ESTRO 36 Abstract Book

S794 ESTRO 36 _______________________________________________________________________________________________

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 Dosimetric effects induced by leaf details were more pronounced for FFF modalities. Due to the leaf tongue- and-groove, abutments of dose profiles using the leaf borders led to underdosages up to 13.7% (6MV), 12.3% (10MV), 15.5% (6MV-FFF), 14.4% (10MV-FFF), with respect to the open field profile (Fig.1, only 6MV and 6FFF are shown). On the other hand, abutments using the rounded leaf tips caused a dose increment up to 8.5% (6MV), 10.6% (10MV), 9.6% (6MV-FFF), 14.0% (10MV-FFF), with respect to the open field profile (Fig.2, only 6MV and 6FFF are shown). MLC-transmission at central axis was 1.2% (6MV), 1.4% (10MV), 1.0% (6FFF), 1.2% (10FFF). Same values were found in case of leaf interdigitation.

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:

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.

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.