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Comparison of Three Treatment Delivery Modalities for Spine Stereotactic Body Radiotherapy. Dosimetric Quantification of Target Coverage and Analysis of Critical Structure Doses.
Curtis Wilgenbusch, MS, CMD, Craig Kozarek, CMD
Department of Radiation Oncology, Miami Cancer Institute, Miami, Florida 33176, USA
Introduction
Stereotactic body radiation therapy (SBRT) for the treatment of spinal metastatic lesions has increased because patients are surviving longer due to improved systemic therapy. Typical doses range from 15 to 27 Gy in 1 to 3 fractions. The use of high, ablative doses (>8Gy per fraction) requires a highly conformal radiotherapy plan with rapid dose fall-off because of the close proximity to organs at risk (OARs). Special attention is required to ensure spinal cord and cauda equina maximum critical volume thresholds are not exceeded. The purpose of this study is to dosimetrically compare various delivery technologies for spine (SBRT): Eclipse volumetric modulated arc therapy (VMAT), CyberKnife (CK) MLC, and Radixact helical tomotherapy utilizing dynamic jaws, 2.5cm collimator (HT).
Results
The three modalities created comparable SBRT plans for the metastatic vertebral body patients. As mentioned the target conformity were all normalized to provide 95% coverage. Differences in the median homogeneity showed difference < 7.0% (VMAT = 1.36, HT = 1.44, CK = 1.35). The conformity numbers for each modality were as follows: (VMAT = 1.30, HT = 1.65, CK = 1.3).
Figure 2: Axial and sagittal isodose distributions of a L3 vertebral body SBRT patient using 6MV for VMAT Cyberknife, and Tomotherapy. The prescription isodose line (red) and 50% isodose line (purple) are noted.
Methods & Materials
Data from 5 previously treated patients was used. Three plans, one using each treatment delivery modality/technology were generated for each patient by different dosimetrists and/or physicists. The vertebral body sites for this study were; T3, T5-T6, T11-T12, L1, and L3. All of the plans were normalized so that 95% of the PTV receives at least 100% of the prescribed dose. All planning utilized 6MV X-ray. All patients were planned for 16 Gy in a single fraction, with the primary objectives being: (a) restricting the maximum dose to the spinal cord to <14 Gy (0.035 cc or less); (b) restricting the cauda equina to <14 Gy (5.0 cc or less); (c) restricting the esophagus to <11.9 Gy (5.0 cc). Metrics for comparison used for Target Volumes were the homogeneity index (HI = D0.2cc/ D98%), conformity number (CN = Vptv,piv)2/(Vptv*Vpiv)), dose fall-off (R50 = V50%/Vptv) using 50% of the prescription dose.
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Figure 1: Normalized dose volume histogram for a sample SBRT patient comparing target coverage, organs at risk, and dose fall off.
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Conclusions
While target coverage was maintained, the median dose fall-off (R50) was best for CK and least favorable for HT, (VMAT = 5.06, HT = 6.53, CK = 4.65). The maximum spinal cord, cauda equina and esophagus doses can be met with all of the modalities.