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The Dosimetric Impacts of Vertebral Implants (pedicle Screw And/or Bone Cement) On Spinal Cord

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B Zhao

B Zhao*, M Zhang , J Yin , X Gao , Department of Radiation Oncology, Peking University First Hospital, Beijing, China


SU-I-GPD-T-387 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The aim is to study the dosimetric impact of vertebral implants on spinal cord.

Methods: An entire lumbar spine specimen was used. The vertebrae were implanted with pedicle screws (PS) and bone cement (BC), and divided into 3 groups: A) PS; B) BC; C) PS combined with BC. The vertebrae were immobilized in water tank for CT simulation, then PTV and spinal cord were contoured. Three treatment plans (5-fields IMRT, 9-fields IMRT and VMAT) were designed with prescription dose of 2Gy, by using different TPS algorithms (MC and AAA). The dose calculation was performed with two image types: 1) no correction; 2) detailed correction (a tool was developed to auto-correct electronic density of artifacts). To verify the accuracy of the planned dose, a semi-conductor in-vivo dose detector was put into the vertebral foramen, at the same position as interest points (before measuring, kV-CBCT was used to assure the consistency of position). The measured doses were compared with the planned doses for different groups, IMRT techniques, image types and algorithms.

Results: The results showed that: 1) planned dose with detailed correction image was more accurate than no correction image (p≤0.05), especially for MC (p≤0.001); 2) planned dose with 9-fields IMRT was more accurate than 5-fields IMRT (p≤0.05), and VMAT was more accurate than 9-fields IMRT (p≤0.05); 3) for the same planning technique and detailed correction image, planned dose with MC was more accurate than AAA (p≤0.05).

Conclusion: To evaluate spinal cord dose accurately for vertebral radiotherapy involving implants, the following three suggestions should be considered: 1) detailed correction of image artifacts; 2) delivering VMAT plan; 3) planning with MC algorithm. Our study also show that the actual delivered dose of spinal cord was higher than the planned dose. So, dose limitation should be more strict to assure spinal cord safety.

Funding Support, Disclosures, and Conflict of Interest: This project was supported by National Natural Science Foundation of China [Grant No. 81502651]. The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

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