ESTRO 2021 Abstract Book

S1335

ESTRO 2021

Conclusion The MU density has impact on the gamma passing rate of patient-specific QA measurements when the MU density becomes very small. For EPID, gamma passing rate dropped when MU density is lower than 12MU/cm; for Delta4, observed in square fields created using MLC, the gamma passing rate was not modified unless the MU density is below 2MU/cm.In patient-specific QA measurements, special attention needs to paid when MU density is really small.

PO-1613 Validation of a full Monte-Carlo beam source for primary standards in Dose Area Product (DAP).

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PO-1614 Analysis of transit in vivo dosimetry for VMAT H&N treatments: weight loss and shoulders position C. Anson Marcos 1 , N. Jornet 1 , A. Latorre-Mussoll 1 , J. Chimeno 1 , P. Gallego 1 , J. Pérez-Alija 1 , M. Barceló 1 , N.

García Apellaniz 1 , M. Adriá 1 , A. Ruiz 1 , H. Vivancos 1 , P. Carrasco de Fez 1 1 Hospital de la Santa Creu i Sant Pau, Medical Physics, Barcelona, Spain

Purpose or Objective The aim of this study is to present the results of transit in vivo dosimetry for head and neck (H&N) patients treated with VMAT. In particular we study the sensitivity of the system to detect patient weight loss and robustness of different treatment plan strategies to shoulders position. Materials and Methods A number of 97 H&N patients were selected retrospectively for the study. All patients were treated with VMAT technique in a TrueBeam with 3-point thermoplastic mask immobilization. The treatment prescription was SIB