Analysis of Intra-fractional Variability of the Internal Target Volume during Stereotactic Radiotherapy in Peripheral Lung Tumors

RapidArc technique was used with 2-4 arcs and a 2 nd CBCT was carried out after the 2 nd arc. Intra-fractional variations were determined for the spinal-column and for the ITV. In addition, intra-fractional variability in ITV position relative to the spinal column was determined. All results were corrected for the finite accuracy of the CBCT registration and couch shifts. Lastly, the sensitivity of the real time monitoring using the ET external body markers for detecting motion of bony structures was investigated. Results: Using a humanoid phantom the inaccuracy in intra-fraction motion measurement due to the combination of CBCT registration and couch corrections inaccuracies was determined to range from 0.8 to 1.2 mm (1 SD) in each direction. The standard deviation (SD) of the intra-fractional drift of the ITV was 1.5, 1.4 and 0.9 mm for the AP, SI and LR direction, respectively. Average time between1 st and 2 nd CBCTwas 12 min.The SD for intra-fraction setup errors of the spinal columnwas1.0 (AP),0.8 (SI) and 0.8 (LR) mm. For the ITV-shift relative to the vertebraethe SDwas 1.1 (AP), 1.1 (SI) and 0.6 (LR) mm. The combinationof all components resulted in an intra-fraction 3D shift of the ITV that was larger than 5 mm in 10% of the fractions. For 25 patients showing a 3D shift of bony anatomy of 3 mm or more, only 3 patients also showed a variation of 2 mm or more in the ET body-marker readings. We found no correlation between the magnitude of the 3D setup error and overall treatment time, PTV size or tumor location. Conclusions: Standard deviations of intrafraction variability of bony structures and ITV relative to spinal column were smaller than 1.1 mm in each direction. Use of image guidance systems with sub-mm accuracy is required for reducing intrafraction variations in order to achieve further reductions in setup margins. This work was performed within a research collaboration with Varian Medical Systems.