ESTRO 2024 - Abstract Book

S3726

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

2998

Digital Poster

Optimizing Collimator Angles in Spine SABR: Dosimetric Effects

Marina Gomes 1 , Stuart Williams 2 , Emily Chan 3

1 GenesisCare UK, Medical Physics, Cambridge, United Kingdom. 2 GenesisCare UK, Medical Physics, Portsmouth, United Kingdom. 3 GenesisCare UK, Medical Physics, Southampton, United Kingdom

Purpose/Objective:

The potential of SABR for addressing spinal metastases is highly promising, however, a significant challenge arises from the necessity to safeguard the spinal cord or cauda equina, which frequently lie in close proximity to or intersect with the target region. Usually, treatment adheres to an isotoxic protocol, where the isodose encompassing the target is incrementally raised until the point of reaching the tolerance dose for critical structures such as the spinal cord, thecal sac, or any neighbouring organs [1]. In order to allow for unavoidable underdosing of the PTV in close proximity to the spinal cord, while maintaining consistency in treatment prescription, it is recommended that a volume be created (PTV_Prescribe) that restricts the PTV by spinal cord PRV plus a small margin [2].

This process can lead to significant variations in target dosimetry due to disparities in the quality of treatment plans, potentially affecting the likelihood of successfully managing the tumour.

This study aims to investigate the dosimetric effects of adjusting the collimator angle in the context of stereotactic body radiotherapy (SBRT) for spinal conditions.

Material/Methods:

In this study, we conducted a retrospective analysis involving ten patients who had previously undergone SABR treatment for spinal metastases at GenesisCare. For each patient, we generated eight new treatment plans, each with varying collimator angles. In seven of these plans, the collimator angles were set at 15-degree intervals, ranging from 0° to 90°. An additional plan was created with the collimator set at 8°, which is the default collimator angle used for VMAT plans at our institution. To ensure a fair comparison, all planning parameters were kept consistent, with adjustments only made to the collimator angle. These plans were generated using the Pinnacle treatment planning system (version 14.0, Philips Medical Systems) and employed a configuration of two VMAT coplanar arcs (6MV). Both arcs had the same collimator angle, taking advantage of Pinnacle's simultaneous optimization feature. These treatment plans were designed for delivery on the Elekta Versa HD machine, which is equipped with an Agility 160MLC.

The prescribed dose for all the plans was 27Gy delivered in three fractions. Our evaluation focused on assessing the coverage of the PTV_Prescribe and the number of monitor units (MU) required for each plan.