Purpose: Upright radiotherapy can lead to improved treatments for lung cancer and more widespread adoption of proton therapy. However, as CT/MRI scanners are orientated horizontally, treatment planning for upright radiotherapy relies on bespoke vertical imaging systems. The purpose of this work was to enable upright radiotherapy by mathematically deforming horizontally acquired images to represent upright anatomy.
Methods: Horizontal (supine) and upright (seated) volunteer lung images were acquired on a 1T prototype open-bore MRI-Linac. Free-breathing MR slices were acquired with using gradient echo sequences (TE/TR=10/124 ms) in the coronal plane. Both volumes were corrected for gradient non-linearity distortion and rigidly aligned. MIM was then used to contour and register the horizontal images to the upright images using the contour-based hybrid deformable image registration (DIR) algorithm. From the DIR output, a novel deformation algorithm was applied to the horizontal images to generate new images that represented upright anatomy. The horizontal to upright transformation was quantified by (1) magnitude of deformation, (2) the structural similarity index measure (SSIM) and (3) the Dice similarity coefficient (DSC) of lung volumes.
Results: The average deformation from horizontal to upright anatomy for lung voxels was 1.0±6.2mm, 4.4±9.9mm and -3.9±7.2mm in the LR, SI and AP directions, respectively. Compared to acquired upright images, the generated upright images had a SSIM=0.36, while a repeated upright acquisition had a SSIM=0.46 and a horizontal acquisition had a SSIM=0.30. Lung volumes on the generated upright images had a DSC=0.86 with lung volumes on acquired upright images. This compared to a DSC=0.90 for repeated upright acquisitions and a DSC=0.76 with horizontal images.
Conclusion: This work is the first to demonstrate the feasibility of using DIR to represent upright anatomy from horizontal images. This technique could be used to create upright treatment plans from horizontal CT/MR images and enable upright radiotherapy treatments.
Funding Support, Disclosures, and Conflict of Interest: The authors acknowledge the financial support of the NHMRC Program Grant and Cancer Council NSW Program Grant. PL and DW acknowledge funding from Cancer Institute NSW Early Career Fellowships.