ESTRO 2020 Abstract book

S739 ESTRO 2020

has been used for gamma radiosurgery. Thus, this work seeks to measure gamma radiosurgery collimator output factors using MCD, for comparison with other results that include SCD, Monte-Carlo simulation and manufacturer recommendations. The 8 mm collimator output factor is The spherical ABS GK phantom with film cassette and positioning jig and was positioned at X=Y=Z=100 mm of GK coordinate system for irradiation. EBT 3 film pieces were irradiated (2 mins,16 mm and 8 mm collimators, all 8 eight sectors). For each collimator, RCF was irradiated in the transverse (X-Y) and coronal (X-Z) planes. RCF calibration was done with the same GK unit with all 16 mm collimators and the same spherical phantom. RCF was calibrated by pairing mean optical density (OD) value of a roughly 30 mm 2 ROI to dose values calculated by the nominal GK dose rate of the day (Fig.1). An unexposed film piece was used to scale scan-to-scan variance between the calibration scan and application scan. All films were from a single lot and digitized on an Epson 10000 XL photo scanner 26 hours after irradiation. Films irradiated by 8 mm and 16 mm were digitized in a single scan to eliminate any scan-to- scan variability (Fig. 2). The MCD method was used for dose map generation compensating any film active layer non- uniformity. Film analysis was performed with MATLAB 2018b. Monte Carlo-simulated output factor was obtained using Penelope codes at this institution. studied as an example. Material and Methods

PO-1309 Dosimetric Impact of Respiratory Motion in IMRT for breast cancer: 3D-Printed Dynamic Phantom Study Y.E. Choi 1 , K. Sung 1 , K.S. Dong 1 , H.J. Kim 1 , Y.K. Lee 1 , Y. Lim 2 1 Gachon University Gil Medical Center, Radiation Oncology, Incheon, Korea Republic of ; 2 Gachon University, Radiological Science, Seongnam, Korea Republic of Purpose or Objective This study aims to evaluate the dosimetric impact of respiratory motion in intensity modulated radiation therapy (IMRT) for breast cancer using a 3D-printed dynamic phantom. Material and Methods Computed Tomography (CT) data from a breast cancer patient were converted to create a 3D-printed model of the patient-specific breast phantom, using the PolyLactic Acid (PLA) as filament material. Different infill densities in 3D-printed dynamic phantom were tested to validate their effect on simulating an average human breast tissue density. This phantom and the motion actuator box of dynamic thorax phantom (CIRS, model 008A) was combined to assemble a dynamic phantom. Multiple CT image datasets were acquired for each position demonstrating respiratory amplitude. Various types of treatment planning, such as IMRT (±SIB) and VMAT (±SIB), were generated using CT images acquired in the lowest position of simulated respiratory motion representing the expiration phase. Plans were delivered 3D-printed dynamic phantom in expiration position and free-breathing motion with 5 mm SI movement and 4 second-respiratory cycle. Gafchromic EBT3 film was inserted in the SI direction of breast phantom and employed for dosimetric verification. The resulting dose map of film measurement of expiration phase was compared with that of free- breathing motion and evaluated using gamma analysis with dose tolerance of 3% within 3 mm. Results The gamma passing-rate of dose distribution delivered in the expiration phase and free-breathing motion was 98.3% - 99.4% for IMRT and 97.0%-98.6% for IMRT with SIB. For the VMAT and VMAT with SIB plans, the gamma passing- rate was 97.9%-99.3% and 99.1%-99.7%, respectively. Conclusion This study determined how respiratory motion affects whole breast IMRT with/without SIB. Although the dose map of film measurement with respiratory motion was shown in blurred dose distribution, it can be delivered without a significant increase in CTV dose heterogeneity and loss in target coverage. Our findings demonstrate that the impact of respiration on breast IMRT for whole breast irradiation was not shown to be significant even with SIB to the tumor bed. However, the impact of respiration during breast IMRT using large field size for regional node irradiation should be verified. PO-1310 Gamma Knife Relative Output Factor Measurements with Multichannel Radiochromic Film Dosimetry M. McKee 1 , R. Tong 1 , J.D. Bourland 1 1 Wake Forest University School of Medicine, Department of Radiation Oncology, Winston-Salem, USA Purpose or Objective The measurement of relative output factors in gamma radiosurgery with the Leksell Gamma Knife (GK) is challenging due to small radiation field sizes, with known volume effects arising for various detectors. Radiochromic film (RCF) dosimetry provides a method with minimal volume effect. The dosimetric accuracy of RCF is affected by active layer inhomogeneities, which can be compensated for using a multichannel dosimetry (MCD) method. Typically, single channel RCF dosimetry (SCD)

Results Average ROI dose values of X-Y and X-Z plane for 16 mm collimators are 591.7±2.9 cGy for MCD, 566.4±3.7 cGy for SCD-red, 569.9±5.5 cGy for SCD-green & 544.0±6.6 cGy for SCD-blue. The dose difference from the nominal dose of 585.6 cGy is 1.0% for MCD, -3.2 % for SCD-red, -2.7 % for SCD-green and -7.1% for SCD-blue. The average output factors in the X-Y and X-Z planes for the 8 mm collimator are summarized in Table I. Table I: 8 mm relative output factors generated by various methods.

Conclusion