2171 - 4-D Dose of Stereotactic Body Radiotherapy to Moving Target in Patients with Non-Small Cell Lung Cancer: Dosimetry Comparison and Potential Clinical Impact

Purpose/Objective(s): Our earlier clinical data showed that the tumor motion volume (TMV) was correlated to local and regional control rate. The aim of this study is to evaluate the dosimetry differences and clinical impact of delivering a 4-dimensional (4-D) dose vs. 3-D dose of stereotactic body radiotherapy (SBRT) to moving targets in the patients with Non-Small Cell Lung Cancer (NSCLC).

Materials/Methods: Six of NSCLC patients treated with SBRT were selected for this study. The SBRT treatment plans were generated based on 10 phases of 4-DCT images, accounting for tumor motion across different phases of the respiratory cycle. Dosimetry comparisons were made between the conventional 3-D dose and 4-D dose plans. The 4-D dose is generated by projecting each phase dose (10% of the prescription dose) to each individual phase targets and recalculated their dose of targets. The dose on each phase target is then transformed back to the average phase base on the deformable register between the average phase and the individual phase. The correlation of the difference in the dose coverage of D100 (dose covering 100% of the planning target) between 4-D dose and 3-D dose with TMV was analyzed. Target motion volume was determined by subtracting gross tumor volume (GTV) from internal target volume (ITV). The rigid motion of GTV centroid represents TMV.

Results: In our retrospective study including106 early-stage NSCLC patients treated with SBRT with median follow up 22 months, the 2-year local control, regional control, nodal control, and freedom from metastasis rates were 93%, 94%, 77% and 81%, respectively. Univariate analysis showed that large TMV was associated with decreased local control (p=0.0013), nodal control (p=0.0089) and freedom from metastasis (p=0.0003). In these six patients, the range of rigid motion is from 0.22 cm to 1.89 cm and the difference of D100% coverage between 4-D dose and 3-D dose is ranged from -0.25 to 20%, respectively. There is a significant correlation between the difference of D100% and rigid motion with Spearman correlation co-efficient of 0.93. D100% coverage loss was approximately 10% for 1 cm rigid motion.

Conclusion: The integration of 4-D dosimetry in SBRT for NSCLC allows for more precise targeting of moving tumors, resulting in improved dosimetry outcomes, and enhanced clinical benefits for the patients with relatively large TMV. Alternatively, it is necessary to implement the active tumor motion management to reduce TMV and avoid potential clinical impact on outcomes of SBRT treatment.