ESTRO 2024 - Abstract Book

S5559

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2024

1830

Digital Poster

Evaluation of the use of CBCT template Thorax_Standard for SABR lung IGRT.

Kirsty Muir, Kirsty Farnan

NHS Tayside, Radiotherapy, Dundee, United Kingdom

Purpose/Objective:

In compliance with IR(ME)R 20171 there is a legal duty to keep doses As Low As Reasonably Practical (ALARP) this includes dose from Imaged Guided Radiotherapy (IGRT). Optimising imaging doses, ensuring justification for all verification related imaging and continual evaluation throughout the patient pathway ensures best practice is followed. The thorax region can present a complex picture when utilising cone beam computed tomography (CBCT) for IGRT2. Therefore it is vital that image quality (IQ) is of a level to inform accurate decision making regarding isocentre verification 3. A balance must be achieved with the quality of the CBCT and the ALARP principal. A local evaluation was undertaken to ascertain if the use of a dose optimised Thorax_Standard CBCT template, resulted in an IQ suitable to facilitate accurate decision making by Therapeutic radiographers performing isocentre verification. Locally Thorax_XLarge template is used for CBCT acquisition as standard for SABR lung IGRT. See Figure 1 for current department practice CBCT template dose information.

Material/Methods:

Fifteen patients were identified that were undergoing SABR lung radiotherapy. The fraction 4 or 7 post-treatment CBCT was acquired using Thorax_Standard template and all other fractions were acquired following current department practice. All CBCT’s were captured on a Varian TrueBeam©. A variety of patient position including standard arms up, arms by side with thermoplastic and one arm by side other arm up were included. The maximum computed tomography dose index volume (CTDIvol max) from each patient’s radiotherapy planning CT was noted. Based on previous departmental evaluation of non-SABR thorax CBCT IQ a CTDIvol max (mGy) threshold was determined which indicates the patients who would benefit from using Thorax_Standard CBCT template. The threshold locally is CTDIvol max ≤ 18.0mGy. The CBCT’s were evaluated offline by two competent radiographers. The aspects evaluated included: PTV borders match, Soft Tissue Match, Bony Anatomy Match, Sharpness, Contrast, an overall evaluation of IQ and if above average artefacts were present. The scoring utilised was a five point scale: 1 = Unacceptable, 2 = Substandard, 3 = Acceptable, 4 = Above Average, 5 = Superior.