Abstract

Justhna Motlib¹, Syed Ali Akber Moinuddin², Sairanne Wickers²

¹University College London Hospital, Radiotherapy, London, United Kingdom; ²University College Hospital London, Radiotherapy, London, United Kingdom

The ability to acquire verification images with 360° rotation maximises full use of the Varian Truebeam™ imaging capabilities. However, for breast radiotherapy the risk of gantry collision with the patient, accessory equipment or couch is high due to patient positioning (inclined and arms raised) and the anterio-lateral isocentre placement. This collision risk is limiting the implementation of 3D soft tissue verification (CBCT) at Varian sites.

Our aim was to identify maximum threshold couch parameters to guide isocentre placement for breast radiotherapy (+/- nodal targets), and enable collision-free 360° gantry rotation for optimal image verification.  

Fifteen staff members were positioned on a 5° inclined wingboard with both arms raised. For each volunteer, the longitudinal isocentre was positioned at the level of the suprasternal notch to represent that of a monoisocentric breast technique.

At couch vertical values; 25cm, 22.5cm, 20cm, 17.5cm, 15cm, 12.5cm and 10cm, the maximum threshold values for the couch lateral were recorded, whereby a 360° rotation was possible without collision, maintaining a 1cm clearance.

The maximum couch threshold values for each volunteer were plotted on a graph and standard deviation calculated for the cohort. 

15 staff members were recruited, 11 females and 4 males with a median height of 164cm (152cm-182cm).

At 25cm couch vertical, the gantry was only able to complete a collision-free 360° rotation with a maximum couch lateral of 1.3cm (±1.2), due to collision between the gantry and the couch.

At couch verticals 22.5cm, 20cm and 17.5cm, the maximum couch lateral values achievable were 2.8cm (±0.9), 4.3cm (±0.6) and 5.6cm (±0.6).

At couch vertical 15cm, 12.5cm and 10cm, the maximum couch lateral values achievable were 6.1cm (±1.2), 5.7cm(±1.7cm) and 4.4cm(±1.8cm) due to collision between the gantry and the patient (most commonly, the elbows).

This study has provided data of the collision-free zone for 360° gantry rotation at specific couch vertical and lateral positions for breast cancer radiotherapy on a Varian Truebeam™ linear accelerator. This informs the threshold values for the lateral and vertical isocentre placement to facilitate collision-free 360° verification.