Assessment of Internal and External Surrogates for Lung Stereotactic Body Radiation Therapy

Purpose: To evaluate internal (lung, heart and diaphragm) and external (nine glass marbles) marker motion in correlation with lung tumor motion, to determine potential surrogate for respiratory gating radiation therapy (RGRT) depending on tumor localization, upper lobe (UL) vs lower lobe (LL).Methods: This study included 58 patients (34 male and 24 female) with small lung cancer (≤ 5cm), who underwent Stereotactic Body Radiation Therapy (SBRT). All patients were scanned and contoured in all ten phases (Varian Eclipse 13.7) after four-dimensional computed tomography simulation (4D-CT). The motion of internal markers and external markers were analyzed and correlated with tumor motion. The Pearson correlation coefficient (PCC) was used to evaluate the correlation between internal and external marker motion with tumor motion. Results: The median (range) values of tumor motion were 3.2 (0.6-11.0) vs 8.6 (4.0-24.0) mm in the UL vs LL. The median (range) values of organs motion and PCC comparing UL vs LL were: 2.0 (0.3-9.1) vs 6.0 (2.8-13.9) mm and 0.46 (0.30-0.95) vs 0.79 (0.50-0.94) to the lung; 11.9 (2.5-16.3) vs 12.5 (5.0-22.5) mm and 0.68 (0.11-0.93) vs 0.89 (0.30-0.99) to the diaphragm; and 3.9 (2.5-6.3) vs 7.6 (4.5-8.6) mm and 0.49 (0.20-0.70) vs 0.59 (0.36-0.83) to the heart. The external marker motion and correlation coefficient for UL and LL were 2.5 (0.9-7.4) vs 2.3 (1.0-5.9) mm and 0.54 (0.09-0.96) vs 0.73 (0.27-0.94).Conclusion: Lung and diaphragm motion correlate better with tumor motion than the external marker. Diaphragm motion can be an excellent indicator for treatment based on RGRT.