ESTRO 2021 Abstract Book

S786

ESTRO 2021

commercial frameless mask, while improving patient’s comfort compared to the invasive fixed frame. However, the moulding procedure requires improved RTT skills and expertise. Our findings show that SRS can be safely delivered in radiotherapy units without dedicated expensive installations, such as CyberKnife and Gamma Knife, thus reducing patient mobility according to Covid-19 restrictions. PD-0943 Set-up reproducibility in an adult brain tumour cohort using Varian 2d-3d co-registration software S. Moinuddin 1 , S. Petkar 1 , M. Kermali 1 , K. Indramohan 1 , E. Wilson 1 , N. Fersht 1 , M. Kosmin 1 1 University College London Hospital NHS Foundation Trust, Radiotherapy, London, United Kingdom Purpose or Objective Daily Cone Beam CT (CBCT) permits a high degree of reproducibility in patient positioning, but results in increased radiation dose delivery to the patient. In scenarios where patients are planned to normal tissue tolerance, this imaging dose could restrict optimal dosimetric strategies. CBCT should be used with caution in clinical situations, such as paediatric cases, where the risk of secondary malignancy induction is higher or where a minimal target dose strategy is employed. One alternative low imaging dose solution available on the Varian Truebeam platform (v2.7) (Varian Medical Systems, Palo Alto, CA) is the use of 2d-3d kV imaging. This work was designed to evaluate the use of this technique for patient set-up reproducibility. Materials and Methods The first 30 adult brain tumour patients whose daily kV imaging had been assessed with the 2d-3d software and a 6 degree of freedom (DOF) couch correction applied were identified. The online 2d-3d matching data reflected the initial set-up of the patients with this immobilisation. The weekly post-treatment CBCT data were evaluated for the residual positional set-up variation. All data were exported from the ARIA v15.5 Offline Review module (Tools, Statistics and Trends). Results A total of 865 orthogonal kV image pairs were acquired (median per patient: 28, range: 27-33), including 8 (0.9%) that were repeated due to initial set-up outside imaging tolerance. A total of 175 CBCT datasets were acquired (median per patient: 6, range: 5-7). Table 1 shows the population mean, systematic and random errors for all translational and rotational directions for both initial set-up and residual position. Applying the translational data to the Van Herk equation (1), margins for initial setup are: Vertical 3.4mm, Longitudinal 4.4mm and Lateral 3.7mm, and for residual error: Vertical 0.6mm, Longitudinal 1.5mm and Lateral 1.5mm. These data are in line with other published data. (2, 3)

Initial Set-up position

Residual position

Vert. (cm)

Long. (cm)

Lat. (cm)

Pitch (deg. )

Roll (deg. )

Rot. (deg.)

Vert. (cm)

Long. (cm)

Lat. (cm)

Pitch (deg. )

Roll (deg. )

Rot. (deg.)

Population mean (µ) Population Systematic Error (Σ) Population Random Error (σ)

0.00 0.03 -0.03 0.09 0.22 -0.11 -0.01 0.01 -0.01 0.33 0.05 -0.11

0.12 0.15 0.13 0.84 0.83 0.73 0.02 0.05 0.06 0.24 0.15 0.20

0.07 0.09 0.07 0.70 0.85 0.68 0.02 0.04 0.05 0.25 0.18 0.22

Conclusion Daily kV imaging and image assessment with the 2d-3d software and a 6DOF couch correction facilitates a low imaging dose correction protocol for brain patients. Further work needs to be undertaken to determine the differences in co-registration error between kV and CBCT modalities. In addition, set-up error margin reduction may be possible with this approach. However kV imaging cannot provide volumetric information regarding ventricular change, midline shift or optic nerve motion.