ESTRO 2021 Abstract Book

S706

ESTRO 2021

A quantitative assessment of cases of a clinically significant advantage of one technique over another in the pairs for given values of planimetric parameters was carried out using Fisher-Freeman-Halton test.

Results All the created plans were satisfying DVH dose constraints for PTV and OAR according QUANTEC and ICRU recommendations. It was revealed that planimetric parameters h max and α correlate with dose difference value to the heart (V 25Gy ), left ventricle of the heart (D mean , V 25Gy ) and LAD-region ( D max , V 30Gy ) in the pairs 3D-CRT/IMRT and 3D-CRT/VMAT (p <0.001) (picture 2).

Picture 2. – Correlation between chest geometry and dose difference value The threshold values for α and h max were determined to be 80° and 5 cm respectively. A quantitative assessment found clinically significant advantage of IMRT and VMAT over 3D-CRT for 3-4 dose constraint criteria when α exceeds 80° and h max is equal to or less than 5 cm. In opposite, for the vast majority of patients with an α equal to or less than 80° and h max value of more than 5 cm, the OAR dose difference value was comparable for all three RT techniques. Conclusion Chest geometry assessment showed to be the useful tool for LSBC RT techniques selection and treatment individualization. PD-0870 Dosimetric comparison of Cranium SRS among VMAT, Tomotherapy and three Cyberknife techniques K.F. Cheng 1 , O.T.H. Lam 1 1 Hong Kong Sanatorium and Hospital, Radiotherapy, Hong Kong, China Purpose or Objective Compare and evaluate the dosimetric characteristics for cranium Stereotactic Radiosurgery (SRS) among Volumetric Modulated Arc Therapy (VMAT), Tomotherapy (Tomo) and Cyberknife (CK) techniques. Materials and Methods In a human phantom image, four spherical targets with the same location but different diameter of 1cm (0.52cc), 1.5cm (1.76cc), 3cm (14.14cc) and 5cm (65.6cc) were created. For each target size, with prescription of 20Gy in 1 fraction, five plans of different techniques, VMAT, Tomo, CK Fixed cone (CK Fixed), CK Iris Variable Aperture Collimator (CK Iris) and CK Multi-leaf Collimator (CK MLC) were created. All the plans were controlled to have at least 98% target volume coverage, Dmax < 25Gy and new conformity index (nCI) < 1.1, excepts 1cm target using VMAT, Tomo and CK-MLC which nCI ranged from 1.5 to 2. Gradient Index (GI**) and the distances of 50% and 70% isodose line (IL) from target edge*** were calculated. Theoretical beam-on time was compared. *nCI = Target Volume x Volume of prescribed isodose / (Target Volume within prescribed isodose) 2 **GI = Volume of 50% prescribed isodose / Volume of prescribed isodose *** The distance of %IL from target edge = 3 √{%IL volume/(4π/3)} – Target diameter/2 Results