Analysis of Intrafractional Organ Motion by Megavoltage Computed Tomography in Patients with Lung Cancer Treated with Image-guided Stereotactic Body Radiotherapy Using Helical Tomotherapy

Aim: To analyze intrafractional organ motion in patients with lung cancer treated with image-guided stereotactic body radiotherapy using helical tomotherapy (SBRT-HT). Patients and Methods: Data from 25 patients with lung cancer who received 50 Gy/5 fractions of SBRT-HT were analyzed. Slow-scan megavoltage computed tomography (MVCT) images were acquired before (pre-MVCT) and after (post-MVCT) each fraction. We analyzed the imaging quality of the 124 post-MVCT images to identify tumor contours using low-density settings. Next we examined tumor contour deviations from the planning target volume (PTV) in post-MVCT images for intrafractional tumor displacement. Results: Image quality was determined as good in 111/124 images from 22 patients (92%). None of the upper lung tumor images were of poor quality (74 images in 15 patients), whereas lower lung tumors yielded 14 poor-quality images Out of the 50 images (3/10 patients). The difference in image quality between upper and lower lung tumors was statistically significant (p<0.01), especially when accompanied by interstitial lung shadows. Deviations in tumor position in post-MVCT images were analyzed in 110 images from 23 patients and revealed 99 images (90%) with tumor contours confined to PTV. In upper lung tumors, 4/74 images in 15 patients (5.4%) showed tumor contour deviations outside PTV. Lower lung tumors showed a higher rate of deviation with 7/36 images in 8 patients (19.4%) showing tumor contour deviations outside PTV (p<0.05). The maximum deviation was 1 mm for upper lung tumors and 2 mm for lower lung tumors. Conclusion: Upper lung tumors are good candidates for MVCT image-guided SBRT-HT. However, lower lung tumors, especially those adjacent to the diaphragm or pleura, can be difficult to assess, warranting precise dose delivery by MVCT image-guided SBRT-HT.