Room: AAPM ePoster Library
Purpose: Spine metal implants introduce dose uncertainty which need to be minimized in spine SBRT as spinal cord is often abutting lesions and set to dose limits for best target coverage. Accurately accounting this uncertainty is difficult in most dose calculation algorithms. This study investigates the dosimetric effect of metal implants on spine SBRT by using a Monte Carlo (MC) simulation, which is well accepted as the most accurate way for dose calculation, and comparison to clinical commonly-used AAA dose calculation.
Methods: Five spine SBRT patients with titanium (Ti) implant were included. VMAT plans were generated using Varian’s Eclipse following common plan criteria. The Ti-implant was contoured and assigned with associated relative electron density and physical density for dose calculation (AAA, ver.13.6). The same plan was converted to a MC model in our commissioned MC system, MCSIM. Ti-implant was assigned with associated ICRU cross-section data and physical density. A hybrid voxel size was used (target & spinal cord region: 1mm; elsewhere: 2mm) with ECUT = 0.7 MeV, PCUT = 0.01 MV, and a 1-s dose uncertainty <1%.
Results: For PTV coverage, MC shows a coverage with a mean value 1.11±1.51% (range: -0.88% to 3.21%) lower than AAA for V100% (volume at 100% Rx dose), and 0.4±0.44% for V95%, 2.05±3.67% for V110%, and 4.65±2.76% for V120%. For spinal cord maximal dose D(0.035cc), MC has a value with a mean of 0.17±0.61 cGy (range: 0.9 to -0.76 cGy) higher than AAA.
Conclusions: The presence of Ti implant in the spine affects the dose coverage for the target significantly which may not be well accounted in commonly-used AAA algorithm. Its effect on spinal cord dose uncertainty seems minimal as the dose difference between MC vs. AAA is small. Further study would be warranted to include more cases.