Abstract Book

S1193

ESTRO 37

Material and Methods The OCTAVIUS 4D system is designed to accommodate variable arrays such as Octavius 729, 1500 & 1000 SRS. The SRS 1000 is widely used for smaller lesions in SRS and SABR treatment for its highest spatial resolution in irregular field and the regions in steep dose gradients. However the maximum field size for this detector array is 10x10cm 2 which is not practical for multiple metastases with single isocenter where the two lesions spaced more than 10cm. As an example here, a single isocentre plan (Figure.1) has been produced in Monaco v5.1 treatment planning system (TPS). In conjunction with PTW verisoft merge function, Octavius 1500 array system has been used as a pre- treatment QA for single isocenter stereotactic treatment of multiple metastases. The measurements were performed on Elekta VersaHD linear accelerator equipped with high-definition MLC, using 6MV dynamic conformal arc therapy (DCAT). Furthermore, the accuracy of the results has been verified with EBT3 Gafchromic film. The film is sandwiched between the rotational unit and the polystyrene plate (replacement for detector unit) to fit in the system with reproducible geometry.

Results For analysis, 3D gamma function was accessed with 3mm/3% and 2mm/3% tolerance limits with a low dose threshold of 10%. Dosemap from the TPS was compared with single and merged measurements. In 3mm/3% criteria the percentage of passing points for merged compared to single measurement was 98.0% to 97.1% and for the 2mm/3% criteria the percentage of passing points was 93.5% to 93.3%. Although there is no significant difference in these g function, while evaluating dose levels (80-100%) the 80% of voxels has not met the passing criteria. For merged measurement the overall passing rate of voxels is significantly higher 81.1% to 70.3 % for 85% isodose level for 2mm/3% (Graph.1) and it gives more information about dose gradient. The measured fluence in coronal plane is compared with the film and shows a good agreement with the planned dose distributions.

Conclusion Head and neck mARC treatments yielded clinically acceptable plans, reduced treatment times and a MU mean reduction of 18.7%. Regarding the PTVs, in mARC treatments a statistically significant improvement was registered in the HI whereas a reduction in conformity was observed. D98% showed an increase of approximately 1 Gy in both PTVs. The mARC plans yielded similar or better sparing of OARs than the IMRT plans. The parotid glands showed better results concerning the mean dose and the V30, though statistical significance was not reached. Regarding the spinal cord mean dose, a statistically significant mean reduction of 8.6 Gy was observed. EP-2160 A practical method of undertaking QA for single Isocenter stereotactic treatment of Oligometastases. A. Jaganathan 1 , S. Fazlic 1 , H. D'Souza 1 , H. Weatherburn 1 1 Cancer Centre London, Physics, London, United Kingdom Purpose or Objective In this study we demonstrated a practical method of implementing Accurate QA for single Isocenter stereotactic radiosurgery (SRS) treatment of multiple metastases using an Octavius 1500 array detector. This is an alternate to using SRS 1000 array detector with the Octavius 4D system.

Conclusion The Octavius 1500 array system is good tool for the single Isocentric with multiple metastases pre-treatment verification for the SRS/SABR treatments without compromising the quality of plan evaluation. The single