Evaluation of dose variation to normal and critical structures for lung hypofractionated stereotactic body radiation therapy

Purpose: To quantify the dose received by normal and critical structures during lung stereotactic body radiation therapy (SBRT) when registered to tumor or bone. Methods and Materials: Sixteen patients with lung cancer receiving a total dose of 50 Gy in 4fractions for lung SBRT were retrospectively studied. Cone-beam computed tomography (CT) was performed for all fractions, and the images obtained were registered with planning CT with respect tosoft tissue for target localization. Isocenter shifts were determined for each fraction from differences between the bony and tumor alignments; doses were then recalculated based on the new isocenters and summed over all 4 fractions to compare against the planned normal and critical tissue dose. The normal and critical structures evaluated were total and ipsilateral lung, spinal cord, and esophagus. The first data collected were isocenter coordinate shifts in all 3 Cartesian coordinates for both tumor andbony alignments. The second were the dose differences to the normal and critical structures fromthe planned and recalculated doses for alignment based on the tumor. Results: The study showed that while the maximum isocenter coordinate shifts in any direction couldbe as much as 1.60 cm, the normal and critical structure dose variations between the original plans and the simulated plans showed almost no change. The mean volume of total lung that receivedat least 20Gy difference for total lung and ipsilateral lung were 0.01% and -0.04%, respectively. For the esophagus, spinal cord, and heart the maximum and mean dose differences were 0.25 Gy and -0.04 Gy, -0.08 Gy and -0.02 Gy, and 0.02 Gy and 0.05 Gy, respectively. Conclusions: Target localization using daily cone-beam CT with soft tissue registration was appropriate for minimizing the dose to the normal and critical structures without the need to re-plan due to the changes in the tumor position. For tumors located close to a critical structure, daily cone-beam CT is recommended to determine the appropriate isocenter shifts.