Novel use of 3D printing for preoperative dose estimation in the first case of GammaTile spine implantation.

Purpose: For a patient who had two previous courses of external beam radiation therapy for rectosigmoid adenocarcinoma and presented with painful, recurrent disease in the sacrum, this study describes the first use of Cs-131 LDR GammaTile therapy outside of the brain and demonstrates a novel use of 3D printing for preoperative dose estimation.

Material and methods: A personalized 3D-printed model of the patient's spine was created using segmented MRI data, differentiating uninvolved bone, tumor, and thecal sac and nerve roots, with a Stratasys J5 MediJet® Printer. This model was used to simulate surgical resection and placement of dummy radioactive sources. A CT scan of the model facilitated preoperative dose calculations, including physical dose using Eclipse planning software and biologically effective dose (BED) using MIM Maestro software. The predicted dose was then compared to the postimplant dosimetry for the actual patient.

Results: For the relevant organ at risk (thecal sac), the max dose (D_0.035cc) was calculated accurately within 8.0% for physical dose and within 10.0% for BED when comparing the dose estimated using our 3D-printed model and the patient's postimplant dosimetry.

Conclusions: 3D printing can be used preoperatively to estimate dose to critical organs at risk for patients receiving surgical resection followed by Cs-131 LDR implantation in the spine and can be especially valuable in the context of reirradiation.