Abstract Book

ESTRO 37

S558

correlations between dosimetric results, TPS/linac and MIs.

The aim of this study is to experimentally quantify (a) by how much an increase in the maximum gantry rotational velocity from 6°/s (standard delivery) to 12°/s (fast delivery) reduces the beam-on time, and (b) to verify whether the dosimetric accuracy is still warranted under the modified delivery conditions. Material and Methods A VersaHD linac (Elekta AB, Stockholm, Sweden) was modified by Elekta to allow for higher maximum gantry rotational velocities of 12°/s. Twelve clinically acceptable lung SBRT treatment sequences with D 95% (PTV)= 12 times 5 Gy were delivered. All deliveries were performed with two 220° VMAT arcs on the ipsilateral side of the target volume with 10 MV flattening filter free (FFF) beams. Treatment plans were generated with the Monaco treatment planning system (Elekta AB). Each sequence was delivered once in fast and once in slow mode. During delivery, point-dose measurements were performed with an ionization chamber (PTW 31010, Freiburg, Germany) that was positioned in the isocenter of a cylindrical phantom (T40015, PTW). Beam-on times were recorded for all deliveries. Axial dose distributions of four treatment sequences were measured using GafChromic EBT3 films (ISP Technologies Inc., Wayne, USA). The gamma pass-rate was calculated using a 2%, 2 mm global gamma evaluation criterion. Results Deliveries with the faster gantry lead to a reduction of beam-on times by (21.3±4.2) s or (22.9±5.0)%. The relative dose difference between the two delivery modes was (0.3±0.3)%. The gamma evaluation showed a high agreement of (98.8±1.2)% (ranging from 97.1 % - 99.9 %) between standard and fast delivery. Conclusion The dosimetric accuracy is still warranted for a dose delivery in faster gantry mode. For high modulated lung SBRT VMAT treatments in DIBH, an increased maximum rotational velocity leads to a mean reduction in beam-on times of 21.3 s. This corresponds to two breath-hold phases less. Furthermore, a fast and highly conformal dose delivery presents a mandatory step towards an effective intra-breath-hold combination of imaging and delivery. This leads to real-time adaptive radiation therapy which eliminates the delivery uncertainties of moving targets. PO-1002 Patient-specific brachytherapy with liquid radioisotope using 3D printer: a Monte Carlo study J.M. Park 1 , J.I. Kim 1 , D. Ryu 2 , S. Lee 2 1 Seoul National University Hospital, Radiation Oncology, Seoul, Korea Republic of 2 Seoul National University Graduate School of Convergence Science and Technology, Program in Biomedical Radiation Sciences- Department of Transdisciplinary Studies, Seoul, Korea Republic of Purpose or Objective To develop a remote afterloading patient-specific brachytherapy technique by utilizing liquid radioisotope for the treatment of skin cancer in the scalp lesion. Material and Methods We designed a device which is composed of liquid radioisotope tank, tube, patient-specific mold, and a thin flexible pouch (a nitrile butadiene rubber, thickness = 0.2 mm). The tank, tube, and the flexible pouch are connected one another to constitute a closed loop system to prevent leaks of liquid radioisotope (Fig. 1).

Table1. Participants to the multicentric study: TPS version, linac and MLC characteristic are shown. Results Data from 28 plans were analyzed. Spearman rank correlation coefficients are shown in Figure 1. In detail, MLspeed, MLacc, MGS, MDR shown significant correlations with TPS and linac used. MItot correlated with target volume (larger PTV required higher modulation), QI (better quality needed higher modulation), and mean liver dose. No correlations were obtained between MItot and linac/TPS or any planning parameters.

Fig1. Spearman correlation between linac/TPS, dosimetric parameters and modulation indexes are shown. Boxes without cross have p<0.05. Conclusion MItot is a promising universal modulation index for liver VMAT SBRT because it is independent of TPS and linac. PO-1001 Implementation and experimental verification of a faster gantry rotation in lung SBRT C. Ohlmann 1 , F. Schneider 1 , A. Arns 1 , V. Steil 1 , H. Wertz 1 , F. Wenz 1 , J. Fleckenstein 1 1 Universitätsmedizin Mannheim- Medical Faculty Mannheim- Heidelberg University, Radiation Oncology, Mannheim, Germany Purpose or Objective A radiation therapy treatment of lesions inside the lung requires advanced treatment delivery techniques. A well- established method to mitigate intrafractional movement of pulmonary targets is a treatment in deep inspiration breath-hold (DIBH). If this technique is used for stereotactic body radiation therapy (SBRT) a faster gantry rotation can potentially lead to a twofold reduction of treatment times: the time reduction during beam-on itself, and additionally, a reduction of the number of required breath-holds. This can reduce positioning errors and thus allow for smaller treatment planning margins.