Abstract book - ESTRO meets Asia

S115 ESTRO meets Asia 2018

during the optimization. Maximum, minimum and mean dose of target structure and OARs were retrieved from Dose-volume histogram (DVH) from all plans that were used for dosimetric analysis. Results Target coverage in both planning techniques were statistically comparable. Aside from the pharyngeal superior constrictor muscle and esophagus, significant lower mean dose was discovered to PCMs in HybridVMAT (HybridVMAT: 37Gy versus 41Gy in IMRT, p<0.01); On average, HybridVMAT contributed less dose to nPTV_PMCM comparing to IMRTs. Mean dose irradiated to PMCM in HybridVMAT dropped significantly by 13.4% when comparing to IMRT (HybridVMAT: 34Gy versus IMRT: 40Gy, p<0.01); HybridVMAT could even deliver further lower mean dose to nPTV_PICM (HybridVMAT: 30Gy versus IMRT: 38Gy, p<0.01). Similar results were found that the lower mean dose in HybridVMAT planning to the esophageal inlet and larynx were 27Gy and 30.4Gy respectively, indicating the superiority of HybridVMAT in sparing midline swallowing structures. Conclusion In this study, results showed that HybridVMAT offered significant benefit in sparing midline PCMs and DARS while not compromising the target coverage. Further studies should be made to confirm the actual clinical benefits for NPC patients. PO-279 Co -relation of 2D-Gamma passing rate and 3D DVH metrics for patient-specific QA in head and neck. S. Bhasker 1 , S. Sharma 1 , S. Chander 1 , V. Subramani 1 , B. Devnani 1 1 All India Institute of Medical Sciences, Radiotherapy, New Delhi, India Purpose or Objective The purpose of this study is to find correlation between 2D-Gamma passing rate and 3D-dose volume histogram (DVH) metrics for pre-treatment patient-specific quality- assurance in the patients of cancers of head and neck region. Material and Methods A total of 25 head and neck cancer patients, treated with VMAT were selected and planned with Elekta VersaHD linear-accelerator using Monaco (5.11) treatment planning system. 2D-planar dose measurements were performed with IBA-I'matriXX evolution detector-array using My-QA- Patients software. For 2D-Gamma index evaluation 3%/3mm and 2%/2mm criteria was used. 3D-dose measurements were performed using IBA-COMPASS system. For 3D-measurement, Monaco and COMPASS doses were compared by means of percentage dose differences to PTV and organ at risks (OAR). For PTV D 95 , D 2 and D 50 (dose received by 95%, 2% and 50% volume), similarly for OARs (spine & brainstem) D 2 was noted. Correlation coefficient with corresponding two tailed p-value (≤0.05, for statistically-significant) was calculated for 2D-gamma passing rate and percentage dose differences of 3D-DVH based metrics (Monaco calculated versus COMPASS measured). Correlation will be considered weak or strong based on r value ≤0.59 and ≥0.60 respectively. Results 2D-Gamma index (global) passing-rate was 98.2±1.6%, 92.1±5.5% for 3%/3mm and 2%/2mm criteria respectively. Results of percentage dose-differences between Monaco calculated versus COMPASS measured and correlation analysis between 2D-gamma passing rate and 3D-DVH metrics are summarized in table-1.

Conclusion No strong correlation (not statistically significant) was seen between 2D-Gamma-index passing rate and 3D-DVH based pre-treatment quality assurance using COMPASS. References: 1. Stasi M, Bresciani S, Miranti A, Maggio A, Sapino V, Gabriele P. (2012) Pretreatment patient-specific IMRT quality assurance: a correlation study between gamma index and patient clinical dose volume histogram. Med Phys. 12:7626-7634. https://doi.org/10.1118/1.4767763 PO-280 Is the blue-color channel useless for correction of radiochromic film dosimetry? H. Mizuno 1 , I. Sumida 2 , Y. Akino 2 , T. Hamada 1 , K. Ogawa 2 , M. Koizumi 1 1 Osaka University, Department of Medical Physics and Engineering Graduate School of Medicine, Suita, Japan 2 Osaka University, Department of Radiation Oncology Graduate School of M edicine, Suita, Japan Purpose or Objective Purpose: Although the blue-color channel has been reported to be useful for correction of uniformity of the radiochromic films, a few reports did not recommend to use it for EBT2 film. Here, we evaluated the impacts of the color channels used for generation of the dose response curves on the accuracy of the film dosimetry by evaluating the intra-variability of the EBT3 films. Material and Methods Materials and Methods: Sheets of EBT3 films were separated into four sections (section A-D), and dose response curve was obtained for each section. Films were irradiated by 10 MV photon beams with doses up to 400 cGy. For each dose, the four pieces of the film were irradiated simultaneously, and the dose inhomogeneity inside radiation field were corrected by the point doses measured by Farmer-type ionization chamber. Each curve was evaluated by 4 methods; pixel value of red channel (R px ), pixel value of red channel divided by blue channel of pixel value (R/B px ), net optical density of Red channel (R netOD ), and netOD of R/B (R/B netOD) . Results Results: The dose difference of each curve analyzed by R px was less than about 6% ranging within 50 cGy, and less than 2% ranging from 75 to 400 cGy, and that analyzed by R/B px was less than about 6% ranging within 50 cGy, and less than 1% ranging from 75 to 400 cGy. On the other hand, the dose difference of each curve analyzed by R netOD was less than about 4% in all range, and that analyzed by R/B netOD was less than about 2% in all range. In pixel value and netOD methods, the dose difference evaluated by R/B was less value than that red channel. These results suggested that R/B method improve the accuracy of dose response curve by inhomogeneity of EBT3.