ESTRO 36 Abstract Book

S966 ESTRO 36 _______________________________________________________________________________________________

Material and Methods To ensure efficient delivery of the audit service, all modalities relevant to a facility’s clinical practice are measured in a single audit visit. The incorporation of new audit modalities requires a consideration of phantom design suitable for multiple modalities and limitations on facility and ACDS workload. Classification of new modalities and choice of associated cases need to take into consideration the utility for clinical trials. Results The Level III audit is an end-to-end test using a humanoid thorax phantom (CIRS, Norfolk, VA). The custom phantom has a central insert for either conformal modality with two farmer chambers, or for IMRT and VMAT with seven CC13 ion chambers as the primary detectors. The IMRT/VMAT central insert includes a film holder for supplementary measurements. The custom phantom includes removable lungs that are replaced with solid water inserts to investigate the effect of inhomogeneity on IMRT and VMAT deliveries. Figure 1 shows the custom phantom. The CC13 chambers are connected to the TomoTherapy ® TomoElectrometer, an 8 channel reference class electrometer for simultaneous measurement on all chambers for each audit case. The IMRT and VMAT planning cases were designed for addition to the current Level III audit. Clinical plans were prepared based on the AAPM Publication TG119 [1] and adapted for use in the ACDS audit program. Table 1 shows an example of how the audit outcomes are reported. Each modality is scored separately, and assigned a Pass (Optimal Level), Pass (Action Level), or Out of Tolerance outcome. Field trials on the IMRT, VMAT and FFF modalities began in September 2016. In 2017 the new modalities scoring criteria will be finalised and the new modalities will go live, and field trials on a SABR modality are scheduled to begin. Table 1 Example of modality scoring in the ACDS Level III audit.

region of the image, it was possible to find the geometry (i.e., ball center and field outline) in much more detail than just subpixel accuracy. Using a set of at least 8 images with various gantry and collimator angles we could accurately obtain the isocentric accuracy per gantry angle. A consecutive set of 16 images allowed for an analysis giving the distance of the table rotation axis to the collimator rotation axis. We were able to adjust the table position slightly to obtain accuracies necessary for stereotactic application. Results The method was tested, and we found an accuracy (1 SD) of 0.01 mm. Four new Elekta accelerators (Versa HD) were analyzed according the procedure. The main contribution to isocentric inaccuracy for Elekta linacs is the gantry sag. By adjusting the table rotation axis to a position between the collimator rotation axes at gantry 0° and 180°, isocentric accuracy can be optimized. The table presents the results that were obtained. Table: r isoc : the size of the isocenter quantified by the radius of the sphere containing rotation axes when applying several gantry angles at zero table angle. d table- coll : the distance between rotation axis of the table and the rotation axis of the collimator. Linac r isoc (mm) d table-coll (mm) A 0.68 0.20 B 0.53 0.14 C 0.77 0.06 6 0.38 0.01 Figure: an EPID image of a 10x10 cm 2 field and the ball bearing. The field outline and the detected ball are overlayed.

Conclusion With our method it is possible to quickly obtain a measure for isocentric accuracy. In combination with table rotation we achieved accuracies better than 0.9 mm, after adjusting the table. EP-1755 Multi-modality end-to-end audit by the ACDS J. Lye 1 , F. Gibbons 1 , M. Shaw 1 , A. Alves 1 , S. Keehan 1 , I. Williams 1 1 Australian Radiation Protection and Nuclear Safety Agency, Australian Clinical Dosimetry Service, Melbourne- Victoria, Australia Purpose or Objective The Australian Clinical Dosimetry Service (ACDS) has commissioned a custom phantom and audit incorporating conformal, IMRT, VMAT, and FFF modalities. The design covers future inclusion of small field and SABR modalities. The vision of the ACDS is to provide a comprehensive suite of audit modalities covering all common clinical practice, ultimately to ensure patient safety and to improve national dosimetry. The ACDS also aims to provide dosimetric information that can be used domestically and globally in the clinical trial setting.

Figure 1 Images of the custom CIRS phantom for the new ACDS Level III audit, showing the removable lungs and removable central insert.