Session Item

Magnetic Resonance (MR) at 3T is used to define targets for stereotactic radiosurgery (SRS) treatments. Images are fused to Computed Tomography (CT) for treatment planning. The delivery accuracy of the CyberKnife system is <1mm. MR images can suffer spatial distortions >1mm, and can be highly variable according to the local MR equipment, protocols and patient setups. For small targets (<0.5cc), or in multiple brain metastases where accurate fusion across the whole brain is required, it is critical to assess these distortions locally and their impact on dosimetric accuracy for SRS.

A phantom was made using 520 LEGO (Billund, Denmark) bricks with each brick manufactured to a tolerance of 20µm. The resulting cuboid (125mm x 126mm x 160mm) of similar dimensions to a human brain was encased in a custom-made Perspex phantom filled with food-grade mineral oil to give a uniform MR signal at 3T. The cuboid was scanned on a Siemens Skyra 3T MR (Erlangen, Germany) using our standard-of-care SRS planning sequence, and on a 1mm Philips CT. Further MR scans were performed with the centre of the phantom offset in 16mm steps (equivalent to 1 Lego brick width) from the centre of the MR bore in the Sup-Inf plane to maximum offset of 64mm. Each MR dataset was fused to CT using the Accuray Precision radiotherapy planning system. This software was used to assess the displacement of 30 bricks from the true position as defined by CT.

LEGO provided the ability to construct a highly uniform phantom to assess the geometric accuracy of CT-MR fusion across a volume similar to that used for intracranial SRS. When the phantom was correctly aligned to MR isocentre; the accuracy was <1mm centrally and <2mm at slices +/-80mm Sup-Inf. A 32mm Sup-Inf offset from MR isocentre resulted in distortions of <4mm at the edges but remained <1mm centrally. The accuracy in the central region was maintained even when the phantom was offset 64mm from the MR isocentre, but became significantly worse at the phantom edges, with distortions up to 7mm. These edge distortions would be significant, since typical dose gradients of up to 7Gy/mm can be achieved in SRS. For multiple targets, the isodose distribution is rarely spherical; in these cases, along certain planes the dose gradient may only be 2Gy/mm. The scanner’s default distortion correction filters were able to correct the extreme edge distortions to <1 mm, but with a loss of MR resolution.

This study highlights the ability to accurately assess CT-MR fusion accuracy for SRS with a LEGO phantom. For patients with multiple metastasis, additional care should be taken if there are lesions at extremes of the dataset. Good MR isocentric positioning of the patient and the use of distortion correction filters reduce these effects, and enables treatment of all lesions with good dosimetric precision. If patient alignment is not ideal there is a risk of significantly under-dosing peripheral targets due to the steep dose gradients used within SRS.