ESTRO 2024 - Abstract Book

S919

Clinical - CNS

ESTRO 2024

2533

Digital Poster

Evaluation of set-up uncertainty in Radiosurgery using ExacTrac for validation of PTV margins

Manoj Tayal, Rashmi Shukla, Kartikeshwar Patro, Mohini Gupta, Akshay Kashyap, Deepak Kumar Arora

Max Superspeciality Hospital, Radiation Oncology, New Delhi, India

Purpose/Objective:

The aim of this study is to explore the inter/intrafraction geometric uncertainties associated with stereotactic radiosurgery (SRS) on a linear accelerator quipped with ExacTrac system (Brainlab AG, Munich). The tolerance for treatment setup errors and margins for PTV were evaluated.

Material/Methods:

In this retrospective study imaging data was evaluated for 34 patients treated with SRS. All the patient’s pretreatment setup was verified with the help of ExacTrac system (ET) equipped with two sets of orthovoltage X-Ray units and flat panel detectors capable of acquiring semi-orthogonal image and delta variation. The calculated shifts were applied in 6 dimension (3 translational and 3 rotational) with the help of Truebeam 6-DoF (degree of freedom) ‘Perfect - Pitch’ couch -top (Varian Inc). Patient position verification was performed by using bony landmarks as visualized on ET and shifts were applied as per ET 6D-fusion software. A verification Cone Beam computed tomography (CBCT) was acquired and evaluated volumetrically. If the shifts in the CBCT are within the limits (0.5mm, 0.3 deg as per our institutional protocol), then SRS started as per plan. Subsequent verification imaging was performed on ET at particular angle/time/MU for each beam irrespective of couch angle. The data values in setup and verification imaging were analyzed in all three translational directions for mean and standard deviation. Same were further evaluated to validate ourinstitutional ET imaging tolerance and PTV (planning target volume) margins. The setup shifts were also analyzed to determine the intercorrelation between all the translational shifts.

Results:

The average shift in all translational directions Lateral, Longitudinal, and Vertical are -0.10±0.05 mm, 0.08±0.05 mm, and -0.17±0.07 mm respectively. All shift values were arranged in normal distribution, with 95% of confidence level the Lateral, Longitudinal and Vertical shifts values are 0.46 mm, 0.45 mm and 0.51 mm respectively. And for 98% of confidence level the shift values for Lateral, Longitudinal and Vertical are 0.57mm, 0.55 mm and 0.66 mm respectively which are more than our tolerance limit. These values justify the 0.5mm Imaging tolerance in all translational direction kept in patient settings with 95% of confidence level also our institutional PTV margin of 1 mm all around. The setup shifts were analyzed to determine the inter-correlation between all the translational shifts and found to have no correlation.

Conclusion: