SU‐E‐T‐675: The Effect of Diaphragm Motion to Dose Calculation During Stereotactic Body Radiotherapy (SBRT) Treatment of Spine Lesions Located at the Superior‐Inferior Level of the Diaphragm

Abstract

Purpose: To investigate the effect of respiratory motion on dose calculation during stereotactic body radiotherapy (SBRT) treatment of spine lesions located at the level of diaphragm. Methods: Helical and four‐dimensional (4D) CT data sets of four patients were used for this study. The 4D sets were binned using 10 respiratory phases (0%–90%) and an average CT was reconstructed from the 4D CT sets. The recorded diaphragm motions were 0.8cm, 1.2cm, 1.4cm, and 1.7cm respectively. The helical CT might represent an arbitrary respiratory phase. The 0% and 50 % CT represent the inhale and exhale phases respectively. One vertebral body (PTV) and the thecal sac at the superior‐inferior level of diaphragm were contoured on the average CT. An SBRT plan, using Rapid Arc, was generated for the average CT. The prescribed dose to the PTV was 24Gy in 3 fractions using V24Gy=95%. The plan was next copied to each of the 10 CT sets for dose calculation. The dose‐volume parameters Dmax and D95% for PTV, and Dmax, D0.25cc, D1.2cc for thecal sac at each respiratory phase CT were compared with those obtained from the average CT. ' Results: The variations of dosimetric parameters for each phase of 4D‐CT relative to the average CT demonstrate similar respiratory shapes. For case #1, all phases, the thecal sac Dmax are higher than that of average CT, and the D0.25cc of thecal sac are lower compared to average CT. For case #4, the thecal sac Dmax for all phases are lower compared with average CT. In this case, the Dmax of thecal sac calculated on the helical CT might be underestimated by 7.3%. Conclusion: Planning on helical CT for spine SBRT at the level of diaphragm might introduce deviations to the thecal sac dose. In such cases, the average CT should be used for dose calculation.