Does a Threshold of Tumor Motion Exist to Indicate Respiratory Gating for Proton Therapy?
J Matney*, J Bluett, H Li, L Court, Z Liao, R Mohan, M.D. Anderson Cancer Center, Houston, TXMO-F-WAB-4 Monday 4:30PM - 6:00PM Room: Wabash Ballroom
Purpose: To determine if a population-based threshold of motion exists to indicate respiratory gating in proton therapy by correlating the dosimetric benefit of gating to the magnitude of tumor motion.
Methods: This research estimates the benefit of respiratory gated passively scattered proton therapy for a cohort of stage III lung patients with range of tumor motion from 3 to 17 mm. Respiratory motion was defined as the tumor centroid excursion between inhale and exhale 4DCT phases. To simulate respiratory gating during free-breathing exhale position, the exhale phase of the planning 4DCT was used for re-planning efforts. For the respiratory gated plan, the original beam angles from non-gated plans are maintained while new contours, beam properties, lateral blocks and compensators are redesigned. Both plans are normalized to the same target coverage. The changes in dose-volume histogram (DVH) indices between the gated and non-gated plans are compared against the extent of respiratory motion.
Results: Gated plans for 19 patients have been completed to date. The only DVH index that correlated (Spearman rank correlation p<0.05) to respiratory motion was the mean lung dose. However, a regression line fit to this data yielded low R-squared value of 0.19. No correlations between motion and dose reduction were found for other DVH indices in the lung, esophagus, heart or spinal cord. In addition, respiratory gating significantly reduced normal tissue dose for most of the patients.
Conclusions: There is no threshold of tumor motion to predict the potential benefits of gated proton radiotherapy. Proton respiratory gating reduces normal tissue dose; however the benefit of gating is not correlated to the magnitude of tumor motion. The benefit may be due to the reduction in anatomy motion as a whole, not just the tumor motion. Other metrics should be explored to determine the need of proton respiratory gating.
Funding Support, Disclosures, and Conflict of Interest: This project is supported by grant P01CA021239 from the National Cancer Institute.