ESTRO 2022 - Abstract Book

S1316

Abstract book

ESTRO 2022

Conclusion A novel method of assessment of the lung SABR treatment delivery was proposed. Accuracy of the dose delivery was assessed inside and outside of the target. The 5# delivery produced best agreement between TPS calculated and TDLs measured dose, both in target and out of field. The discrepancy between the predicted and measured doses for single fraction deliveries are likely the result of the interplay between the target and the delivery system respective motions. The measurements results gave confidence in TPS beam model and confirmed viability of using TLD beads to test the delivery.

PO-1536 Imaging dose distributions from CyberKnife robotic image guided radiotherapy

P. Archontakis 1 , P. Papagiannis 2 , I. Seimenis 2 , E. Pantelis 2,3

1 National and Kapodistrian University of Athens, Medical Physics Laboratory, Medical School, Athens, Greece; 2 National and Kapodistrian University of Athens, Medical Physics Laboratory , Medical School, Athens, Greece; 3 Iatropolis Clinic, Radiotherapy Department, Athens, Greece Purpose or Objective In frameless radiosurgery, in-room x-ray images and automated software routines are used to register the planned dose distributions to the treated lesions with submillimeter accuracy. These x-ray images are associated with an imaging dose to the patient. The purpose of this work is to calculate the dose distributions from the image guidance x-ray based system of the CyberKnife robotic radiosurgery system (Accuray TM Inc., Sunnyvale, USA). Doses to the eye lenses and thyroid radiosensitive organs are reported for typical intracranial treatments. In addition, beam hardening filters of variable thickness were studied to minimize imaging dose. Materials and Methods The CyberKnife system employs two ceiling mounted x-ray kV tubes (40 - 150 kV) and two in-floor aSi detectors. The imaging field is confined to (17x17) cm 2 at isocenter using trapezoidal collimators in order to cover the active surface of the detectors. The image guidance system of the CyberKnife was modeled using the C++ class library (egs++) of the EGSnrc Monte Carlo software package. The geometrical characteristics of the developed model were based on the information shared by the vendor. The total tube filtration was calculated based on Halve Value Layer (HVL) measurements with the XR solid state detector (IBA Dosimetry, Germany) positioned at the system isocenter. The x-ray spectrums used for sampling the energy of the emitted photons were calculated using the SpekPy software toolkit. Absorbed dose was calculated on digital patient models created using corresponding Computed Tomography (CT) images and appropriate software tools of the EGSnrc package. To minimize the imaging dose, additional simulations using Tin beam hardening filters of variable thickness were studied. Results The absorbed imaging dose distributions in an intracranial CyberKnife application is presented in Figure 1 using nominal imaging parameters of 120 kVp and 10 mAs. The dose to the eye lenses from both x-rays was found equal to 0.8 mGy per acquisition. The dose to the thyroid which is outside the imaging field of view for intracranial applications was found equal to 0.01 mGy per acquisition. When Tin filter was used to harden the imaging photon energy spectrum, the dose was found to decrease reaching up to 20% for the lenses and for Tin filter thickness of 1mm. Conclusion The absorbed dose to the eye lenses from the image guidance in typical CyberKnife intracranial applications (i.e., using 120 kVp and 10 mAs x-ray image acquisition parameters) was found equal to 0.8 mGy per acquisition. The presence of Tin filter was found to decrease the imaging dose to the lenses by up to 20%. However, simulation findings should be verified by corresponding measurements using optimized imaging parameters.