ESTRO 36 Abstract Book

S986 ESTRO 36 2017 _______________________________________________________________________________________________

Varian Standard Couch(VSC) by modeling with Treatment Planning System (TPS) Material and Methods VSC was scanned by CBCT (Cone Beam Computed Tomography) of the Linac(Clinac IX, VARIAN, USA), following the three conditions of VSC, Side Rail Out Grid(SROG), Side Rail In Grid(SRIG), Side Rail In Out Spine Down Bar(SRIOS). After scan, the data was transferred to TPS and modeled by contouring Side Rail, Side Bar Upper, Side Bar Lower, Spine Down Bar automatically. We scanned the Cheese Phantom(Middelton, USA) using Computed Tomography(Light Speed RT 16, GE, USA) and transfer the data to TPS, and apply VSC modeled previously with TPS to it. Dose was measured at the isocenter of Ion Chamber(A1SL, Standard imaging, USA) in Cheese Phantom using 4 and 10 MV radiation for every 5° gantry angle in a different filed size(3X3cm², 10X10cm²) without any change of MU(=100), and then we compared the calculated dose and measured dose. Also we included dose at the 127° in SRIG to compare the attenuation by Side Bar Upper. Results The density of VSC by CBCT in TPS was 0.9g/cm³, and in the case of Spine Down Bar, it was 0.7g/cm³. The radiation was attenuated by 17.49%, 16.49%, 8.54%, and 7.59% at the Side Rail, Side Bar Upper, Side Bar Lower, and Spine Down Bar. For the accuracy of modeling, calculated dose and measured dose were compared. The average error was 1.13% and the maximum error was 1.98% at the 170°beam crossing the Spine Down Bar. Conclusion To evaluate the usability for the VSC modeled by TPS, the maximum error was 1.98% as a result of compassion between calculated dose and measured dose. We found out that VSC modeling helped expect the dose, so we think that it will be helpful for the more accurate treatment. EP-1826 Analysis of dose distribution with change of the air gap when proton therapy using line scanning S. Seo 1 1 samsung medical center, radiation oncology, Seoul, Korea Republic of Purpose or Objective When proton therapy of cranio-spinal irradiation (with prone position) using line scanning technique, there is a motion of spinal code caused by breathing. According to our clinical experience, we could find out the length of breathing motion is up to 20mm. Because of this motion, the air gap(defined as the distance from nozzle to surface) could be changed. In this study, we are going to find out the target dose distribution in various air gap. Material and Methods Sumitomo proton therapy machine(SHI, JAPAN) and robotic couch(Forte, USA) have been used for this study. CT scans were performed using GE Discovery CT-590 RT. In order to measure the target dose, glass dosimeter(AGC Techno, JAPAN) and atom phantom(Norflok, CIRS, USA) were used. For treatment planning, Ray Station(Ray search ver. 5.0, USA) has been used. Mean and D95 were analyzed shifted points from isocenter in PTV of lower spine which has significant effect from patient’s respiratory motion. The shifted pitch was ±10mm, ±20mm. Results As a result of analysis, D95 dose at each depth are as follows: iso-center: 2347.4(100%), iso-10mm: 2302.3(98.1%), iso+10mm: 2341.9(99.7%), iso-20mm: 2281.4(97.2%), iso+20mm: 2361.7(100.6%). And mean dose at each depth are as follows: iso-center: 2389.2(100%), iso-10mm: 2355.1(98.6%), iso+10mm: 2394.7(100.2%), iso- 20mm: 2335.3(97.7%), iso+20mm: 2415.6(101.1%). Conclusion The main purpose of this study is to confirm that the air gap changes may affect the target dose or not when proton therapy using line scanning. The difference of

prescribed dose due to a change of air gap is only D95: - 2.8% ~ 0.6%, mean: -2.3% to 1.1%. So even though the air gap changes, it dose not affect to target dose. Therefore, line scanning proton therapy can be seen clinically useful. EP-1827 Dosimetric comparison of 3D-CRT, IMRT and VMAT for bilateral breast irradiation H.M. Hung 1 1 Princess Margaret Hospital, Department of Oncology- Radiotherapy Centre, Kowloon, Hong Kong SAR China Purpose or Objective Breast cancer is the most common cancer worldwide amongst females and ranked the 5th cause of cancer death in 2012. There were 1.67 million new breast cancer cases diagnosed, and contributed to more than 25% of the total number of new cases of cancer diagnosed. The incidence rate of female breast cancer in Hong Kong has been tripled in the past twenty years. Some of the patients may suffered from synchronous bilateral breast cancer which involved a more complex radiotherapy planning as both lungs, the heart and a large irradiated volume are involved. The aim of this study was to evaluate the dosimetric difference between three breast irradiation techniques: 3-Dimensional Conformal Radiotherapy (3D- CRT), Intensity-modulated Radiotherapy (IMRT) and Volumetric-modulated Arc Therapy (VMAT) on bilateral 20 breast cancer patients were previously treated with adjuvant radiotherapy, with the prescription of 50Gy over 25 daily fractions to 100% isodose level. 3-dimensional CT- based treatment planning of the three bilateral breast irradiation techniques was performed and all plans were calculated by Acuros External Beam (AXB) algorithm. The cardiac dose, lung dose, conformity, homogeneity, low dose spillage, the overall planning, simulation and treatment time were analyzed and compared. Results IMRT and VMAT significantly improved the conformity to the Planning Target Volume (PTV) and increased the volume of PTV covered with 50Gy. This effect was clearly illustrated in the region of the lateral field edges since it is difficult to achieve in 3D-CRT due to the dose building up effect. The lung volume received 20Gy in IMRT and VMAT were higher than that of 3D-CRT but the result was found to be not significant. The treatment time in terms of number of monitor unit (MU) was found to be significantly lower in 3D-CRT but the time required for setting up the patient to the treatment position was almost doubled to other two techniques. Conclusion IMRT and VMRT are well-accepted techniques and able to provide encouraging dosimetric results for bilateral breast irradiation. VMAT could further reduce the overall treatment time because of the simple setup procedure and the relatively small number of MU. This study suggested that VMAT technique is feasible and is recommended for bilateral breast irradiation. EP-1828 Mean Dose in healthy lung for chest tumors treated with Stereotactic Body Radiation Therapy (SBRT) J.l.Monroy Anton 1 , M. Asensio Martinez 2 , V. Borreda Talon 2 , J. Hernandis Barbera 2 , M. Marco Collado 2 1 Hospital universitario de la ribera, radiation oncology, alzira, spain 2 hospital universitario de la ribera, radiation protection and radiophysics dpt, Alzira, Spain Purpose or Objective Stereotactic body radiotherapy (SBRT) is considered treatment of choice in patients with pulmonary lesions not candidates for surgical treatment. High doses of radiation, both total and per fraction , breast radiotherapy. Material and Methods