ESTRO 2020 Abstract book

S789 ESTRO 2020

dimensional conformal radiotherapy (3DCRT), field in field, intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) was applied to whether the bolus was used and whether the artifact was corrected. Difference at each point between measured dose and calculated dose was analyzed to determine their association with radiotherapy techniques. Results The effect of metal port on radiation dose was related to the treatment technique ( p = 0.018); dose difference between measured and calculated occurred in 6.8% (191.6 cGy vs. 179.5 cGy) of 3DCRT with bolus and artifact correction followed by 5.8% (190.2 cGy vs. 179.8 cGy) of VMAT with artifact correction. IMRT with the bolus showed a minimum difference of 0.3% (180.7 cGy vs. 181.3 cGy). There were no significant dose differences at the points except skin surface and metal port location. Conclusion The metal port within TTE resulted in increased dose difference to breast during radiotherapy, but absolute dose difference was minimal. Future analyses need assess whether this dose difference could affect clinical outcomes. PO‐1396 Dosimetric comparison of AAA and Acuros XB algorithm for breast DIBH VMAT treatment S. Deshpande 1 , S. Naidu 1 , K.N. Chavan 1 , V. Kannan 1 1 P.D. Hinduja National Hospital, oncology, Mumbai, India Purpose or Objective Radiotherapy of chest wall and nodes involves complex anatomy and many times VMAT is preferred to get uniform dose distribution and to avoid junction problem. In order to limit the radiation dose to the heart respiratory gating technique such as deep inspiration breath hold (DIBH) is used. The lung density may substantially decrease when utilizing DIBH We investigated the impact of change in lung density on dose distribution of VMAT plans. In the study, calculations performed with deterministic particle transport algorithm Acuros XB are evaluated against the model based Anisotropic Analytical Algorithm (AAA) Material and Methods CT data from 10 patients presenting left sided post mastectomy patients involving regional nodes were selected for the study. For all patients two scanning acquisition sets were available: the first patient with normal breath (free breathing, FB), the second obtained by scanning patients under deep inspiration breath hold. Gating and breath tracking during scanning were determined by means of the Respiratory Gating RPM system (Varian Medical System, Palo Alto, CA); adjacent slices with 2.5 mm thickness were acquired on a GE CT scanner. The lung HU was determined for each CT scan as the average lung HU in a two dimensional rectangular region of interest (ROI) in axial plane DIBH CT scan set was used for VMAT plans for all the patients. VMAT plans were generated with four partial arcs using Eclipse v13.6 with 6MV energy and calculated using AAA algorithm. Same plans were recalculated using Acuros XB algorithm. Two sets of plan were compared using PTV- Eval (5mm crop from body) for target coverage (V95%), Homogeneity index (HI). For ipsilateral lung V20Gy, Mean dose and V5Gy and heart mean dose were compared Results Lung HU variations between -796 and -861 and between - 680 and -750 were observed for DIBH CT scan set and FB CT scan set respectively. PTV- Eval dose coverage was less by average 3% and average HI index increased by 12% (Std dev 4.7, P value 0.027) when plans were calculated using Acuros XB algorithm The V20Gy and mean lung dose deviated by less than 2% for both algorithms. Larger difference was observed for low lung dose V5Gy, the correction based AAA algorithm overestimated V5Gy.

Commercial software packages that make use of the portal imager to automatically analyze such tests do exist. Unfortunately, they do not fully address our needs. We have therefore developed a solution that is a) complete enough to perform all current mechanical machine QA tests, b) versatile enough to add possible future tests, c) intuitive enough to allow easy (geometric and dosimetric) interpretation of the test results with respect to clinical tolerances and d) independent of the practitioner. Material and Methods The QA procedure was developed on a Varian TrueBeam. In order to limit the amount of extra equipment required, we have made use of the MPC phantom, standard included with all TrueBeams. We only added three small accessories to allow a comprehensive mechanical QA program: an IsoBall insert, a tungsten crosshair frame and a crossroad floor board. These should be used in combination with an appropriate series of test plans. A number of tests are inevitably performed by the physicist in the treatment room (cross-hair, light field, lasers, ...), but most consist of irradiating the portal panel with pre- programmed test fields. The obtained precision can be quantified off-line afterwards but a simple visual on-line assessment of the images during acquisition suffices to instantly check if the mechanical precision is within the set tolerance levels. For the routine dosimetric QA, we rely on a 2D array as well as on two simple, yet versatile solid water phantoms (one for photons, one for electrons) and single ion chambers. Dosimetric QA requires setup of reference values by cross-validation to water phantom measurements. Results Our adapted QA program allows us to quickly assess the mechanical precision of the treatment unit and its on- board imaging with high precision. Although the idea sounded simple enough, it was challenging to work out a methodology that systematically assesses one parameter at a time, so that in case of failure the guilty component can be instantly identified. During the actual development process itself, we encountered a number of issues that highlighted the importance of the order in which the test plans are executed to obtain unambiguous results. The quick on-line visual assessment during the image acquisition is enough to assess a pass/fail. Dosimetric QA is now also fast and efficient. Conclusion Our newly implemented treatment unit QA allows us to quickly assess the mechanical precision of the treatment unit and its on-board imaging with high precision through simple, quasi-instantaneous visual analysis of the images during acquisition. Dosimetric QA is now succinct yet complete. PO‐1395 Dosimetric analysis of the effects of a temporary tissue expander on the radiotherapy technique S.H. Park 1 , Y.S. Kim 1 , J. Choi 1 1 Jeju National University Hospital, Department of Radiation Oncology, Jeju, Korea Republic of Purpose or Objective Temporary tissue expander (TTE) has metal port with high density. The aim of this study was to evaluate the dose effect of TTE according to the radiotherapy techniques for The CT images of 3D printed breast phantom with a TTE were acquired for dosimetric analysis. For dose measurement during 180 cGy of radiotherapy, 13 to 15 EBT3 films were attached to the TTE including metal port area. The treatment planning generated three- breast cancer patients. Material and Methods