radiation oncology residents.
Symposium
00:00 - 00:00
Are the universal limits recommended by TG218 suitable for VMAT QA regardless of technical solution?
PO-1392

Abstract

Are the universal limits recommended by TG218 suitable for VMAT QA regardless of technical solution?
Authors: DELLA GALA|, Giuseppe(1)*[giuseppe.dellagala@gmail.com];Russo|, Serenella(2);Esposito|, Marco(2);Pini|, Silvia(2);Ghirelli|, Alessandro(2);Ghafour|, Hawbir Omer(3);Zatelli|, Giovanna(2);
(1)Università degli Studi di Firenze, Specialization School in Medical Physics, Firenze, Italy;(2)Azienda USL Toscana Centro, Medical Physics Unit, Firenze, Italy;(3)Zhianawa Cancer Center, Sulaymani directorate of health, Sulaymaniyha, Iraq;
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Purpose or Objective

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.

Material and Methods

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.



Results

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.

Conclusion

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.