ESTRO 36 Abstract Book

S426 ESTRO 36 2017 _______________________________________________________________________________________________

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.

dosimetric end-to-end procedures for protons based on customized anthropomorphic phantoms and different

dosimetric techniques. Material and Methods

A homogeneous polystyrene phantom and two anthropomorphic phantoms (pelvis and head phantom) have been customized to allocate different detectors such as radiochromic films, ionization chambers and alanine pellets. During testing, the phantoms were moving through the workflow as real patients to simulate the entire clinical procedure. The CT scans were acquired with pre-defined scan protocols used at MA for cranial and pelvic treatments. All treatment planning steps were performed with RayStation v5.0.2 treatment planning system (TPS). A physical dose of 10 Gy was planned to clinically shaped target volumes in order to achieve uniformity better than 0.5% on the dose delivered to the alanine pellets. In the treatment room the plans were delivered to the phantoms loaded either with alanine pellets and radiochromic EBT3 films (figure 1) or two Farmer chambers. The alanine pellets (5.0 mm diameter and 2.3 mm thickness) and their read-out were provided by the National Physical Laboratory (NPL). One of the challenges of alanine for dosimetry in particle beams is the known response dependency (quenching) on the charge, the fluence and the energy of the particles constituting the mixed radiation field. Corrections for this were derived by a Monte Carlo dose calculation platform implemented in a non-clinical version of RayStation.

Results The measured absolute dose to water obtained with the Farmer chamber in all delivered plans was within 2% of the TPS calculated dose. A lateral 2D homogeneity of 3% inside the treatment field was measured with EBT films. Doses determined with the alanine pellets after correction for the quenching effect showed a mean deviation within 3% and a maximum deviation below 7% in the homogeneous and anthropomorphic phantoms. Conclusion The end-to-end test procedures developed at MedAustron showed that the entire chain of radiation treatment works efficiently and with accurate dosimetric results. Our experience shows that alanine pellets are suitable detectors for dosimetry audits and developed procedures can be used to support implementation of scanning beam delivery technology in clinical practice .

Figure 1 Figure 2