ESTRO 37 Abstract book

ESTRO 37

S559

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).

The flexible pouch is located inside the patient-specific mold which is solid, therefore, when the liquid radioisotope is injected into the flexible pouch, the pouch is inflated and form shape according to the mold shape. For the treatment, [atient’s head is scanned with a 3D scanner. With this information, solid patient-specific mold is fabricated with a 3D printer. Inside the mold, the thin flexible pouch is located and connected with a tube for injecting the liquid radioisotope. When injecting the liquid radioisotope through the tube from the liquid radioisotope tank to the flexible pouch inside the solid mold, the pouch fills inside the mold. The liquid radioisotope stays in the pouch to deliver prescription doses to the scalp. To examine which liquid radioisotope would be appropriate for this system, we performed Monte Carlo simulation using GEANT4 version 10.3 patch- 2. The physics list of the simulation was QGSP_BIC_LIV. The cutoff range was set to 0.001 mm. The energy deposition was save to each voxel of which dimension was 0.2 mm 0.2 mm 0.2 mm. Liquid radioisotopes of P- 32, Sr-89 and Y-90 were tested in this study. The water- equivalent rectangular-shaped phantoms (target volumes) were covered with each of the liquid isotopes (thickness of 10 mm) contained in the pouch. After evaluating the percent depth doses (PDDs) in the phantoms by each liquid radioisotope, we selected the best appropriate liquid radioisotope for the suggested system. After that, we acquired PDDs of the selected radioisotope varying the thicknesses from 1 mm to 5 mm with spherical phantoms with radii of 77 mm (77Sph_phantom) and 91 mm (91Sph_phantom). Results The PDD of the Y-90 at the depth of 2 mm in the phantom was 1.97 times higher than that of the P-32 and 2.63 times higher than that of the Sr-89. The PDD of the Y-90 was less than 1% at the depth of 6 mm. The PDD values of the Y-90 in the 77Sph_phantom and the 91Sph_phantom at the depth of 2 mm were 17.4% and 16.9%, respectively. As increasing the thickness of the Y-90 from 1 mm to 5 mm, the dose rates at the depth of 2 mm increased from 0.0015 Gy/s to 0.0027 Gy/s (Fig. 2).

No noticeable differences in the dose rates were observed between the thickness of 4 mm and 5 mm of Y- 90.