ESTRO 2021 Abstract Book

S1600

ESTRO 2021

Conclusion PSQA and MCS results shows that a DIL dose escalation plan is more complex and is subjected to higher delivery errors. However, calculated dose distribution with the reconstructed plan shows minimal difference in dose coverage of DIL lesion. Thus, the dose degradation due to delivery error is clinically insignificant in the DIL boost plan. PO-1880 Dosimetric accuracy of dual isocenter irradiation in magnetic resonance guided radiotherapy system L. Placidi 1 , M. Nardini 1 , D. Cusumano 1 , L. Boldrini 1 , F. Catucci 1 , G. Chiloiro 1 , C. Votta 1 , V. Valentini 1 , L. Indovina 1 1 Fondazione Policlinico Universitario A. Gemelli IRCCS, Radiation Oncology, Rome, Italy Purpose or Objective Due to limited field size of Magnetic Resonance Linear Accelerators (MR-Linac), some treatments could require a dual- isocenter planning approach to achieve a complete target coverage and thus exploit the benefits of the online adaptation. This study evaluates the dosimetric accuracy of the dual-isocenter intensity modulated radiation therapy (IMRT) delivery Dual-isocenter multi leaf collimator (MLC) and couch accuracy tests have been performed to evaluate the delivery accuracy of the system. In particular a homogeneous rectangular-shape (10x30 cm 2 ) IMRT plan has been delivered on a gafchromic film (GAF) to evaluate the field junction. A fixed conformal open square field of known size has additionally been irradiated on each isocenter, in order to evaluate the accuracy of both the MLC and the table shift, computing and comparing the measured and calculated size and the distance between the two fields. A clinical case of splenomegaly treated in our department with a mono-isocenter plan has been retrospectively selected .It has been re-planned with the following combination: employing dual isocenter (Dip), computing the dose related only to the first isocenter (ISO1p), computing the dose related only to the second isocenter (ISO2p) and combining the ISO1p and ISO2p dose distribution (ISO1p+ISO2p). The dual-isocenter plans have been re-calculated and delivered on a 3-dimensional (3D) ArcCHECK phantom and 2-dimensional (2D) films to assess its dosimetric accuracy in terms of gamma analysis, comparing the delivered and the calculated dose distributions. Clinical and planning target volume (CTV and PTV respectively) coverage robustness was then investigated after the introduction of ±2mm and ±5mm positioning errors by shifting the couch. Results MLC and couch accuracy tests confirmed the system accuracy in delivering a dual-isocenter irradiation. homogeneous rectangular-shape plan reports a flatness more than 99.5% while the difference between the measured and the calculated size and distance of the open square fields where less than 0.07 cm and 0.06 cm respectively. 2D/3D gamma analysis results occurred always to be above 95% if considered a gamma criteria 1%/2mm and 1%/1mm respectively for the 2D and 3D analysis. Detailed results are listed in Table 1 and 2. technique for MR-Linac. Materials and Methods