Accumulated Dose Estimation in Lung Stereotactic Body Radiation Therapy using Deformable Image Registration Algorithm
Wu Q, Liu C, Devpura S, Ajlouni M, Movsas B, and Chetty I. Accumulated dose estimation in lung stereotactic body radiation therapy using deformable image registration algorithm. Med Phys 2017; 44(6):3222-3223.
Purpose: To estimate accumulated dose to targets and OARs for lung cancer patients treated with SBRT. Methods: CBCT and planning CT (pCT) image datasets for lung cancer patients treated with SBRT (12Gyx4), were deformably registered. ITV-PTV margin was uniformly 5 mm. The pCT was deformed and resampled based on the displacement vector field (DVF) from the pCT/CBCT registration, so as to minimize HU issues associated with CBCT. Deformable image registration (DIR) was based on a multi-resolution b-spline algorithm with mutual-information as the similarity metric. Dose-ofthe- day was calculated on the deformed pCT datasets, for each of the four fractions, and dose was accumulated using an energy-mass-mapping algorithm. Results: Mean (standard deviation) of cumulative mean dose minus plan mean dose was -0.6% (1.2%) for the GTV, -0.9% (1.3%) for the PTV, -1.8% (1.2%) for the spinal cord, and -1.6% (4.1%) for total lung-PTV. Mean (standard deviation) of cumulative D95 minus plan D95 was -0.5% (1.3%) for the GTV and -2.9% (2.6%) for the PTV. Mean (standard deviation) of cumulative max. dose minus plan max. dose was -1.1% (1.0%) for the GTV, -1.1% (0.9%) for the PTV, 3.4% (11.7%) for the spinal cord, and 1.1% (2.0%) for total lung-PTV. Mean (standard deviation) of cumulative min. dose minus plan min. dose was 2.1% (3.5%) for the GTV and -14.2% (5.9%) for the PTV. Conclusion: Cumulative doses using deformable dose accumulation were found to be less than planned doses. The planned dose degradation is noted in the DVH shoulders, where min. differences >10% are observed for the PTV, albeit with the GTV min. dose maintained within (2- 4%). OAR max. doses for critical structures such as the spinal cord are shown to increase >10%, which is an important consideration for this serial organ, in the context of SBRT lung treatment.