Sci-Thur PM Therapy-12: Use of patient-specific planning target volumes in conformal radiotherapy of lung cancer: simple estimates of potential dosimetric benefit in tumor motion management

Purpose: To estimate the volume of lung receiving at least 20 Gy (V20) for three tumor‐motion management strategies: population‐based uniform 15 mm margin, free‐breathing individualized margins, and reduced margins for gating with a 0.33 duty cycle. Methods: We have used a simple formula to combine set‐up error with target motion and to individualize planning target volumes (PTV) for four non‐small cell lungcancer patients. Previous measurements of set‐up error and fluoroscopy data synchronized to a spirometer were used as inputs for our calculations. Tumor motion was characterized in terms of the standard deviation of superior‐inferior (sup‐inf) displacements in each fluoroscopyimage. Single‐phase treatment plans were generated such that at least 97% of the PTV was enclosed by the 60 Gy isodose, spinal cord dose was kept below 45 Gy, and V20 was minimized. Results: The table shows the PTV margins in the sup‐inf direction and the corresponding V20 values, as read from the dose‐volume histograms. Conclusion: Reductions in lung V20 and spinal cord dose are possible by use of patient‐specific margins. For more mobile tumors, gating may provide additional gains. Since tumor motion is predominantly in the superior‐inferior direction, uniform margins are not optimal.