5B02a-Near Simultaneous Computed Tomography Image-Guided

Stereotactic Spinal Radiotherapy

Almon S. Shiu, Ph.D

Associate Professor

Departments of Radiation Physics,

The University of Texas M. D. Anderson Cancer Center,

Houston, TX

The implementation of the treatment setup from an emerging technique using a near-simultaneous computed tomography (CT) image-guided stereotactic radiotherapy (IMSRT) for the treatment of spinal and paraspinal tumors is the focus of this presentation. In addition, the treatment setup accuracy and a comparison of planned and delivered dose distributions using IMSRT will also be discussed.

A targeting system that integrates a CT-on-rails scanner with a linear accelerator (LINAC) was evaluated in the lead-in portion of a Phase I/II protocol for treating patients with paraspinal metastases. Patients were immobilized in the supine position by a moldable body cushion vacuum wrapped with a plastic fixation sheet. Planning CT and immediately repeated CT were performed on the LINAC/CT-on-rails unit to assess respiratory-related vertebral body motion. Coplanar intensity modulated radiotherapy (IMRT) using 7-9 beams was used to deliver 30 Gy in five fractions to the target volume, while limiting the spinal cord dose to <10 Gy. Pretreatment CT scans were fused with the planning CT scans to determine the correct target isocenter by accounting for any translational and roll (axial) rotational discrepancies from the planning CT. The reproducibility of the treatment isocenter as compared with the planned isocenter was measured with digitally reconstructed radiographs (DRRs), portal film imaging, and immediate post-treatment verification with CT scans. Phantom measurements were taken for dose verification for each IMRT plan. An assessment of dose coverage to the clinical target volume (CTV) for each patient was evaluated based on the delivered dose distributions represented on the post-treatment CT scans.

Based on a total of 180 CT scans (fifteen for planning, fifteen for respiration study, 75 pretreatment, and 75 post-treatment) from fifteen patients, no respiration-associated vertebral body motion was seen. A comparison of the corrected daily anterior-posterior (AP) and lateral (LAT) digital portal images with the planning AP and LAT DRRs confirmed that the isocenter setup accuracy for the 90 treatments (3 patients had 2 isocenters) was within 1 mm of the planning isocenter. The results from the immediate post-treatment CT scans reconfirmed the findings from the portal images and verified the absence of spinal movement during the treatment. The ion-chamber measurement for the high-dose region was within 2% of the planning dose for fifteen patient treatment plans. Film dose measurement in an IMRT QA phantom demonstrated good agreement from 90% to 30% isodose lines between the planned and measured results.

The recalculated dose distributions on the post-treatment CT revealed that the CTV received the desired dose coverage as originally planned while meeting the dose constraints to the spinal cord (<10 Gy). On the other hand, the use of orthogonal DRRs alone without taking axial rotation (up to 3°) into account was found that the maximum cord dose is approximate 2 Gy greater than that of original plan, even though the CTV coverage is reasonable. The actual delivered DVH versus the planned DVH will be presented in the meeting.

Preliminary experience suggests that near-simultaneous CT image-guided verification technique can be used as a new platform technology for extracranial applications of stereotactic radiotherapy and radiosurgery to spinal and paraspinal tumors.