Abstract Book

S1053

ESTRO 37

with a more pronounced deviation from the original dose for TI. The mean standard deviations inside the PTV increased e.g. by Δσ TI (PTV)=7.8±4.2% and Δσ CA (PTV)=2.4±0.9% for SP=3.0.

OAR for 97 patients were evaluated. The dose for Oars and for targets obtained for AU and MA plans were calculated and compared. Paired t test was used for statistical analysis of AU and MA plan. Results There is no significant difference in the coverage rate of target between AU and MA Treatment plans. But there are significant differences between AU and MA plan for most of OARs in 97 patients: the average mean dose of OARs generated with AU plan is 270cGy to 1870cGy lower than those generated with MA plan. For OARs for which no significant difference between AU and MA plans is highlighted, the mean dose of OARs in AU plans were at least not higher than MA plans. Conclusion Nasopharyngeal carcinoma IMRT plans made by automatic planning tool meet the clinical requirements for target prescription dose, moreover, the dose of normal tissues was lower, and therefore the quality of radiotherapy planning was significantly enhanced. AU treatment plans were generated with higher quality without any human intervention; the time of dosimetrists can be saved and the influence of factors as the lack of experience on the treatment planning can be avoided. In conclusion, for radiotherapy of nasopharyngeal carcinoma AU plans might accurately replace MA plans. EP-1939 The Impact of Tumor Volume on Dose Fall-Off of Cyberknife With MLC vs. Fixed Collimator in Brain SRS K.S. Park 1 , J.H. Kim 1 , J.Y. Woo 1 , J.G. Baek 1 , M.J. Kim 1 , S.Y. Kim 1 , R.H. Park 1 , T.S. Jeong 1 , D.M. Jung 1 , J.H. Cho 1 1 Yonsei Cancer Center- Yonsei University, Radiation Oncology, Seoul, Korea Republic of Purpose or Objective The sharp dose fall-off is a feature of brain stereotactic radiosurgery (SRS) that deliver high radiation dose to subject with minimal dose to brain normal tissue. The aim of this study is to compare treatment plans using Cyberknife with InCiseTM multi-leaf collimator (CK_M) with treatment plans using Cyberknife with fixed collimator (CK_F) for brain SRS in various tumor volumes. Material and Methods Fifteen patients with brain SRS were planned us ing CK_M and CK_F. Prescription was 24 Gy in 1 fraction covering at least 95% of the target volume. The dose was prescribed at the 70% isodose level. Homogeneity index (HI), Conformity index (CI) and dose gradient index (DGI) were used as dosimetric indices. DGI is the distance from the average radius enclosed by the original prescription isodose to the average radius enclosed by the isodose at 50% of the prescription dose. Results The mean tumor volume was 1.41 cm3 (range, 0.27- 6.19). Overall, comparing CK_M and CK_F, the respective parametric values (mean ± standard deviation) were : same HI (1.44 ± 0.02 vs. 1.44 ± 0.01); similar CI (1.11 ± 0.06 vs. 1.20 ± 0.09, p=0.005); non statistically significant DGI (2.99 ± 0.55 vs. 3.12± 0.88 mm, p=0.206). Comparing CK_M and CK_F by tumor volume, CI (1.15 ± 0.05 vs. 1.26 ± 0.11, p=0.063), DGI (2.60 ± 0.16 vs. 2.37 ± 0.14 mm, p=0.014) for tumor volume less than 0.5 cm3(N=7, mean 0.38 ± 0.07); CI (1.07 ± 0.04 vs. 1.15 ± 0.03, p=0.028), DGI (3.31 ± 0.57 vs. 3.78 ± 0.69 mm, p=0.011) at more than 0.5 cm3(N=8, mean 2.31 ± 1.77) .

Conclusion Dosimetric uncertainties given by inaccurate HU assignment and its conversion to SP are reduced by utilizing carbon based implants instead of pure titanium, which decreases the risk of tumor underdosage. However, due to the remaining metallic component, some uncertainties in dose distributions still remain. Generally, decreasing the fraction of metal does not only improve dosimetric accuracy, but is also considered to be beneficial for RT planning through more accurate contouring. EP-1938 Comparative study of Auto plan and manual plan for nasopharyngeal carcinoma IMRT radiotherapy X. Xin 1 1 Sichuan Cancer Hospital, Radiotherapy center, Chengdu, China Purpose or Objective Auto planning might reduces the manual time required for optimization and could also potentially improve the overall plan quality. This study is aim to demonstrate the statistical comparison of automatic (AU) and manually (MA) generated nasopharyngeal carcinoma intensity modulated radiation therapy (IMRT) plans. Material and Methods The study included in total, 97 nasopharyngeal carcinoma patients, admitted to our hospital during 2015–2016. The patients underwent IMRT treatments. The clinical and delivered plans were performed with Eclipse (Version 11.0) using manual optimization. The same plans were optimized successively in Pinnacle TM3 (version 9.20) treatment planning system using auto plan software package module (provided by MD Anderson Cancer Center) and then exported to MATLAB software. D95, D98 were calculated for the targets and Dmax and Dmean for the OARs, moreover the average doses of each target and