Purpose: Tumor motion due to the breathing cycle affects the dosimetry of the lung treatment especially for SBRT cases. We retrospectively studied the correlation between the dose coverage and the tumor motion including rigid motion and deformable motion for SBRT lung treatment.
Methods: Six patients are enrolled into this study. Simulation 4D-CT is acquired and divided to 10 individual phases. The gross tumor volume (GTV) is contoured on each phase of the breathing cycle. The original treatment plan is generated on the average phase. This plan is copied to each individual phase CT and recalculated with same beams orientation and with a weighting factor of 10% prescription dose. The dose on each phase is transformed back to the average phase based on the deformable registration. A 4D dose is then determined by combining the weighted dose from all 10 phases. The displacement of the GTV centroid is used as the index of rigid motion; and the GTV volume change is used as the index of the deformable motion.
Results: The 4D composite dose is compared with the clinical plan. With rigid motion from 0.22 cm to 1.89 cm, the 4D PTV dose coverage dropped by up to 20% from the clinical plan. The PTV coverage loss gets higher while the tumor motion is higher, on the order of ~10% for 1 cm rigid motion and R2 coefficient is 0.93.When the tumor has similar rigid motion, the tumor with higher deformable motion gets worse dose coverage, with ~ 3% PTV coverage loss for 20% GTV volume change.
Conclusion: For SBRT lung cases, breath motion causes worse dose coverage and lower maximum dose. Rigid motion plays a dominant factor for motion effect on dose coverage. Deformable motion can change the dose coverage too at a smaller scale.
Not Applicable / None Entered.