ESTRO 36 Abstract Book

S793 ESTRO 36 _______________________________________________________________________________________________

Material and Methods Five preliminary sensitivity test scenarios were created from a baseline plan modifying each MLC segment by introducing increasing shifts in leaves positions (0.5 mm - 2 mm). The baseline and test plans were delivered to an Octavius 1000SRS array (PTW) as well as to EBT3 films. An average correction was applied to 1000SRS results to account for the response dependence on source-detector- distance (SDD) [O. Blanck et.al. Phys Med 2016]. The same five test plans were delivered a second time to the 1000SRS re-orienting all beams perpendicularly to the array (nominal position) to eliminate SDD and angle dependence. As a second step 40 clinical MLC plans optimized for various treatment sites (liver, spine, prostate) were delivered to the liquid ion chamber array for patient-specific QA using both the clinical beam orientations and the beam “nominal position”. For the latter only a subset of segments(18-21) was selected. Finally, for 15 out of 40 clinical plans a film-based DQA was also performed. All results were analyzed using (2%, 2mm),( 3%, 1mm) and (2%, 1mm) gamma index criteria [O. Blanck et.al. Phys Med 2016]. Results The pass-rate reductions from the baseline,obtained delivering the five test plans, are shown in fig.1 for (3%, 1mm) gamma criteria. The Octavius 1000SRS showed a good sensitivity to simulated delivery errors with pass-rate reductions increasing from 1.7% to a maximum of 43% with increasing leaves shifts (0.5 mm - 2 mm). Similar sensitivity was observed when the beams were re-oriented in the nominal position geometry. The pass-rate reductions observed with films showed a more irregular trend, and the maximum reduction was 16%. The average pass-rates obtained over clinical plans are shown in fig.2, for the three gamma index criteria. The mean values obtained by the 1000 SRS array, using both the clinical and nominal beam geometry, and by film-dosimetry are all above 92%, when using 3%, 1mm criteria. Differences among the mean pass-rates observed for the three measurement modalities were not statistically significant (p> 0.1, t-test)

Conclusion The results confirm that the 1000SRS array is reliable for pre-treatment QA of CyberKnife MLC plans. The test scenarios highlighted a higher sensitivity to small leaves shifts than what observed by film dosimetry. The gamma pass-rates obtained for clinical plans DQA were comparable to film pass-rates. The possibility to use the beam nominal position was validated and can be an alternative to eliminate SDD and angle dependence. EP-1484 Validation of ptw’s diamond as alternative method for the imrt-vmat pretreatment verification A. Niebla Piñero 1 , B.C. Portas Ferradás 1 , P. Rosa Menéndez 1 , J.A. Merino Gestoso 1 , M. Rodríguez Pazos 1 , D. Pardo Calvo 1 , B. Sobral Aspérez 1 1 Hospital Universitario Nuestra Señora de Candelaria, Medical Physics, Santa Cruz de Tenerife, Spain Purpose or Objective The aim of the study is the validation of the software DIAMOND, as alternative method to ion chamber point dose measurements to verify prostate’s IMRT-VMAT plans. For this purpose, we have selected 109 IMRT and 65 VMAT treated plans between the 12/09/2013 to 08/16/2016. We have compared the results using Diamond with the ion chamber results in the pretreatment verification. Using a ROC analysis we have obtained the new tolerances to apply in our QA program. Material and Methods We have selected 109 IMRT step & shoot plans calculated with CMS XiO and 65 VMAT calculated with CMS MONACO. This plans were calculated over the own patient’s CT and also over the I’mRT phantom’s CT used for the point dose verification. These plans were sent to DIAMOND to make the recalculation in two points: One of them in a high dose-low gradient region (P1), and the other in a high