Variability of Inter-Fraction Target Motion during Hypofractionated Lung Radiation Therapy

Abstract

<h3>Purpose/Objective(s)</h3> Pre-treatment 4D-CTs are used in planning for motion management of lung RT. The target excursion (E) at the time of planning may differ between planning and treatment. Additionally, the tumor excursion can change between fraction deliveries. The objective of this study is to investigate patterns of excursion variability (∆E) during hypofractionated radiotherapy (HRT) and the clinical factors that correlate with ∆E. <h3>Materials/Methods</h3> 41 patients with primary or metastatic lung tumors underwent HRT. For each patient, planning was performed on 4D-CT imaging. Patients underwent daily pre- and post- treatment 4D-CBCT for setup conformation and motion assessment. Respiration-induced excursion (E) was quantified in the planning 4D-CT, and in each daily excursion 4D-CBCT. All excursion values were measured in 3 dimensions: AP, LR, and SI. ∆E was calculated as the difference between daily 4D-CBCT excursion values and the planning 4D-CT excursion value. The clinical statistics and excursion statistics were evaluated for all fractions. Finally, Pearson correlation was performed to associate clinical factors with ∆E. <h3>Results</h3> 377 4D-CBCTs were analyzed. ∆E was largest in the SI direction, with ∆E_avg in the SI, LR, and AP directions being 0.16, 0.05, and 0.08 cm respectively. Only 7.1% of all 4D-CBCT's displayed ∆E ≥ 4mm. The average age of patients with ∆E ≥ 4mm was 58.63 y/o, and that of patients with ∆E<4mm was 71.73 y/o (P<0.001). The average baseline weight of patients with ∆E ≥ 4mm and ∆E<4mm were 84.94 kg and 72.82 kg respectively (P<0.001). 71.1% of males and 28.9% of females gave ∆E ≥ 4mm (P=0.003). 58.3% of tumors with ∆E ≥ 4mm were in the right lower lobe (RLL) and 20.8% of tumors with ∆E ≥ 4mm were in the left lower lobe (LLL) (P<0.001). An average AP planning excursion of 0.34cm gave ∆E<4mm and that of 0.53 cm gave ∆E ≥ 4mm (P =0.008). The average Charlson Comorbidity Index (CCI) was 3.58 for tumors with ∆E<4mm, and 5.17 for tumors with ∆E ≥ 4mm (P=0.004). Pearson correlation between the left and right lungs didn't yield statistically significant results (P=0.76). <h3>Conclusion</h3> Large ∆E seems predictable, with ∆E ≥ 4mm seen more in young patients with higher baseline weight, male gender, tumors in the lower lobes, and with large 4D-CT excursion displayed in the AP direction. There was no correlation found between ∆E and right versus left lung. Additionally, CCI did not correlate with excursion variability. Finally, the largest degree of ∆E is displayed in the SI direction.