ESTRO 36 Abstract Book

S431 ESTRO 36 _______________________________________________________________________________________________

PO-0807 Practical advantages of a transmission chamber in relative dosimetry of Brainlab conical applicators B. Tang 1 , J. Li 1 , S. Kang 1 , P. WANG 1 , L.C. Orlandini 1 1 Sichuan Cancer Hospital, Radiation Oncology, Chengdu, China Purpose or Objective The commissioning of a radiosurgery unit requires the acquisition of specific detectors able to characterize the geometry and dosimetry of small fields. The acquisition of the equipment for absolute dosimetry remains the priority for the Hospitals, considering that relative measurements can be performed without a reference chamber using long acquisition time. The commissioning results therefore in a tedious procedure. In this study, a new transmission chamber (Stealth Chamber, IBA Dosimetry) was used as a reference chamber (RC) in relative dosimetry of Brainlab cone applicators. The timing of the practical procedure and dosimetry results with and without the reference chamber, will be analyzed and compared. Material and Methods IBA SFD3G diode detector was used to measure the 6MV photon beam of a Varian Novalis used with Brainlab cone applicators. Inline and crossline profiles at different depths and central axis depths doses (PDDs) were measured with a motorized water phantom (Blue phantom, IBA Dosimetry) and OmniPro v7.4 software for every cone. The measurements were acquired with the transmission reference chamber positioned on the gantry head in a continuous mode and without RC in a step by step mode. The details of the acquisition parameters were reported in Table 1. The total measurements time for each procedure was registered. Table 1 The Acquistion parameters with and without stealth chamber Acquisition parameters/method No reference Stealth Chamber Scan mode step by step continuous Scan speed - 5 mm/s In-scan positioning speed 5 mm/s - Positioning speed 10 mm/s 10 mm/s Acquisition time 5 s - Stabilzation time 1 s 0.08 s Results Profiles at depth 10 cm for 4/15 mm diameter cones and the depth doses acquired with the two procedures (Figure 1&Figure 2) shown a good agreement. The total measurement time registered was 490 seconds for the PDDs acquisition without RC and only 64 seconds when the scan mode change from “step by step” to “continuous” after stealth chamber was in place. The overall measurement time for 4mm diameter was 575 s and 12 s without and with RC respectively, 735 s and 17 s for the 15 mm diameter cone.

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 Figure 2