Clinical evaluation of 4D dynamic dose for thoracic tumor stereotactic body radiation therapy with variable parameters

Objective

To evaluate the dose uncertainty in stereotactic body radiation therapy induced by respiratory motion using a 4D dynamic dose (4DDD) reconstruction method.

Methods

A retrospective analysis was conducted on five lung cancer patients who received static intensity-modulated radiation therapy. The 4DDD was constructed using beam delivery log files, four-dimensional computed tomography (4DCT) scans, and treatment plans. To evaluate the impact of respiratory motion, 4DDD calculations were performed with 10 starting phases for each field. A total of 270 field doses were simulated and calculated. The differences between the cumulative volume histogram in whole-course treatment and the field doses' gamma passing rate (GPR) were compared. The correlations between plan complexity metrics and the dose deviation caused by respiratory motion were evaluated independently. The phase distributions of 398 subfields were calculated and evaluated for the influence of dose rate and breathing frequency.

Results

The GPRs of all fields were different among various starting phases, with the highest range from 62.20% to 76.87% for 2 mm/3% GPR. The deviation of mean point dose was (5.42 ​± ​5.21) %, and the deviation in the mean dose and $D_{98\%}$ within the internal gross tumor volume were (0.97 ​± ​0.71) % and (0.77 ​± ​0.53) %, respectively. There was a significant correlation between the beam aperture-to-volume (BA2V) ratio and the average 2 mm/2% GPR (R ​= 0.601, P ​< ​0.01). Lower dose rates led to a more homogeneous distribution of phases among subfields (t ​= ​44.100, P ​< ​0.001).

Conclusions

Different beam starting phases had a limited impact on the overall treatment evaluation. However, the respiratory motion could be observed to induce dose deviations using the 4DDD reconstruction model, particularly for fields with small BA2V.