Purpose: Successful stereotactic body radiation therapy (SBRT) requires both accurate patient positioning and adequate motion management. Spine SBRT, although generally considered as treating a rigid target with minimum motion, could nonetheless be affected by breathing rhythms due to changes of tissue intersected in the beam path. This study focuses on dosimetric variation of VMAT SBRT treating spines approximate to diaphragm.
Methods: Spine SBRT plans were generated on images of three previously treated patients who had both 4D-CT and free breathing CT performed in the same session. Two spinal vertebrae parallel to the diaphragm apex were identified as target. A treatment plan of 16Gy in 1 fraction was generated on the free breathing CT, following TG101 constraints for spinal cord and esophagus. The treatment plan was then projected onto each phase of the 4D-CT, and the calculated doses were accumulated through deformable dose composition and considered as the ‘treated’ dose on the free breathing CT. Differences between planned dose and ‘treated’ dose were analyzed.
Results: PTV volumes were between 100.9cc (T6-7) and 126.5cc (T10-11). Diaphragm motion in the superior-inferior direction were between 4.8mm and 5.9mm. Compared with the planned dose, ‘treated’ dose differed -3.3~0.2%, -2.5~0.4% for PTV V100% and CTV V100%, -0.12~0.51Gy for cord D0.03cc, -0.24~0.17Gy for esophagus D0.03cc, and -4.8~9.7cc for lung V5Gy. Global Dmax varied less than 0.15Gy and stayed within PTV. Locally, the largest dose differences were observed in lung and liver where doses increased up to 2.74Gy. Noticeable continuous dose variations approximately coincided with the superior and inferior field boundaries, as well as domes of diaphragm.
Conclusion: Statistic parameters revealed discernable difference between the planned and ‘treated’ dose, although small. Treatment planning should strive to leave rooms for such dosimetric variations. In addition, extent of dose perturbation effect on different planning techniques should be included into analysis.