Room: AAPM ePoster Library
Purpose: Metal implants are a challenge in radiation therapy and particularly in proton therapy. However, contouring uncertainties, artifact handling and the dosimetric impact of spinal implants to date have not been quantitatively studied for proton SBRT. This study aimed to investigate those uncertainties for spinal titanium alloy implants placed in vertebrae treated with proton beams.
Methods: Titanium alloy rods and screws with known geometry dimensions and mass density were placed in a mini-water tank phantom and scanned using clinical protocols. The contouring uncertainties were studied based on a statistical analysis of the volume delineations by different dosimetrists/physicists on iMAR and iMAR extended CT images using different window levels. Plan delivering 8Gy/fraction with field sizes of 2x2cm² were generated in the treatment planning system(TPS) to irradiate target through the implants. The dosimetry effects were studied by placing multiple-layer EBT3 film beyond the implants.
Results: The contouring uncertainties, as the standard deviation/mean volume, for implants, high- and low-density artifacts were quantified as 15%, 19%, and 18%, respectively. The measurement demonstrated that the scattered proton after the implants can enhance field dose by ~10%; however, TPS analytical algorithm cannot predict the scattered dose accurately. The artifacts and implants delineation uncertainties can introduce a range deviation up to ~6mm, which translates to significant dose errors beyond the implants.
Conclusion: Both the extended and non-extended CT images can provide sufficient quality to contour the implants precisely by selecting the appropriate window level. The contouring and CT override can introduce large errors in the proton range and dose calculation that can be clinically significant for proton spine SBRT. The scattered photons can enhance the field dose by a factor of ~10% beyond the implants.