ESTRO 36 Abstract Book

S427 ESTRO 36 2017 _______________________________________________________________________________________________

different equipment to simulated machine errors and explores the role of different planning approaches to this Material and Methods VMAT plans were generated for a selected patient in Pinnacle 3 at three institutions, as per their local protocol. An automated VMAT plan was also generated by institution 3 using Pinnacle 3 Autoplanning. Simulated machine errors were deliberately introduced to the plans utilising Python. These included collimator (°), MLC field size (mm) and MLC shift (mm) errors of 5, 2, 1, -1, -2 and -5 units. Error- introduced plans were then recalculated and reviewed. The DVH metrics listed in Table 1 were deemed unacceptable if their differences relative to the relevant baseline plan were above the tolerances listed. Plans were considered unacceptable if any one or more of the limits were exceeded. Table 1. DVH metrics and limits. For each error type (i.e. in Collimator, C; MLC shift, S; MLC Field Size, FS), the smallest error plans that were deemed unacceptable were delivered within the given institution; on an Elekta Linac, measured using an Arccheck for institutions 1 and 3, and on a Varian Linac , measured using a Delta4 for institution 2. Gamma analysis was performed in SNC Patient version 6.6 or Delta4 software respectively, utilising a 3%/3mm and 2%/2mm global gamma pass rate (10% isodose threshold with correction off). Before each set of measurements, MLC checks and a complex benchmark patient test were used to ensure the Linacs' performances were within normal range. Results The global 3%/3mm gamma pass is able to detect the majority of unacceptable plans; however some plans with significant errors still pass. Interestingly the error type/s that passed differed at differing institutions (Figure 1).

Conclusion Traditionally, there is no way of applying a reference detector when measuring small fields, especially for SRS Brainlab conical collimators. The lack of reference signal usually requires to acquire more signals in each measured point to suppress the linac output fluctuation, which results into a long measurement procedure. However, by the introduction of stealth chamber,“continuous mode” became available to us which substantially shorten the measurement time while a good agreement between measurements with and without stealth chamber for both PDDs and Profiles was still reached. The use of stealth chamber is a good solution to spare time during small field dosimetry measurements. This aspect is important during the commissioning of the stereotactic unit but it becomes fundamental for the frequently quality control performed. PO-0808 Comparison of multi-institutional QA for VMAT of Nasopharynx with simulated delivery errors D.I. Thwaites 1 , E.M. Pogson 1 , S. Arumugam 2 , C.R. Hansen 3 , M. Currie 4 , S. Blake 1 , N. Roberts 5 , M. Carolan 4 , P. Vial 2 , J. Juresic 2 , C. Ochoa 2 , J. Yakobi 2 , A. Haman 2 , A. Trtovac 2 , T. Al-Harthi 1 , L. Holloway 2 1 University of Sydney, Institute of Medical Physics- School of Physics, Camperdown, Australia 2 Liverpool and Macarthur Cancer Treatment Centres, Medical Physics-Radiation Oncology, Liverpool, Australia 3 Odense University Hospital, Laboratory of Radiation Physics, Odense, Denmark 4 Illawarra Cancer Care Centre, Medical Physics - Radiation Oncology, Wollongong, Australia 5 University of Wollongong, Centre of Medical Radiation Physics, Wollongong, Australia Purpose or Objective Quality assurance of individual treatment plans is often performed using phantom measurement and analysing acceptable delivery accuracy by gamma analysis with a required pass rate. Simplifying a complex treatment plan and measurement into a single number is problematic. This study evaluates the sensitivity of

Figure 1. The smallest error plans (including Collimator (C), MLC shift (S), and MLC Field Size (FS) error) which exceeded global gamma pass rates. Errors detected if the gamma pass rate was < 95% (for 3%/3mm) or <88% (2%/2mm). Plans that passed are illustrated above the red