ESTRO 36 Abstract Book

S1000 ESTRO 36 _______________________________________________________________________________________________

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. breast radiotherapy. Material and Methods

Conclusion Both the 6-field IMRT and the VMAT technique can be used to spare the humeral head and surrounding tissues as aimed for in the ESTRO guidelines while still achieving proper target coverage. The IMRT technique discussed in our study resulted in a lower dose in the OARs and consequently this technique has been implemented in our institute. EP-1825 Evaluation for the usability of the Varian Standard Couch modeling using Treatment Planning System Y.M. Yang 1 , Y.M. Song 1 , J.M. Kim 1 , J.M. Choi 1 , B.K. Choi 1 1 Samsung Medical Center, Radiation Oncology, seoul, Korea Republic of Purpose or Objective When a radiation treatment, there is an attenuation by Carbon Fiber Couch. In this study, we tried to evaluate the usability of the Varian Standard Couch(VSC) by modeling with Treatment 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 Planning System (TPS) Material and Methods