Purpose: To quantify the impact of respiratory motion variations on the delivered dose, using a previously validated 4D Monte Carlo-based dose reconstruction tool, for a cohort of non-small cell lung cancer patients undergoing curative radiation therapy treatments.
Methods: Surface motion measurements recorded during treatment delivery using the RADPOS 4D dosimetry system were used for input in the 4D dose reconstruction tool, 4Ddefdosxyznrc, along with linac log files and the deformation vectors between the exhale and inhale phases of the 4D CT. Deformation vectors were calculated in Velocity AI (V3.2.1), and the reconstructed doses were compared to the clinical VMAT plans calculated on the average CT scan set using Monaco (V5.51.02).
Results: We present here the results for two patients, for whom we have collected and analyzed the data over five treatment fractions. Each of the patients had the tumor located in the upper right lobe. No motion control device was used during planning CT scans and treatments. The average peak-to-peak breathing amplitude, r, was 5.23 ± 1.35 mm and 4.74 ± 0.21 mm for Patients 1 and 2, respectively.The 4D reconstructed doses show less than 0.5% inter-fraction variation for the CTV D98% for both Patient 1 and 2 and are within 3.0% of the CTV V66Gy planned in Monaco. These results are consistent with relatively small breathing motion amplitude measured for these patients. However, percent change of up to 22.3% in the mean dose to the esophagus was found for Patient 2 when comparing the planned dose to the reconstructed doses.
Conclusion: This preliminary study demonstrated the use of a 4D reconstruction tool for a small patient sample to evaluate the effects of respiratory motion on delivered dose. Future studies will include larger number of lung patients and variety of tumor locations.
Funding Support, Disclosures, and Conflict of Interest: We acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (NSERC).