ESTRO 36 Abstract Book

S784 ESTRO 36 2017 _______________________________________________________________________________________________

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 dose-high gradient region (P2). In our QA program for ion chamber verification, we have established a 3% of deviation in P1 points, and 5% deviation in P2 points. Results We have recalculated the dose using DIAMOND in P1 and P2 points over the definitive patient plan, and also over the definitive plan simulated in the phantom. The results were compared with the TPS values. Furthermore, we have compared this deviation with the deviation between the measurements and the dose calculated in the TPS. The following table shows some results:

As we show in figure 1, differences may appear between phantom and patient results for P2 points in VMAT plans. These differences could be associated with differents algorithms used in the TPS (Monte Carlo) and the Diamond (Clarkson), and their differences in a heterogeneity medium. Although differences exist, we can correlate the results between the new and the old method, over the phantom’s plan and also over the patient’s plan. Even though, we have obtained good results in the global plan analysis, we have seen that it’s possible to obtain big differences in a field. This can be explained because the calculated point may be in a penumbra region, where the uncertainties in the calculation are bigger than the established tolerances. Conclusion We have checked the DIAMOND’s viability to verify IMRT- VMAT plans, also we have calculated tolerances to apply in clinical use. With this new method, we will decrease the time in the verification and also decrease the time between the moment that the plan is calculated and the beginning of the treatment. EP-1485 Dosimetric characterization of an high definition MLC for stereotactic radiotherapy treatments. F. Rosica 1 , F. Bartolucci 1 , C. Fidanza 1 , A. Savini 1 , D. Ciuffetelli 1 , A. Rastelli 1 , G. Orlandi 1 1 USL di Teramo Azienda ASL 4, Medical Physics, Teramo TE, Italy Purpose or Objective High definition multi leaf collimators (MLCs) with reduced leaf width are beneficial for treating small lesions in modern stereotactic radiotherapy. In general, leaves have special design details that may have a strong impact on the delivered dose. The aim of this study was to characterize the dosimetric impact of such details in the Varian HD120 MLC for several beam qualities including flattening-filter-free (FFF) modalities. Material and Methods A set of MLC-collimated fields was irradiated using a Varian TrueBeam STx linear accelerator equipped with the HD120 MLC (beam qualities: 6MV, 10MV, 6MV-FFF, 10MV- FFF). These fields were designed using several abutment configurations (e.g. picket fence) in order to enhance the dosimetric impact of the MLC design details such as tongue-and-groove and rounded leaf tip. Dose profile scans were measured in a motorized water phantom using small detectors (IBA-Razor stereotactic diode and PTW- microDiamond 60019). Dose profiles of the abutted fields were summed and compared with the dose profiles of the corresponding open fields. In addition, average MLC transmission was measured using a Farmer ion chamber (IBA-FC 65-G). Results

To establish new tolerances, we have looked into the 95% confidence intervals for dose deviation, and also we have done a ROC analysis between the new method (DIAMOND) and the old (ion chamber). Even though, we show in our results that 95% confidence intervals are asymmetric, we have chosen our tolerances in a symmetric interval. We believe that this decision will make the analysis more clear and will avoid errors in the future.