Dosimetric Effect of Intrafraction Tumor Motion in Lung Stereotactic Body Radiotherapy Using CyberKnife Static Tracking System

PURPOSE: We investigated the dosimetric effect of intrafraction tumor motion in lung stereotactic body radiotherapy using the CyberKnife static tracking system. METHODS: Four-dimensional computed tomography scans of a dynamic thorax phantom were acquired. Two motion ranges, 3 collimator sizes, and 4 treatment starting phases were investigated. Monte Carlo dose distributions were calculated on internal target volume with a treatment-specific setup margin for 6 Gy/1 fraction. Dosimetric effects of intrafractional tumor motion were assessed with Gafchromic films. γ (5%/3 mm), dose differences, and distance to agreement were analyzed. RESULTS: With 30 mm collimator plans, the measured dose passed the criteria γ (5%/3 mm) in all tumor motion ranges. The γ passing rates of the plans using 20 mm or 20+35 mm collimators were much lower than that with 30 mm collimator, especially with the 30 mm tumor motion range. The measured dose of 10 mm tumor motion ranges all passed the 90% criteria of γ (5%/3 mm), the results being much better than those of 30 mm tumor motion ranges, which were below 80%. The results of same delivered plan but treated with different starting phases varies greatly. CONCLUSION: Xsight Lung Tracking technique should be used with caution in lung cancer stereotactic body radiation therapy because the temporal dose variations can be significant.