Purpose: AAPM TG 76 report suggests 5mm tumor motion as the threshold of motion mitigating techniques such as gating or breath‐holding. However, for proton therapy, plan quality can change significantly due to change in tissue density along the beam path independent of tumor motion. In this work, we introduce our in‐house software used for respiratory motion assessment and demonstrate its application for selected patients. Methods: During the 4DCT simulation, an observer estimated the magnitude of tumor motion. Later, the max‐inhale (T0) and max‐exhale (T50) phases were registered using in‐house deformable image registration (DIR) software. The deformation vectors of voxels contained in the internal target volume (ITV) plus 5mm space were used to calculate the mean distance traveled by the tissue in direction that is both perpendicular and parallel to the beam axis. Furthermore, tissue density perturbation was measured as follow: the difference in water‐equivalent thickness (WET) between T0 and T50 were calculated by ray‐tracing along the beam path from the patent skin to the distal surface of the ITV. The observer and DIR measured tumor motion, and the absolute mean differences of WET were compared using Spearman rank correlation. Results: A total of 15 lung cases were studied. The observer measured motion was ranged from 6‐13mm while DIR measurement showed 4‐17mm. The average WET difference was range from 3–9mm. No correlation was observed between observer measured and WET change (rho = 0.16). A positive correlation was observed between DIR and the WET change (rho = 0.7). Conclusion: Our Result: indicates that the change in tissue density due to the breathing motion is as important as tumor motion itself when considering treatment planning strategy for lung proton therapy. Therefore, tissue perturbation due to breathing motion should be measured along with tumor motion when planning for respiratory management. This project is supported by grant P01CA021239 from the National Cancer Institute.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging