ESTRO 2021 Abstract Book

S1391

ESTRO 2021

Conclusion In conclusion, we found that no ion can maximize all image quality metrics simultaneously. Low energy/low charge ions produce the highest SNR for a low dose at a high spatial resolution cost. On the other hand, high energy/high charge ions produce the best spatial resolution for a higher dose. Finally, this model can inform the user on the optimal choice of ion species for imaging based on the application need. PO-1668 Impact of immobilization devices on planning target volume for pelvic radiotherapy patients P. Agarwal 1 , N. Upadhyay 1 , S.S. Nanda 1 , L. Pujari 1 , R. Shrivastava 1 , A. GV 1 , A. Mukherji 1 , S. Pradhan 1 1 Homi Bhabha Cancer Hospital, Radiation Oncology, Varanasi, India Purpose or Objective With the advanced technologies, Setup error analysis plays an important role in patient treatment accuracy for a new established department. Immobilization devices is a key to reduce the inter-fractional setup error. However, the patient comfort remains pivotal for accurate set up and reducing random errors while using immobilization devises. Therefore, in order to reduce the setup error, and balancing the immobilization devices, we have compared the setup margins, in terms of systematic and random errors, for pelvic radiotherapy patients with immobilization devices, with semi immobilization devices and without immobilization along with clinical target volume (CTV) to planning target volume (PTV) margin. Materials and Methods This is a retrospective study performed in a tertiary care center, all the patients were divided into three groups, first the patients were simulated without immobilization devices (Group A) using only knee rest and U-