Organ deformation and dose coverage in robotic respiratory-tracking radiotherapy.

Purpose: Respiratory motion presents a significant challenge in stereotactic body radiosurgery. Respiratory tracking that follows the translational movement of the internal fiducials minimizes the uncertainties in dose delivery. However, the effect of deformation, defined as any changes in the body and organs relative to the center of fiducials, remains unanswered. This study investigated this problem and a possible solution.

Methods And Materials: Dose delivery using a robotic respiratory-tracking system was studied with clinical data. Each treatment plan was designed with the computed tomography scan in the end-expiration phase. The planned beams were applied to the computed tomography scan in end-inspiration following the shift of the fiducials. The dose coverage was compared with the initial plan, and the uncertainty due to the deformation was estimated. A necessary margin from the clinical target volume to the planning target volume was determined to account for this and other sources of uncertainty.

Results: We studied 12 lung and 5 upper abdomen lesions. Our results demonstrated that for lung patients with properly implanted fiducials a 3-mm margin is required to compensate for the deformation and a 5-mm margin is required to compensate for all uncertainties. Our results for the upper abdomen tumors were still preliminary but indicated a similar result, although a larger margin might be required.

Conclusion: The effect of body deformation was studied. We found that adequate dose coverage for lung tumors can be ensured with proper fiducial placement and a 5-mm planning target volume margin. This approach is more practical and effective than a recent proposal to combine four-dimensional planning with respiratory tracking.