Author: Edward A. Opalko, Matthew B. Podgorsak, Dheerendra Prasad 👨🔬
Affiliation: Roswell Park Comprehensive Cancer Center 🌍
Purpose: To design a 3D printed insert for Varian’s SRS Head Phantom that allows for multi-planar film dosimetry and to evaluate its ability in end-to-end testing for Gamma Knife treatments.
Methods: A modeling software was used to 3D print a cylindrical polylactic acid (PLA) insert that fits within Varian’s SRS Phantom. The design includes 18 circular slabs with Vitamin D softgels stacked vertically around four positional rods which allows for gafchromic film to be placed anywhere along the z-axis of the phantom and be positionally identifiable via the softgels. Three multimodality commercial fiducial markers were placed within slabs and used for image co-registration. CT, MRI and CBCT scans were taken of the completed phantom and co-registered. Shot coordinates from 10 previously treated Gamma Knife patients with various disease types were extracted, projected onto the phantom, and irradiated using EBT3 gafchromic film. The measured film was scanned using a flatbed scanner and compared to the plan using the film dosimetry modules within SNC Patient. Gamma analysis with a 2%/2mm criteria was used to evaluate the phantom’s ability to perform dose comparisons. An evaluation of the dosimetric effects of PLA was also done by creating an optical density curve using both solid water and PLA as buildup.
Results:
The commercial fiducial markers were successfully able to co-register images of the phantom between each imaging modality. All 10 patient plans produced gamma passing rates above 94% using a 2%/2mm criteria. Thresholds between 20-30% were used to reduce signal errors caused by hand cutting the gafchromic film. Optical density measurements between the solid water and PLA blocks were equivalent for multiple MU values.
Conclusion: The 3D printed insert demonstrated the ability to perform accurately in all facets of Gamma Knife end-to-end testing and expand upon the capabilities of Varian’s commercial phantom.